Biology Ch 13
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Transcript of Biology Ch 13
AN INTERVIEW WITH
Terry l. Orr-WeaverHow the daughter cells resulting from celldivision end up with equal numbers of chromosomes is the focus of much of Terry OrrWeaver's research; she also studies how cellscontrol the DNA replication that precedescell division. Her research group has identi·fied a number of proteins involved in theseprocesses, A professor of biology at MIT andthe first woman to become a member of theWhitehead Institute for Biomedical Research,Dr. Orr-Weaver has an undergraduate degree in chemistry from UC San Diego and aPh.D. in biological chemistry from Harvard.She is a member of the U.S. National Academy of SCiences and a past president of theGenetics Society of America.
After receiving your Ph.D., you switchedfrom using single-celled yeast as your modl'lorganism to thl' multicelled fruit flyDrosophila melanogaster. Why?[ was excited by the possibility of working atthe interface of two fields. At that time, the cellcycle field was exploding from work with single cells-mainly yeasts and mammalian cellsgrown in culture-and biochemical experiments using extracts. Meanwhile, developmental biology was entering a new era withthe discovery of pattern formation genes. [ realized there was a key question at the interfaceof these two fields that was being ignored: If anorganism starts as a single cell and ends up as amulticellular entity, how are pattern and celldivisions coordinated? And I realized therehad to be intrinsic regulation from the cell cycle components but also extrinsic developmental control feeding into that. This wasclearly going to be an important area of study.
In choosing a multicellular model organism,why use Drosophila?A lot of fundamental discoveries in biologyhave been made with fruit nil's. For example, itwas research with DroS<Jph.i1a that establishedin 1916 that chromosomes were the physical
,%
basis of inheritance, the structures that carriedthe genes. Decades later, the master regulatorygenes that set up the animal body plan in embryonic development were discovered inDrosoph.ila. These genes determine, for example, where the fly's head is and where its legsare. Then, to the amazement of biologists. itturned out that exactly the same genes controlhow the human body plan is set up! $0Drosophila has a really rich heritage. When Istarted working on Drosophila, it had a 70
year-long history of use in genetics, prOVidingan incredible array of knowledge and methods.
How do geneticists approach biologicalquestions?Geneticists want to discover the genes that areinvolved in a biological process and then figureout what the genes do. First. you decide whatprocess you're interested in-in our case, howchromosomes gel partitioned during cell division. Next, you figure out what you would seeif you had a mutant in which that process wasperturbed-how would you recognize it? Thenyou generate mutations, usually with the helpof high-energy radiation or a chemical, hopingthat one or more of the resulting mutants areaffected in the process of interest. \vt1at you'redoing by making a mutation is generating adisease state in your organism. \vt1en you'vefound a mutant you're interested in. you canfind out what gene has been made defective,and you know that that gene has to play an important role in the process you're interested in.
Now that we have the genome sequences ofmany organisms, including the fruit fly and thehuman, you might think that all their genes areknown-so what's to discover? But it turned outthat we didn't know the functions of many of thegenes that showed up in genome sequences.Even in an organism as simple and well-studiedas yeast, at the publication of the genome sequence we didn't know what 70% ofthe genes did.And we still don't have a clue about halfofthehuman genes. So having genome sequences provides a foundation, but we must still find outwhat all the genes do, and we geneticists feelthe best way to go about this is 10 use genetics.
A big and useful surprise in this genediscovery enterprise has been how many geneshave been 'conserved" through evolutionaryhistory and are still verysimiJar in organisms asdistantly related as fruit flies and humans. It'svery hard to figure out directly what humangenes do. But when we discover a new gene inDrosophila, invariably it turns out thai there's asimilar gene in humans that does the samething. So Drosophila turns out to be an evenbetter model organism than we had guessed.
Besides genetics, what other approachesand methods do you use?A combination ofgenetics, biochemistry, and cellbiology turns out to be incredibly powerful forour research. For instance, with the microscope,a tool ofcell biology. we can literally watch thechromosomes as they undergo mitosis or meiosis.Ifwe have a mutation causing a defect in oneofthose processes, we can look directly at howthe chromosomes behave in mutant cells. And inour research on DNA replication, we can look di
rectly at proteins that attach to the DNA duringDNA replication. In this lab, we try to do thingsas directly as possible!
Here's an example from our work on DNAreplication in Drosophila. Using a genetic approach. we discovered a mutant that couldn'tcarry out DNA replication-although such amutant can live and grow for a while by usingproteins its mother stockpiled in the egg. Itlooked like the gene affected might code for areally important protein. But we wanted to findout if it was a protein that played a direct role inDNA replication, and we couldn't tell that fromthe genetics alone. However, by labeling thenormal version of the protein with a fluorescent tag, we could use the microscope to seewhere the protein was located in cells, and wesaw that the protein got on the DNA right at aplace where DNA synthesis starts. That established that this protein was directly involved.
How is meiosis different from mitosis?Let me first review mitosis. After a cell duplicates its DNA, mitosis ensures that each duplicated chromosome gets partitioned to the
daughter cells so that you end up with twocells that have exactly the same DNA contentand chromosome number. That's what happens in normal cell division.
But what about making a sperm or an egg?In fertilization, a sperm and egg are going tofuse and give rise to a new progeny. [fthe organism is diploid-that is, has two similarcopies of each chromosome, as nil'S and humans do-and the progeny is going to bediploid, then you've got to make sperm andeggs that have only one copy of each chromosome, so that when those sperm and egg cometogether you restore the right chromosomenumber. And to produce sperm and eggs withonly half the diploid chromosome number, youneed a special kind of cell division-meiosis.There has to be a way to bring together thetwo similar copies of each chromosome andthen pull them apart, with each going to a different daughter cell. [n meiosis, there is an
extra round of chromosome partitioningwhere what we call the homologous pairs ofchromosomes-the chromosome that initially
came from dad and the similar chromosomethat came from mom-get separated fromeach other. To sum up, in mitosis you're separating identical copies of each chromosome,whereas in meiosis you have an extra round ofdivision stuck in there, where the copy ofeachchromosome from dad and the copy frommom get separated from each other.
What imporlanl questions about meiosisstill need to be answeredlWhat we don't understand at all is how the
chromosomes of a homologous pair find eachother. That pairing is unique to meiosis; itdoesn't happen in mitosis. The two strands ofthe DNA don't come apart, so although thehomologous chromosomes have very similarDNA sequences, it's not base-pairing that
brings the homologous chromosomes together. Given the relatively gigantic volume ofthe nucleus and the huge mass of chromatin ina eukaryotic cell, how do the right chromo
somes find each other? That's the number onemystery about meiosis. And [would say thesecond big mystery is why humans are so unbelievably bad at carrying out meiosis.
What makes you say thaB Are we worsethan fruit flies?We're about a thousand times worse. Here are
some amazing statistics: Twenty percent ofrecognized pregnancies end in spontaneousmiscarriage, and out of those at least half aredue to a mistake during meiosis. And for everypregnancy that proceeds far enough to be recognized, there have been many that endedwithout being recognized. So about 10-20% ofthe time meiosis doesn't work properly in humans. In fruit flies, meiosis seems to occur inaccurately only 0.01-0.05% ofthe time. Evenmice, which are mammals like ourselves, do
much better than humans. So I would call thisthe second big mystery regarding meiosis:Why are humans so bad at it?
