Chapter 13: Meiosis and Sexual Life Cycles

Cell Theory

• Cell Theory: Living things are made of cells and cells reproduce from other cells

• Asexual Reproduction: Cells divide creating identical cells– A clone is a group of genetically identical individuals

from the same parent• Sexual Reproduction: Gametes fuse to create

zygote – Unique combination of chromosomes gives rise to

offspring with unique set of characteristics

Asexual Reproduction• All prokaryotes• Some protists, invertebrates, plants– Some multicellular organisms

reproduce both sexually and asexually• Budding: creation of new individuals

from growths of existing individuals• Fission: splitting of one organism

into 2• Fragmentation and Regeneration:

Breaking of body part, followed by growth and development

Asexual Reproduction

• Advantages:– Sessile animals can reproduce without mate– Produce many offspring quickly, without energy

invested in sperm and eggs or in mating– Produce offspring with optimal genetics for

environment• Disadvantages:– Genetically uniform populations• Prevents adaptation of population in

changing environment

Sexual Reproduction

• Most animals and plants reproduce by sexual reproduction

• Offspring created by fertilization= fusion of 2 haploid gametes to produce a diploid zygote– Egg, Sperm

• Offspring has unique genetic characteristics

Sexual Reproduction

• Advantages– High genetic variability among offspring– High variability= greater chance for adaption for

changing environments• Evolution by natural selection

• Disadvantages– Must find mate– High energy requirements for courtship, gamete

production

Living Cells

• Genetics is the scientific study of heredity and variation

• Heredity is the transmission of traits from one generation to the next

• Variation is demonstrated by the differences in appearance that offspring show from parents and siblings

Living Cells

• Sexual Reproduction: Gametes fuse to create zygote – Gametes= egg and sperm cells (Haploid)

• Genes are the units of heredity, and are made up of segments of DNA

• Genes are passed from parent to offspring through gametes

– Fertilization= Fusion of egg and sperm cell– Zygote= Fertilized egg (Diploid)

• After fertilization, zygote goes from a single cell to a multicellular organism through mitosis

Gametes

• Gametes contain haploid number of chromosomes– Single set of autosomes + one sex chromosome

• Haploid gametes conserve original number of chromosomes in offspring– If two diploid cells (2n) fused, resulting offspring

would be 4n– If two haploid cells (n) fused, resulting offspring

would be 2n

Chromosomes

• Sex chromosomes– Determine sex of offspring

• Genes can have other functions too

– Mammals= Males have XY chromosomes, Females have XX chromosomes

– Female gametes= all contain X chromosome– Male gametes= 50% contain X and 50% contain Y

• Autosomes– Non-sex chromosomes– In humans, 22 pairs of autosomes

Human Karyotype

Haploid (n)=23 Diploid (2n)=46

Chromosome Terminology• Diploid Cell: 2n = 6– Haploid= n= 3

• Following DNA synthesis, each of the six chromosomes consists of 2 sister chromatids, connected at the centromere

Sister Chromatids

Centromere

HomologousChromosome

Blue: Paternal ChromosomesPink: Maternal Chromosomes

Chromosome Terminology• Locus: location of gene

on chromosome• Maternal and paternal

chromosomes can have different versions of the same gene

• Allele: alternative version of a gene

• Plants and some algae exhibit an alternation of generations• Life cycle has a diploid

and haploid multicellular stage

• The diploid organism, called the sporophyte, makes haploid spores by meiosis• Each spore grows by

mitosis into a haploid organism called a gametophyte

• A gametophyte makes haploid gametes by mitosis• Fertilization results in a

diploid sporophyte

• In most fungi and some protists, the only diploid stage is the single-celled zygote; there is no multicellular diploid stage

• The zygote produces haploid cells by meiosis

• Each haploid cell grows by mitosis into a haploid multicellular organism

• The haploid adult produces gametes by mitosis

Life Cycles

• Depending on the type of life cycle, either haploid or diploid cells can divide by mitosis

• However, only diploid cells can undergo meiosis

• In all three life cycles, the halving and doubling of chromosomes contributes to genetic variation in offspring

Meiosis

• Like mitosis, meiosis is preceded by the replication of chromosomes

• Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II

• The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis

• Each daughter cell has only half as many chromosomes as the parent cell

Meiosis

• Cell division– Cells go through G1, S, and G2 phases prior to meiosis– All chromosomes duplicated during S phase

• Two consecutive nuclear divisions + cytokinesis– Meiosis 1: separates homologous chromosomes