Tell us more about what happens whenmeiosis doesn't work correctly.Most of the time the result is early death oftheembryo. In humans, if any chromosome otherthan X or Y is present in only one copy ormore than two copies, the embryo dies veryearly. The only exceptions are for chromosomes number 13, 18, and 21: When oneofthese is present in three copies, the embyo usually dies at an early stage, but it may survive.Even in the case of Down syndrome, a relatively common condition where a person hasthree copies of chromosome 21, only a minority of fetuses survive to term, And when the individual does survive, there are serious effects,
In fact, errors in meiosis are the leading causeof menial retardation in the United States.Some scientists argue that, in evolutionaryterms, the human species is able to cope with
such a high rate of meiotic errors because mostof the pregnancies are lost very early,
Whal do you like besl about research?What's so great about research is that you getto unwrap presents all the time! \'ifhen youfind an interesting mutant, it's like a beautifullywrapped present. And then you unwrap it, and
it can be a big surprise to learn which gene isaffected and what its protein product is. Sometimes you unwrap it and it's beautiful and satisfying and completely makes sense; othertimes you don't at first understand what thegift is, and then you have to figure that out, [want to tell students how wonderful it is getting these presents to unwrap. But they alsoneed to realize that it might take three years toget the ribbon and all the tape off. It can takesome real patience to unwrap the present!
In your view, how important is il forscientists to reach out beyond the scientificcommunity?I think scientists do have an obligation to educate the public. It's especially critical for people to understand the importance of basicresearch and the study of model organisms,Because if I'll' shortchange basic research, inthe long run I'll' won't be able to benefit frommedical applications. The reason we've doneso well in this country is our willingness to invest in basic research science.
The challenge is figuring out how to convey amessage like this in an era ofSQundbites. The
media and the public want to hear that a piece ofresearch is directly going to cure a disease. Sohow can we get a message across that requires anexplanation more complex than a catchphrase?There are many stories I'll' can tell where discoveries in basic research have led to very importantapplications, but these stories take at lea;,t acouple of paragraphs to communicate. Also,there's a real need for scientists to try to build a
bridge between science and the public.It's especially important to educate our rep
resentatives in government. A coalition of several biology societies, including the GeneticsSociety of America, has a Joint Steering Committee for Public Policy, which has been terrific in educating members of Congress andtheir staffs. They bring in scientists to explaintheir research and why investing in research isimportant. Maybe that's a good starting point.
Learn about an experiment by Terry OrrWeaver in Inquiry Figure 16,22 on page 322,
Jane Reece and Terry Orr-Weaver
247
Mei"",,-,SexualCycles
KEY CONCEPTS
anife
.... Figure 13.1 What accounts for family resemblance?
13.1 Offspring acquire genes from parents byinheriting chromosomes
13.2 Fertilization and meiosis alternate in sexual lifecycles
13.3 Meiosis reduces the number of chromosomesets from diploid to haploid
13.4 Genetic variation produced in sexual life cyclescontributes to evolution
Most people who send out birth announcementsmention the sex of the baby, but they don't feel theneed to specify that their offspring is a human be
ing! One of the characteristics of life is the ability of organisms to reproduce their own kind-elephants produce littleelephants, and oak trees generate oak saplings. Exceptionsto this rule show up only as sensational but highly suspectstories in tabloid newspapers.
Another rule often taken for granted is that offspring resemble their parents more than they do Wlrelated individuals. If youexamine the famUy members shown in Figure 13.1-SissySpacek and Jack Fisk with daughters Madison and SchuylerFisk-you can pick out some similar features among them. Thetransmission oftraits from one generation to the next is called inheritance, or heredity (from the Latin heres, heir). However, sonsand daughters are not identical copies ofeither parent or oftheirsiblings. Along with inherited similarity, there is also variation.Farmers have exploited the principles of heredity and variationfor thousands of years, breeding plants and animals for desiredtraits. But what are the biological mechanisms leading to thehereditary similarity and variation that we call a "family resemblance"? This question eluded biologists until the development ofgenetics in the 20th century.
248
Genetics is the scientific study of heredity and hereditaryvariation. In this unit, you will learn about genetics at multiplelevels, from organisms to cells to molecules. On the practicalside, you will see how genetics continues to revolutionize medicineand agriculture, and you will be asked to consider somesocial and ethical questions raised by our ability to manipulateDNA, the genetic material. At the end of the unit, you will beable to stand back and consider the whole genome, an organism's entire complement of DNA. The rapid accumulation ofthe genome sequences of many species, including our own, hastaught us a great deal about evolution on the molecular levelin other words, evolution of the genome itself. In fact, geneticmethods and discoveries are catalyzing progress in all areas ofbiology, from cell biology to physiology, developmental biology,behavior, and even ecology.
We begin our study of genetics in this chapter by examining how chromosomes pass from parents to offspring in sexually reproducing organisms. The processes of meiosis (aspecial type of cell division) and fertilization (the fusion ofsperm and egg) maintain a species' chromosome count duringthe sexual life cycle. We will describe the cellular mechanics ofmeiosis and how this process differs from mitosis. Finally, wewill consider how both meiosis and fertilization contribute togenetic variation, such as the variation obvious in the familyshown in Figure 13.1.
~7;::~~g~'~ire genesfrom parents by inheritingchromosomes
Family friends may tell you that you have your mother's freckles or your father's eyes. However, parents do not, in any literal sense, give their children freckles, eyes, hair, or any othertraits. What, then, is actually inherited?
... Figure 13.2 Asexual reproduction in two multicellularorganisms. (a) This relatively simple animal. a hydra. reproduces bybudding. The bud. a localized mass of mitotically dividing cells. developsinto a small hydra. whICh detaches from the parent (LM). (b) Each tree inthis circle of redwoods grew from a single parent tree. whose stump is inthe center of the circle.
I. How are the traits of parents (such as hair color)
transmitted to their offspring?2. Explain how asexually reproducing organisms pro
duce offspring that are genetically identical to eachother and to their parents.
3.•i,'!lfU1jM A horticulturalist breeds orchids, trying
to obtain a plant with a unique combination of desirable traits. After many years, she finally succeeds. Toproduce more plants like this one, should she breed itor clone it? Why?
For suggested answers, see Appendix A.
capable of reproducing asexually (Figure 13.2). Because thecells of the offspring are derived by mitosis in the parent, the~chip off the old block" is usually genetically identical to itsparent. An individual that reproduces asexually gives rise to adone, a group ofgenetically identical individuals. Genetic differences occasionally arise in asexually reproducing organisms as a result of changes in the DNA called mutations,which we will discuss in Chapter 17.
In sexual reproduction, two parents give rise to offspringthat have unique combinations ofgenes inherited from the m'oparents. In contrast to a clone, offspring ofsexual reproductionvary genetically from their siblings and both parents: They arevariations on a common theme of family resemblance, not exact replicas. Genetic variation like that shown in Figure 13.1 isan important consequence of sexual reproduction. Whatmechanisms generate this genetic variation? The key is the behavior of chromosomes during the sexual life cycle.