• Cells are haploid after end of Meiosis 1

– Meiosis 2: separates sister chromatids• Similar to Mitosis

• End Result: 4 haploid gametes

Meiosis in Humans

N=46 N=46

N=23

N=23

N=23

N=23

N=23

N=23

G1 S phase/G2

Meiosis 1

Meiosis 2

Prophase I• Prophase I= ~90% of the time

required for meiosis• Chromosomes begin to condense• In synapsis, homologous

chromosomes loosely pair up, aligned gene by gene– Each pair of chromosomes forms a

tetrad, a group of four chromatids• Crossing over can occur

– Nonsister chromatids exchange DNA segments

• Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred

Metaphase I• Tetrads line up at the

metaphase plate, with one chromosome facing each pole

• Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad

• Microtubules from the other pole are attached to the kinetochore of the other chromosome

Anaphase I• Pairs of homologous

chromosomes separate• One chromosome moves

toward each pole, guided by the spindle apparatus

• Sister chromatids remain attached at the centromere and move as one unit toward the pole

Telophase and Cytokinesis I• In the beginning of telophase I,

each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids

• Cytokinesis usually occurs simultaneously, forming two haploid daughter cells– Animal cells= Cleavage furrow – Plant cells= Cell plate

Meiosis II

• Two haploid daughter cells undergo another round of cell division in Meiosis II

• No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated

• Meiosis II is very similar to mitosis– Separation of sister chromatids

Meiosis II

Meiosis II

• At the end of meiosis, there are four daughter cells, each with a haploid set of individual chromosomes

• Each daughter cell is genetically distinct from the others and from the parent cell– Crossing over causes differences in sister

chromatids

Mitosis vs Meiosis

• Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell

• Meiosis reduces the number of chromosome sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell

Mitosis Meiosis

End Result:2 Identical Cells

End Result:4 Unique Gametes

Mitosis vs Meiosis

Mitosis vs Meiosis

• Three events are unique to meiosis, and all three occur in meiosis l– Synapsis and crossing over in prophase I: Homologous

chromosomes physically connect and exchange genetic information

– At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes

– At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate

Meiosis and Genetic Diversity

Natural selection results in the accumulation of genetic variations favored by the environment

Sexual reproduction contributes to the genetic variation in a population, which originates from mutations, through multiple processes:1. Independent orientation of chromosomes2. Random Fertilization3. Homologous chromosomes can have different versions

of genes4. Crossing Over

1. Independent orientation of chromosomes

The number of combinations possible when chromosomes sort independently into gametes is 2n, where n is the haploid number

2. Random Fertilization

• Potential for any chromosome combination to be fused with any other chromosome combination

• Fusion of 2 human gametes: 70 trillion possible combinations

• Each zygote has a unique genetic identity

3. Homologous chromosomes can have different versions of a gene

• Mutations (changes in an organism’s DNA) are the original source of genetic diversity

• Mutations create different versions of genes called alleles

• Reshuffling of alleles during sexual reproduction produces genetic variation

3. Homologous chromosomes can have different versions of a gene

G1

S phase

Meiosis 1 Meiosis 2

4. Crossing Over: Exchange of genetic information

• Occurs during Prophase 1 in meiosis

• Homologous chromosomes pair up and form tetrads

• Sister chromatids from 2 chromosomes “cross-over” at chiasmata

• Exchange of DNA• Genetic Recombination=

production of gene combinations different from parent cell

Homologous ChromosomesSister

Chromatids

Genetic Recombination

What happens when meiosis goes wrong?

Failure to Separate

• Nondisjunction= chromosome pair does not separate in meiosis 1 or 2

• Gamete created with extra chromosome– Majority of gametes with wrong number of chromosomes will

not produce viable offspring• Trisomy 21= Downs Syndrome

– 1 out of 700 children born– Chance increases with age of mother

• Produces physical effects– Round face, flattened nose, small teeth, short stature, failure to

sexually develop– Susceptible to heart defects, respiratory infections, leukemia,

Alzheimer’s disease

What about sex cells?

• Abnormal numbers of sex cells do not usually have an effect on survival

• XXY= Klinefelter Syndrome– Small testes, sterile, develops female body characteristics

• XYY= Normal male– Taller than normal

• XXX= Normal female– No obvious effects

• XO= Turner Syndrome– Short stature, web of skin between shoulders and neck,

sterile and lack of sexual development– Only case where 45 chromosomes is not fatal in humans

Errors can give rise to new species• Plants– Polyploidy= errors in meiosis resulted in more than 2 sets of

chromosomes• Plants can self-fertilize, preventing the altered chromosome number from

disrupting reproduction• Creates a new species

– ~50% of plants are polyploid• Wheat, potatoes, apples, cotton

• Animals– Less common in animals– Fishes and amphibians– Rat= recent discovery

in ChileRainbow Trout 2n=60