(b) Redwoods
13.1CONCEPT CHECK
(a) Hydra
Inheritance of Genes
Only organisms that reproduce asexually produce offspringthat are exact copies ofthemselves. In asexual reproduction,a single individual is the sole parent and passes copies of all itsgenes to its offspring. For example, single-celled eukaryoticorganisms can reproduce asexually by mitotic cell division, inwhich DNA is copied and allocated equally to two daughtercells. The genomes of the offspring are virtually exact copiesofthe parent's genome. Some multicellular organisms are also
Parents endow their offspring with coded information in theform of hereditary units called genes. The genes we inherit
from our mothers and fathers are our genetic link to our parents, and they account for family resemblances such as sharedeye color or freckles. OUf genes program the specific traitsthat emerge as we develop from fertilized eggs into adults.
The genetic program is written in the language of DNA,the polymer of four different nuc1eotides you learned aboutin Chapters 1 and 5. Inherited information is passed on inthe form of each gene's specific sequence of DNA nudeotides, much as printed information is communicated inthe form of meaningful sequences of letters. In both cases,the language is symbolic. Just as your brain translates theword apple into a mental image of the fruit, cells translategenes into freckles and other features. Most genes programcells to synthesize specific enzymes and other proteins,whose cumulative action produces an organism's inherited
traits. The programming of these traits in the form of DNAis one of the unifying themes of biology.
The transmission of hereditary traits has its molecular basis in the precise replication of DNA, which produces copiesofgenes that can be passed along from parents to offspring. Inanimals and plants, reproductive cells called gametes are thevehicles that transmit genes from one generation to the next.During fertilization, male and female gametes (sperm andeggs) unite, thereby passing on genes of both parents to theiroffspring.
Except for small amounts of DNA in mitochondria andchloroplasts, the DNA of a eukaryotic cell is packaged intochromosomes within the nucleus. Every living species has acharacteristic number of chromosomes. For example, hu
mans have 46 chromosomes in almost all of their cells. Eachchromosome consists of a single long DNA molecule elaborately coiled in association with various proteins. Onechromosome includes several hundred to a few thousandgenes, each of which is a specific sequence of nuc1eotideswithin the DNA molecule. A gene's specific location alongthe length of a chromosome is called the gene's locus (fromthe Latin, meaning "placen
; plural, loci). Our genetic endowment consists of the genes carried on the chromosomeswe inherited from our parents.
Comparison of Asexualand Sexual Reproduction
(HAPTE~ THIRTEEN Meiosis and Sexual Life Cycles 249
Centromere
[illk I ,,' Of "~ '. • • .'
• •• , .-,,, Ii", ' ,
'-" i i H I ; l(, \ ,,~ #i.. .. ,·, E ii. '.
" II ~\·, ••," "
, "0' H II ! I
" D•• " • •". II " H.,• • " ..
•
RESULTS This karyotype shows the chromosomes froma normal human male. The size of the chromosome, position ofthe centromere, and pattern of stained bands help identify specific chromosomes. Although difficult to discern in the karyotype,each metaphase chromosome consists of two closely attachedsister chromatids (see the diagram of a pair of homologousreplicated chromosomes).
13.3
Preparing a karyotype
T Sisterchromatids
Metaphasechromosome
TECH Nt QUE Karyotypes are prepared from isolated somaticcells, which are treated with adrug to stimulate mitosis and thengrown in culture for several days. Cells arrested in metaphase arestained and then viewed with a microscope equipped with a digital camera. A photograph of the chromosomes is displayed on acomputer monitor, and the images of the chromosomes arearranged Into pairs according to size and shape.
Pair of homologousreplicated chromosomes~
APPLICATION A karyotype is adisplay of condensed chromo·somes arranged in pairs. Karyotyping can be used to screen forabnormal numbers of chromosomes or defective chromosomesassociated with certain congenital disorders, such as Downsyndrome.
In humans, each somatic cell-any cell other than those in
volved in gamete formation-has 46 chromosomes. Duringmitosis, the chromosomes become condensed enough to bevisible in a light microscope. Because chromosomes differ insize, in the positions of their centromeres, and in the patternof colored bands produced by certain stains, they can be dis~
tinguished from one another by microscopic examinationwhen sufficiently condensed.
Careful examination of a micrograph of the 46 humanchromosomes from a single cell in mitosis reveals that thereare two chromosomes of each of 23 types. This becomesdear when images of the chromosomes are arranged inpairs, starting with the longest chromosomes. The resultingordered display is called a karyotype (Figure 13.3). The twochromosomes composing a pair have the same length, centromere position, and staining pattern: These are calledhomologous chromosomes, or homologs. Both chromosomes of each pair carry genes controlling the same inherited characters. For example, if a gene for eye color issituated at a particular locus on a certain chromosome, thenthe homolog of that chromosome will also have a gene spec
ifying eye color at the equivalent locus.The two distinct chromosomes referred to as X and Yare
an important exception to the general pattern of homologous chromosomes in human somatic cells. Human femaleshave a homologous pair of X chromosomes (XX), but maleshave one Xand one Ychromosome (XY). Only small parts ofthe X and Yare homologous. Most of the genes carried onthe X chromosome do not have counterparts on the tiny Y,and the Y chromosome has genes lacking on the X. Becausethey determine an individual's sex, the X and Y chromosomes are called sex chromosomes. The other chromosomes are called autosomcs.
The occurrence of homologous pairs of chromosomes ineach human somatic cell is a consequence ofour sexual origins.We inherit one chromosome of each pair from each parentThus, the 46 chromosomes in our somatic cells are achlally twosets of 23 chromosomes-a maternal set (from our mother)
Sets of Chromosomes in Human Cells
A life cycle is the generation-ta-generation sequence ofstages in the reproductive history of an organism, from conception to production of its own offspring. In this section, weuse humans as an example to track the behavior of chromosomes through sexual life cycles. We begin by considering thechromosome count in human somatic cells and gametes; wewill then explore how the behavior ofchromosomes relates tothe human life cycle and other types ofsexual life cycles.
r;:~~~I~::i~~'~nd meiosisalternate in sexual life cycles
250 UNIT THREE Genetics
Behavior of Chromosome Setsin the Human Life Cycle
The human life cycle begins when a haploid sperm from thefather fuses with a haploid egg from the mother. This unionof gametes, culminating in fusion of their nuclei, is calledfertilization. The resulting fertilized egg, or zygote, isdiploid because it contains two haploid sets of chromosomesbearing genes representing the maternal and paternal familylines. As a human develops into a sexually mature adult, mitosisofthe zygote and its descendants generates all the somatic ceUsof the body. Both chromosome sets in the zygote and all thegenes they carry are passed with precision to the somatic cells.
The only cells of the human body not produced by mitosisare the gametes, which de\'elop from specialized reUs calledgerm cells in the gonads-ovaries in females and testes inmales (Figure 13.5). Imagine what would happen if humangametes were made by mitosis: They would be diploid likethe somatic cells. At the next round offertilization, when twogametes fused, the normal chromosome number of46 woulddouble to 92, and each subsequentgeneration would double the
Key Haploid gametes (n " 23)
and a paternal set (from our father). The number of chromosomes in a single set is represented by n. Any cell with twochromosome sets is called a diploid cell and has a diploidnumber of chromosomes, abbreviated 2n. For humans, thediploid number is 46 (2n = 46), the number ofchromosomesin our somatic cells. In a cell in which DNA synthesis has occurred, all the chromosomes are replicated, and therefore eachconsists oftwo identical sister chromatids, associated closelyatthe centromere and along the arms. Figure 13.4 helps clarifythe various terms that we use in describing replicated chromosomes in a diploid ceiL Study this figure so that you understandthe differences between homologous chromosomes, sisterchromatids, nonsister chromatids, and chromosome sets.
Unlike somatic cells, gametes (sperm and eggs) contain asingle chromosome set. Such cells are called haploid cells,and each has a haploid number of chromosomes (n). For humans, the haploid number is 23 (n = 23). The set of 23 consists of the 22 autosomes plus a single sex chromosome. Anunfertilized egg contains an X chromosome, but a sperm maycontain an X or a Ychromosome.
Note that each sexually reproducing species has a characteristic diploid number and haploid number. For example, thefruit ny, DrosoplJila melanogaster, has a diploid number of 8and a haploid number of 4, while dogs have a diploid numberof78 and a haploid numberof39.
Now that you have learned the concepts ofdiploid and haploid numbers of chromosomes, let's consider chromosomebehavior during sexual life cycles. We11 use the human life cycle as an example.
Haploid (n)
DiplOId (2n)Egg (n)
• Figure 13.4 Desaibing chromosomes. Acell with a dipbdnumber of 6 (2n = 6) is depicted here followang chromosome replicatJOnand condensa1JOfl. Each of the SIX replicated chromosomes conSISts oftwo SiSler chromatids assoaated dosefy along their lengths. Eachhomologous pair IS composed of one chromosome from the maternalset (~ and one from the paternal set (blue). Each set is made up ofthr~ chromosomes Ir'I thIS example. NonSlSter chromatids are any twochromatids In a pa... of homologous chromosomes that a~ not SISter
"""""""n What is the haploid number of this celf? ~ a ~set~ of.. chromosomes haploid Of diploid?
Diploidzygote(2n = 46)
Mitosis anddevelopment
Mulllcellular diploidadults (2n = 46)
• Figure 13.5 The human life cycle. In each generation, thenumber of chromosome sets doubles at fertilization. but IS halveddunnq meIOSIS. For humans, the number of chromosomes in a haplOIdcell is 23. COOSlSllnq of ooe set (n = 23); the number of chromosomesIn the diploid lY90te and all somatIC cells ansmg from It is 46,Consrstlnq of two sets (2n = 46).
ThIS figure IntrodllCe5 a color code that wia be IlSf'C:f for other lifecydes later In thIS book. The teal arrows highlight haploid stages of atife~, and the betge arrows highlight dIploid stages.
Centromere
.., ,L-_7'_palf of homologous!!I chromosomes
(one from each set)
Koy
{
• Maternal set of2n = 6 chromosomes (n = 3)
Paternal set of• chromosomes (n = 3)
Two sister chromatidsof one replicatedchromosome
Two nonSlSter ~""::.l,,--.tlduomalldsma homologous pair
CHAHII THIRTUN Meiosis il.nd Sexua.l Life Cycles 251
number of chromosomes yet again. This does not happen,however, because in sexually reproducing organisms, the gametes are formed by a modified type of cell division calledmeiosis. This type ofcell division reduces the number ofsets of
chromosomes from two to one in the gametes, counterbalanc
ing the doubling that occurs at fertilization. In animals, meiosisoccurs only in the ovaries or testes. As a result of meiosis, eachhuman sperm and egg is haploid (n = 23). Fertilization restores
the diploid condition by combining two haploid sets of chromosomes, and the human life cycle is repeated, generation after generation (see Figure 13.5). You will learn more about theproduction of sperm and eggs in Chapter 46.
In general, the steps of the human life cycle are typical ofmany sexually reproducing animals. Indeed, the processesof fertilization and meiosis are the unique trademarks ofsexual reproduction, in plants as well as animals. Fertilization and meiosis alternate in sexual life cycles, maintaininga constant number of chromosomes in each species from
one generation to the next.
The Variety of Sexual Life Cycles
Although the alternation of meiosis and fertilization is common to all organisms that reproduce sexually, the timing ofthese two events in the life cycle varies, depending on thespecies. These variations can be grouped into three maintypes of life cycles. In the type that occurs in humans and mostother animals, gametes are the only haploid cells. Meiosis occurs in germ cells during the production of gametes, whichundergo no further cell division prior to fertilization. After
fertilization, the diploid zygote divides by mitosis, producinga multicellular organism that is diploid (Figure 13.6a).
Plants and some spe<ies of algae exhibit a second type oflifecycle called alternation of generations. This type includesboth diploid and haploid stages that are multicellular. The multicellular diploid stage is called the sporophyte. Meiosis in the
sporophyte produces haploid cells called spores. Unlike a gamete, a haploid spore doesn't fuse with another cell but divides
mitotically, generating a multicellular haploid stage called thegametophyte. Cells of the gametophyte give rise to gametes bymitosis. Fusion of two haploid gametes at fertilization resultsin a diploid zygote, which develops into the next sporophytegeneration. Therefore, in this type ofHfe cycle, the sporophytegeneration produces a gametophyte as its offspring, and thegametophyte generation produces the next sporophyte generation (Figure 13.6b). Clearly, the term alternation ofgenerations is a fitting name for this type of life cycle.
A third type oflife cycle occurs in most fungi and some protists, including some algae. After gametes fuse and form adiploid zygote, meiosis occurs without a multicellular diploid
offspring developing. Meiosis produces not gametes but haploid cells that then divide by mitosis and give rise to either unicellular descendants or a haploid multicellular adult organism.Subsequently, the haploid organism carries out further mitoses, producing the cells that develop into gametes. The onlydiploid stage found in these species is the single-celled zygote(Figure 13.6c).
Note that either haploid or diploid cells can divide by mitosis, depending on the type oftife cycle. Only diploid cells, however, can undergo meiosis because haploid cells have a singleset of chromosomes that cannot be further reduced. Though
the three types ofsexual life cycles differ in the timing of meiosis and fertilization, they share a fundamental result: genetic
K.y
Zygote
Haploid unicellular ormultICellular organism
n
n
Gametes
20
n
Diploidmulticellular J.~ Mitosisorganism / ~
(sporophyte)
n
FERTILIZATION
o
Gametes
Haploid (n)
• Diploid (2n)
DiPIOid~~-::~::;::::::multicellularorganism
(a) Animals (b) Plants and some algae (c) Most fungi and some protists
.. Figure 13.6 Three types of sexual life cycles. The common feature of all three cycles is the alternation ofmeiosis and fertilization, key events that contribute to genetic variation among offspring. The cycles differ in the limingof these two key events.
252 UNIT THREE Genetics
variation among offspring. A closer look at meiosis will revealthe sources of this variation.
r:·~~::~s·r::~~es the numberof chromosome sets fromdiploid to haploid
Many of the steps of meiosis closely resemble correspondingsteps in mitosis. Meiosis, like mitosis, is preceded by the replication of chromosomes. However, this single replication isfollowed by not one but two consecutive cell divisions, calledmeiosis I and meiosis II. These two divisions result in fourdaughter cells (rather than the two daughter cells of mitosis),each with only half as many chromosomes as the parent cell.
)Diploid cell withreplicatedchromosomes
Chromosomesreplicate
8 Sister chromatids
D'P'"'' c~
1JHomologous pair of replicated chromosomes
/\Si"" mchrom~
;\X 0 Homolog,", X
chromosomesseparate
Haploid cells withreplicated chromosomes
Homologous pair of chromosomes
io diploid P'A'
~ ~ )
13.ZCONCEPT CHECK
I. How does the karyotype ofa human female differfrom that ofa human male?
2. How does the alternation of meiosis and fertilizationin the life cycles of sexually reproducing organismsmaintain the normal chromosome count for eachspecies?
3. Each sperm ofa pea plant contains seven chromosomes. What are the haploid and diploid numbers forpeas?
4. • i,'llfu1iM A certain eukaryote lives as a unicellular organism, but during environmental stress, its cellsproduce gametes. The gametes fuse, and the resultingzygote undergoes meiosis, generating new single cells.What type of organism could this be?
For suggested answers, see Appendix A.
The Stages of Meiosis Haploid cells with unrepllCated chromosomes
The overview of meiosis in Figure 13.7 shows that both members ofa single homologous pair of chromosomes in a diploidcell are replicated and that the copies are then sorted into fourhaploid daughter cells. Recall that sister chromatids are m'O
copies of one chromosome, closely associated all along theirlengths; this association is called sisterchromatid cohesion. Together, the sister chromatids make up one replicated chromosome (see Figure 13.4). In contrast, the two chromosomes ofahomologous pair are individual chromosomes that were inherited from different parents. Homologs appear alike in themicroscope. but they may have different versions of genes,called alleles, at corresponding loci (for example, an allele forfreckles on one chromosome and an allele for the absence offreckles at the same locus on the homolog). Homologs are notassociated with each other except during meiosis, as you willsoon see.
... Figure 13.7 Overview of meiosis: how meiosis reduceschromosome number. After the chromosomes replicate ininterphase. the diploid cell divides twice. yielding four haploiddaughter cells This overview tracks just one pair of homologouschromosomes, which for the sake of simplicity are drawn in thecondensed state throughout (they would not normally be condensedduring interphase), The red chromosome was inherited from thefemale parent, the blue chromosome from the male parent.••l.f.WIII Redraw the cells in this figure using a simple DNAdouble helix (0 represent each DNA molecule
Figure 13.8, on the next rn'o pages, describes in detail thestages of the two divisions of meiosis for an animal cell whosediploid number is 6. Meiosis halves the total number of chromosomes in a very specific way, reducing the number of setsfrom two to one. with each daughter cell receiving one set ofchromosomes. Study Figure 13.8 thoroughly before going on.
CHAPTE~ THI~HEN Meiosis and Sexual Life Cycles 253
• FIguro 13.1
Telophase I andCytokinesis
Anaphase IMetaphase I
..........MEIOSiS I: Separates homologous chromosomes"
Prophase I
@:j[;mmim·TheMeioticDivisionofanAnimaICell
"omologoo,chromosomes
Two haploid cellsform; each chromosomestill consists of twosister chromatids
SISter chromatidsremdlll attached
Homologouschromosomesseparate
Each pair of homologouschromosomes separates
Metaphasep"~
Centromere(WIth kinetochore)
Microtubuleattached tokinetochore
Chromosomes line upby homologous pairs
Fragmentsof nuclearenvelope
Replkated homologouschromosomes (red and blue)pair and exchange segments;2n '" 6 in this example
Centrosome(WIth centnole pair)
Sister ChlilSlTlillachromattds
Prophase I• Chromosomes begin to
condense, and homologs looselypair along their lengths. alignedgene by gene.
• Crossing over (the exchange ofcorresponding seglTl(!nts of DNAmole<ules by nonsisler chromatids)is completed while homologs alein synapsis, held tightly togetherby proteins along their lengths(before the stage shown).
• Synapsis ends in mid·prophase.and the chromosomes in eachpair move apart slightly, asshown above.
• Each homologous pair has one 01more chiasmata, points whereGossing over has occuued andthe homologs are still associateddue 10 cohesion betweef1 sisterdllomalids (sister chromatidcohesion).
• Centrosome movement, spindleformation, and nuclear envelopeb/eakdown occur as in mitosis.
• In late prophase I(after the stageshown), microtubules from onepole or the other attach to thetwo kinetochores, proteinstructures at the centromeres ofthe two homologs. Thehomologous pairs then movetoward the metaphase plate.
Metaphase I• Pairs of homologouschromoso~ are now arrangedon the metaphase plate, withone chromosome in each pairfacing each pole.
• Bolh dwomatids ofall! tonoIog areattad'led 10 kinetochoreiOOouDJIes frrm all! jXIIe; those ofthe other hlmolog are attathed 10
iOOoniluIes frrm the oppo5ite pole.
Anaphase I• Breakdown 01 proteins
responsible for sister chromatidcohesion along chromatid armsallows homologs to separate.
• The homologs move towardopposite poles. guided by thespindle apparatus.
• Sister chromatid cohesion persistsat the centromere. causingchromatids to move as a unittoward the same pole.
Telophase I andCytokinesis• At the beginning of telophase I,
each half of the cell has acomplete haploid set ofreplicated chromosomes. Eachchromosome is composed of twosister chromatids; one or bothchromatids include regions ofnonsister chromatid DNA.
• Cytokinesis (division of thecytoplasm) usually occurssimultaneously with telophase I.forming two haploid daughter ceI~
• In animal cells. a cleavage furrowIOlms.. (In plant cells. a cell plateIOlms..)
• In some spe<ies. chromoso~deconr:lense aoo the nudearenvtlope re-forms..
• No replication occurs betweenmeiosis Iand meiosis II.
254 UNIT TUff Genetics
Telophase II andCytokinesis
Anaphase 11Metaphase n-l,.--1111.................. MEIOSIS II: Separates sister chromatids .....
Prophase 11
During another round of (ell division, the sister dlromatids finally separate:four haploid daughter cells result., containing unrepficated chromosomes
Haploid daughtef cellsfOfml"9
SISler chromatidsseparate
Prophase II• Aspindle apparatus forms.
• In late prophase II (nOI shownhere), chromosomes, each stillcomposed of two chromatidsasso<iated <lIthe centromere,move toward the metaphase IIplale.
Metaphase II• The chromosomes are positioned
on the metaphase plate as inmitosis.
• Because of crossing over inmeiosis I, the two sisterchromatids of each chromosomeare not genetically identical.
• The kinetochores of sislerchromatids afe attached tomicrotubules extl'nding fromopposite poles,
Anaphase II• Breakdown of proteins holding
the sister chromatids togethel atthe centromere allows thechromatids to separate, Thechromatids move towardopposite poles as individualchromosomes.
Telophase IIand Cytokinesis• Nuclei form, the chlOmosomes
begin de(Ondensing, andcytokinesis occurs.
• The meiotic division of oneparent cell produces fourdaughter cells, each with ahaploid set of (unreplicatedlchlOmosomes.
• Each of the fOUl daughter cells isgeflelically distinct hom theolhel daughter cells and flOmthe parent cell.
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(HAHUI THIRTUH Meiosis and Sexual Life C)"des 255
MITOSIS MEIOSIS
Parent cell(before chromosome replication) MEIOSIS I
Metaphase I
Prophase I
Homologous chromosomepair held together bychiasma and sisterchromatid cohesion
Chiasma (site ofcrossing over)
.Ai\,LcOh-,-,m-,,-,-m-,-( =- t
replication
Chromosomes lineup by homologouspairs at themetaphase plate
2n= 6
Chromosomesline up Individually at themetaphase plate
Chromosomereplication
ReplICated chromosome(two sister chromatids)
Metaphase
Prophase
AnaphaseTelophase
®-..... --" 20
Daughter cellsof mitosis
Sister chromatidsseparate duringanaphase
@-.... -......20
Homologsseparateduringanaphase I;sisterchromatidsremainattached atcentromere
Sisterchromatidsseparatedunnganaphase II
oDaughter cells of meiosis II
Anaphase ITelophase I
Haploidn=3
MEIOSIS II
SUMMARY
Property MitOSIS MeIOSIS
DNAreplication
Number ofdivisions
Occurs during interphase before mitosis begins
One, including prophase, metaphase,anaphase, and telophase
Occurs during interphase before meiosis I begins
Two, each including prophase, metaphase, anaphase, andtelophase
Synapsis ofhomologouschromosomes
Does not occur Occurs during prophase I along with crOSSing overbetween nonsister chromatids; resulting chiasmatahold pairs together due to sister chromatid cohesion
Number ofdaughter cellsand geneticcomposition
Role in theanimal body
Two, each diploid (2n) and geneticallyidentical to the parent cell
Enables multicellular adult to arise fromzygote; produces cells for growth, repair, and,in some species, asexual reproduction
Four, each haploid (n), containing half as many chromosomesas the parent cell; genetically different from the parentcell and from each other
Produces gametes; reduces number of chromosomes by halfand introduces genetic variability among the gametes
... Figure 13.9 A comparison of mitosis and meiosis in diploid cells.••I;t-W'I' Could any other combinations of chromosomes be generated during meiosis II from the specific cellsshown in telophase P Explain. (Hint: Draw fhe cells as they would appear in metaphase II.)
256 UNIT THREE Genetics
What prevents the separation of sisterchromatids at anaphase I of meiosis?
N'mu". Draw agraph showing what you expect happened tothe chromatids of the unlabeled chromosome in both strains of cells,
# 10080
~ 60~ 40&. 20~ 0 .t,2~::-;;c
Shugoshin+ Shugoshin-
SOURCE
CONCLUSION The researchers concluded that shugoshin pro·tects cohesins at the centromere at anaphase I. thus maintainingthe attachment between sister chromatids and ensuring that theyseparate properly during meiosis II,
Shugoshin+ (normal) Shugoshin-
Spore case Fluorescent label
~tL - -Metaphase I
-)('-- t -1/ ~.--Anaphase I
kt
II l.Metaphase II
t OR
-I • ,II' ,II'-.... Anaphase II ""- ""-
~t
Maturespores
Two of three possible arrange-Sporements of labeled chromosomes
T. S, KitaJ'III<l, S. A. KiJWa'>hlma, and Y. Watanabe, Theconserved konetochore proteon shugoshin protects centromeric coheSIon during
meIOsis. N~lure 427510-517 aOO4)
Yoshinori Watanabe and colleagues knew thatduring anaphase I. the protein shugoshin is present only aroundcentromeres, They wondered whether it protects cohesins therefrom degradation in meiosis I. ensuring that chromatids stay together while homologs separate. To test this hypothesis. they useda species of yeast in which meiosis produces haploid spores linedup in aspecific order inSide a spore case To follow the movementof chromosomes. they fluorescent/y labeled a region near the centromere of both chromatids in one homolog, leaving the other homolog unlabeled. They then disabled the gene coding forshugoshin and compared this yeast strain (shugoshin ) with normal yeast cells (shugoshin +), The researchers expected that the twolabeled chromosomes ariSing from the labeled chromatids in normal cells would end up in separate spores at one end of the sporecase. They further predicted that if shugoshin does protect cohesins from cleavage at the centromere at anaphase I, then the labeled chromosomes in shugoshin- cells would separate randomlyin meiosis II, sometimes ending up in the same spore.
EXPERIMENT
RESULTS In shugoshin +cells, the two labeledchromosomes ended up indifferent spores in almost allcases, In shugoshin- cells.they were in the same sporein about half the cases.
AComparison of Mitosis and Meiosis
Figure 13.9 summarizes the key differences between meiosis
and mitosis in diploid cells. Basically, meiosis reduces thenumber of chromosome sets from two (diploid) to one (hap[oid), whereas mitosis conserves the number ofchromosomesets. Therefore, meiosis produces cells that differ geneticallyfrom their parent cell and from each other, whereas mitosis
produces daughter cells that are genetically identical to their
parent cell and to each other.Three events unique to meiosis occur during meiosis I:
1. Synapsis and crossing over. During prophase I, replicated homologs pair up and be<ome physically connectedalong their lengths by a zipper-like protein structure, thesynaptonemal complex; this process is called synapsis.
Genetic rearrangement between nonsister chromatids,
known as crossing over, is completed during this stage.
Following disassembly of the synaptonemal complex in
late prophase, the hvo homologs pull apart slightly but re
main connected by at least one X-shaped region called a
chiasma (plural, chiasmata). A chiasma is the physical
manifestation of crossing over; it appears as a cross be
cause sister chromatid cohesion still holds the m'o origi
nal sister chromatids together, even in regions where one
of them is now part of the other homolog. Synapsis and
crossing over normally do not occur during mitosis.
2, Homologs on the metaphase plate, At metaphase I of
meiosis, chromosomes are positioned on the metaphase
plate as pairs of homologs, rather than individual chro
mosomes, as in metaphase of mitosis.
3. Separation of homologs. At anaphase I of meiosis, the
replicated chromosomes ofeach homologous pair move
toward opposite poles, but the sister chromatids of each
replicated chromosome remain attached. In anaphase of
mitosis, by contrast, sister chromatids separate.
How do sister chromatids stay together through meiosis I
but separate from each other in meiosis II and mitosis? Sister
chromatids are attached along their lengths by protein com
plexes called cohesins. In mitosis, this attachment lasts until the
end of metaphase, when enzymes cleave the cohesins, freeing
the sister chromatids to move to opposite poles of the cell. In
meiosis, sister chromatid cohesion is released in two steps. In
metaphase I, homologs are held together by cohesion between
sister chromatid arms in regions where DNA has been ex
changed. At anaphase I, cohesins are cleaved along the arms,
allowing homologs to separate. At anaphase II, cohesins are
cleaved at the centromeres, allowing chromatids to separate.
Figure 13.10 shows one of a series of experiments carried
out by Yoshinori Watanabe and colleagues at the University of
Tokyo. They knew that similar proteins were present in co
hesin complexes during mitosis and meiosis, and they won
dered what was responsible for preventing cohesin cleavage at
the centromere while it was occurring along sister chromatid
CHAPTER THIRTEEN Meiosis and Sexual Life Cycles 257
~:~':t~;v~~i~~on producedin sexual life cycles contributesto evolution
I. How are the chromosomes in a cell at metaphase ofmitosis similar to and different from the chromo
somes in a cell at metaphase of meiosis II?
2. -'M"'I. Given that the synaptonemal complexdisappears by the end of prophase, how would thetwo homologs be associated if crossing over did notoccur? What effect might this ultimately have ongamete formation?
For suggested answers, see Appendi~ A.
arms at the end of metaphase I. They found a protein theynamed shugoshin (Japanese for "guardian spirit") that protectscohesins from cleavage at the centromere during meiosis I.Shugoshin is similar to a fruit fly protein identified lOyears ear~
lier by Terry Orr-Weaver, this unit's interviewee.
Meiosis I is called the reductional division because ithalves the number of chromosome sets per cell-a reduc·tion from two sets (the diploid state) to one set (the haploidstate). During the second meiotic division, meiosis II (some
times called the equational division), the sister chromatidsseparate, producing haploid daughter cells. The mechanismfor separating sister chromatids is virtually identical inmeiosis II and mitosis. The molecular basis of chromosomebehavior during meiosis continues to be a focus of intenseresearch interest.
CONCEPT CHECI( 13.3
Origins of Genetic Variation Among Offspring
In species that reproduce sexually, the behavior of chromosomes during meiosis and fertilization is responsible for most
of the variation that arises each generation. Let's examinethree mechanisms that contribute to the genetic variationarising from sexual reproduction: independent assortment ofchromosomes, crossing over, and random fertilization.
Independent Assortment of Chromosomes
One aspect ofsexual reproduction that generates genetic variation is the random orientation of homologous pairs of chromosomes at metaphase of meiosis L At metaphase I, thehomologous pairs, each consisting of one maternal and onepaternal chromosome, are situated on the metaphase plate.(Note that the terms maternal and paternal refer, respectively,
to the mother and father of the individual whose cells are un·dergoing meiosis.) Each pair may orient with either its mater·nal or paternal homolog closer to a given pole-its orientationis as random as the flip of a coin. Thus, there is a 50% chancethat a particular daughter cell of meiosis I will get the maternal chromosome of a certain homologous pair and a 50%chance that it will get the paternal chromosome.
Because each homologous pair of chromosomes is positioned independently ofthe other pairs at metaphase I, the firstmeiotic division results in each pair sorting its maternal andpaternal homologs into daughter cells independently of every
other pair. This is called independent assortment. Each daugh·ter cell represents one outcome of all possible combina·tions of maternal and paternal chromosomes. As shown inFigure 13.11, the number of combinations possible fordaughter cells formed by meiosis of a diploid cell with twohomologous pairs of chromosomes is four (two possiblearrangements for the first pair times two possible arrangements for the second pair). Note that only two of the four
Possibility 2How do we account for the genetic variation illustrated in Figure 13.1? As you willlearn in more detail in later chapters, mutations are the original source of geneticdiversity. These changes in an organism'sDNA create the different versions ofgenes known as alleles. Once these differ~
ences arise, reshuffling of the alleles dur
ing sexual reproduction produces thevariation that results in each member of aspecies having its own unique combination of traits.
.... Figure 13.11 The independentassortment of homologouschromosomes in meiosis.
258 UNIT THREE Genetics
Possibility 1
Two equally probablearrangements ofchromosomes at
metaphase I
Metaphase II
Daughtercells
Combination 3 Combination 4
... Figure 13.12 The results of crossing over during meiosis.
Nonsist~r chromatidsh~ld tog~ther
during synapsis
f.) Chiasmata andatlachm~nts betweensister chromatids holdhomologs together;they move to themetaphase I plate,
o In prophas~ t synapsisand crossing overoccur; then homologsmov~ apart slightly.
oBreakdown of prot~ins
holding sister chroma·tid arms togetherallows homologs withrecombinant chromatids to separate,
Pair of --''-'';:0-/homologs
Recombinantchromosomes
Chiasma.site ofcrossingover
Centromere
IEM
Anaphase [
Anaphase II
Prophas~ Iof m~iosis
Daughtercells
ing meiosis II further increases the number of genetic types of
daughter cells that can result from meiosis.
You will learn more about crossing over in Chapter 15. The
important point for now is that crossing over, by combining
DNA inherited from two parents into asinglechromosome, is
an important source of genetic variation in sexual life cycles.
The random nature of fertilization adds to the genetic vari·
ation arising from meiosis. In humans, each male and fe
male gamete represents one of about 8.4 million (223)
possible chromosome combinations due to independent as
sortment. The fusion of a male gamete with a female gamete
during fertilization will produce a zygote with any of about70 trillion (223 x 223
) diploid combinations. If we factor in
the variation brought about by crossing over, the number of
possibilities is truly astronomical. It may sound trite, but
you really are unique.
Random Fertilization
combinations of daughter cells shown in the figure would result
from meiosis of a single diploid cell, because a single parent cell
would have one or the other possible chromosomal arrangement
at metaphase I, but not both. However, the population of daugh
ter cells resulting from meiosis of a large number of diploid cells
contains aU four types in approximatelyequal numbers. In the caseof n = 3, eight combinations of chromosomes are possible for
daughter cells. More generally, the number of possible combina
tions when chromosomes sort independently during meiosis is 2n,
where n is the haploid number of the organism.
In the case of humans (n = 23), the number of possible
combinations of maternal and paternal chromosomes in the
resulting gametes is 223, or about 8.4 million. Each gamete that
you produce in your lifetime contains one of roughly 8.4 mil
lion possible combinations of chromosomes.
Crossing Over
As a consequence of the independent assortment of chro
mosomes during meiosis, each of us produces a collection of
gametes differing greatly in their combinations of the chro
mosomes we inherited from our two parents. Figure 13.11
suggests that each individual chromosome in a gamete is ex
clusively maternal or paternal in origin. In fact, this is notthe case, because crossing over produces recombinant
chromosomes, individual chromosomes that carry genes
(DNA) derived from two different parents (Figure 13.12).In meiosis in humans, an average of one to three crossover
events occur per chromosome pair, depending on the size of
the chromosomes and the position oftheir centromeres.
Crossing over begins very early in prophase I, as homolo
gous chromosomes pair loosely along their lengths. Each gene
on one homolog is aligned precisely with the corresponding
gene on the other homolog. In a single crossover event, spe
cific proteins orchestrate an exchange of corresponding seg
ments of two nonsister chromatids-one maternal and one
paternal chromatid ofa homologous pair. In this way, crossing
over produces chromosomes with new combinations of ma
ternal and paternal alleles (see Figure 13. I2).
In humans and most other organisms studied so far, cross
ing over also plays an essential role in the lining up ofhomolo
gous chromosomes during metaphase I. As seen in Figure 13.8,
a chiasma forms as the result of a crossover occurring while
sister chromatid cohesion is present along the arms. Chias
mata hold homologs together as the spindle forms for the first
meiotic division. During anaphase I, the release of cohesion
along sister chromatid arms allows homologs to separate.
During anaphase II, the release ofsister chromatid cohesion at
the centromeres allows the sister chromatids to separate.
At metaphase II, chromosomes that contain one or more re
combinantchromatids can be oriented in two alternative, non
equivalent ways with respect to other chromosomes, because
their sister chromatids are no longer identical. The different
possible arrangements of nonidentical sister chromatids dur-
CHAPTE~ THIRTEEN Meiosis and Sexual Life Cycles 259
CONCEPT CHECK
The Evolutionary Significance of GeneticVariation Within Populations
Now that you've learned how new combinations of genesarise among offspring in a sexually reproducing population, let's see how the genetic variation in a population relates to evolution. Darwin recognized that a populationevolves through the differential reproductive success of itsvariant members. On average, those individuals bestsuited to the local environment leave the most offspring,thus transmitting their genes. This natural selection re·suits in the accumulation of those genetic variations favored by the environment. As the environment changes,the population may survive if, in each generation, at leastsome of its members can cope effectively with the newconditions. Different combinations of alleles may work
better than those that previously prevailed. Mutations are
the original source of different alleles, which are then
mixed and matched during meiosis. In this chapter, we
have seen how sexual reproduction greatly increases the
genetic variation present in a population. In fact, the abil
ity of sexual reproduction to generate genetic variation is
one of the most commonly proposed explanations for the
persistence of sexual reproduction.
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SUMMARY OF KEY CONCEPTS
.i.III1I'_ 13.1Offspring acquire genes from parents by inheritingchromosomes (pp. 248-249).. Inheritance of Genes Each gene in an organism's DNA exists
at a specific locus on a certain chromosome. We inherit one setof chromosomes from our mother and one set from our father.
.. Comparison of Asexual and Sexual Reproduction Inasexual reproduction, a single parent produces geneticallyidentical offspring by mitosis. Sexual reproduction combinessets of genes from two different parents. forming geneticallydiverse offspring.
Acti\ity Asexual and Sexual Life Cycles
.i.IIIII'-13.2Fertilization and meiosis alternate in sexual life cycles(pp.250-253)
.. Sets of Chromosomes in Human Cells Normal human somatic cells are diploid. They have 46 chromosomes made up
260 UNIT THREE Genetics
Although Darwin realized that heritable variation is what
makes evolution possible, he could not explain why offspring
resemble-but are not identical to-their parents. Ironically,
Gregor Mendel, a contemporary ofDarwin, published a theory
of inheritance that helps explain genetic variation, but his dis
coveries had no impact on biologists until 1900, more than 15years after Darwin (1809-1882) and Mendel (l822-1884) had
died. In the next chapter, you will learn how Mendel discovered
the basic rules governing the inheritance of specific traits.
13.41. What is the original source of all the different aneles
of a gene?
2. The diploid number for fruit flies is 8, while that for
grasshoppers is 46. If no crossing over took place,
would the genetic variation among offspring from a
given pair of parents be greater in fruit flies or
grasshoppers? Explain.
3. -Q@i1IM Under what circumstances would
crossing over during meiosis not contribute to genetic
variation among daughter cells?
For $ugg~sted an$w~rs, see Appendix A.
of two sets of23-one set from each parent. In human diploidcells, there are 22 homologous pairs of autosomes, each witha maternal and a paternal homolog. The 23rd pair, the sexchromosomes, determines whether the person is female (XX)or male (XY).
.. Behavior of Chromosome Sets in the Human Life CycleAt sexual maturity, ovaries and testes (the gonads) producehaploid gametes by meiosis, each gamete containing a Singleset of 23 chromosomes (n == 23). During fertilization, anegg and sperm unite, forming a diploid (2n = 46) singlecelled zygote, which develops into a multicellular organismby mitosis.
.. The Variety of Sexual life Cycles Sexual life cycles differin the timing of meiosis relative to fertilization and in thepoint(s) of the cycle at which a multicellular organism isproduced by mitosis.
.ill"I'-13.3Meiosis reduces the number of chromosome sets fromdiploid 10 haploid (pp. 253-258)
.. The Stages of Meiosis The two cell divisions of meiosis produce four haploid daughter cells. The number of chromosomesets is reduced from two (diploid) to one (haploid) duringmeiosis I, the reductional division.
.. A Comparison of Mitosis and Meiosis Meiosis is distinguished from mitosis by three events of meiosis I, as shownon the next page:
Prophase [: EilCh homologous pa,r undergoes sYT1~psis
~nd crossmg over l>elween nClnSJster chrom~1lds
c. the daughter cells are diploid.
d. homologous chromosomes synapse.
e. the chromosome number is reduced.
Metaphase I: Chromosomes line up ~s homologouspairs o<l1he mel~pnase pl~1e.
Anaphase I: Homologs separate from eilCh other; sislerchroma1ids fem~ln joined at lhe centromere.
5. If the DNA content of a diploid cell in the G1 phase of the cell
cycle is x, then the DNA content ofthe same cell at metaphase
of meiosis I would be
a.0.25x. b. D.5.\:. c. x. d. 2x. e. 4x.
6. Ifwe continued to follow the cell lineage from question 5, then the
DNA content ofa single cell at metaphase of meiosis II would be
a. 0,25x. b. D.5.\:. c. x. d. 2x. e. 4x.
d. fertilization.
e. binary fission.
Meiosis II separates the sister chromatids.
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Acthity Meiosis Animation
_',11'''''-13.4Genetic variation produced in sexual life cyclescontributes to evolution (pp. 258-260).. Origins of Genetic Variation Among Offspring Three
events in sexual reproduction contribute to genetic variationin a population: independent assortment of chromosomesduring meiosis, crossing over during meiosis I, and randomfertilization of egg cells by sperm. Due to sister chromatid cohesion. crossing over leads to chiasmata. which hold homologs together until anaphase I.
.. The Evolutionary Significance of Genetic Variation WithinPopulations Genetic variation is the raw material for evolutionby natural selection. Mutations are the original source of thisvariation; the production of new combinations ofvariant genesin sexual reproduction generates additional genetic diversity.
-tiN',·Acthity Origins ofGenetk Variation
In,'estigation How Can the FrequencyofCrossing Over Be Estimated?
TESTING YOUR KNOWLEDGE
SELF·QUIZ
I. A human cell containing 22 autosomes and a Y chromosome is
a. a sperm, d. a somatic cell of a male.
b. an egg. e. a somatic cell of a female.
c. a zygote.
2. \Vhich life cycle stage is found in plants but not animals?
a. gamete d. multicellular haploid
b. zygote e. unicellular diploid
c. multicellular diploid
3. Homologous chromosomes move toward opposite poles of a
dividing cell during
a. mitosis.
b. meiosis I.c. meiosis II.
4. Meiosis 11 is similar to mitosis in that
a. sister chromatids separate during anaphase.b. DNA replicates before the division.
7. How many different combinations of maternal and paternal
chromosomes can be packaged in gametes made by an organism with a diploid number of 8 (2n = 8)?
a. 2 b. 4 c. 8 d. 16 e. 32
Use the diagram of a cell below to answer questions 8-10.
8. How can you tell this cell is under
going meiosis, not mitosis?
9. Identify the stage of meiosis
shown.
to. ••i;flWIl. Copy the
drawing to a sepamte sheet
of paper and label appropri.
ate structures with these
terms, drawing lines or brack-
ets as needed: chromosome (label
as replicated or unreplicated), cen-
tromere, kinetochore, sister chromatids, nonsister chromatids,
homologous pair, homologs, chiasma, sister chromatid cohesion.
Describe the makeup of a haploid set and a diploid set.
For Self-Qui~ answers, see Appendix A.
em If·. Visit the Study Area at www.masteringbio.com for aPractice Test.
EVOLUTION CONNECTION
II. Many species can reproduce either asexually or sexually. What
might be the evolutionary significance of the switch from asex
ual to sexual reproduction that occurs in some organisms when
the environment becomes unfavorable?
SCIENTIFIC INQUIRY
12. The diagram accompanying questions 8-10 represents a meiotic
cell in a certain individual. A previous study has shown that the
freckles gene is located at the locus marked F, and the hair color
gene is located at the locus marked H, both on the long chromo·
some. The individual from whom this cell was taken has inherited different alleles for each gene ("freckles" and "black hair"
from one parent, and "no freddes" and "blond hair" from the
other). Predict allele combinations in the gametes resulting from
this meiotic event. (It will help ifyou draw out the rest of meiosis,
labeling alleles by name.) List other possible combinations of
these alleles in this individual's gametes.
CHAPTE~ THIRTEEN Meiosis and Sexual Life Cycles 261