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Lectures by Kathleen FitzpatrickSimon Fraser University
Copyright 2012 Pearson Education Inc.Mark F. Sanders John L. Bowman
G E N E T I C A N I N T E G R A T E D A P P R O A C H
A N A LY S I S Chapter 3Cell Division and
Chromosome Heredity
Slides adapted from lectures byKathleen Fitzpatrick
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Cell Division
Mitosis produces two identical daughter cells that are exactreplicas of the parental cell
Most body cells are somatic cells (non-reproductive), usuallywith chromosomes present in pairs, the number ofchromosomes is the diploid number (2n )
The haploid chromosome number includes one of eachchromosome pair ( n)
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Reproductive Cells
Gametes are produced from germ-line , or reproductive cells
Meiosis produces gametes that have half the number ofchromosomes as the original cell
Sex chromosomes determine sex and differ between sexes
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3.1 Mitosis Divides Somatic Cells
Mitosis is the process of cell division that produces twogenetically identical daughter cells from one original parentalcell
It is precisely controlled to prevent either an excess orinsufficient number of cells
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Stages of the Cell CycleCell division is regulated by control of the cell cycle , a cycle ofDNA replication and division common to all eukaryotes
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Substages of M Phase
M phase is divided intoProphase
Prometaphase
Metaphase
Anaphase
Telophase
M phase accomplishes karyokinesis , partitioning of DNAinto daughter cell nuclei and cytokinesis , the partitioning ofthe cytoplasm
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Mitosis separates replicatedcopies of sister chromatidsinto identical nuclei, formingtwo genetically identicaldaughter cells
Mitosis Produces Identical Daughter Cells
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Cell Cycle CheckpointsCell cycle checkpoints are monitored by proteininteractions for readiness to progress to the next stage
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3.2 Meiosis produces gametes for sexualreproduction
Reproduction can be divided into two broad categories
In asexual reproduction, organisms reproduce without matingand produce genetically identical offspring
In sexual reproduction, gametes (reproductive cells) areproduced; these unite during fertilization
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Multicellular Eukaryotes Reproduce MainlySexually
Males and females carry distinct reproductivetissues and structures
Mating requires the production of haploid gametesfrom both male and female
The union of haploid gametes produces diploidprogeny
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Meiosis versus Mitosis
Many features of meiosis are similar or identical to mitosis, e.g.,interphase
Meiosis is distinguished from mitosis on the basis of events
during meiotic M phase and the production of four haploidgametes
Meiotic interphase is followed by two division stages calledmeiosis I and meiosis II with no DNA replication betweenthem
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Meiosis I and II
In meiosis I homologous chromosomes separate from oneanother, reducing the diploid number of chromosomes to thehaploid number
In meiosis II, sister chromatids separate from one another toproduce four haploid gametes, each with one chromosome ofthe original diploid pair
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Meiosis I
Three hallmark events occur in meiosis I1. Homologous chromosome pairing
2. Crossing over between homologous chromosomes
3. Segregation (separation) of homologous
chromosomes, which reduces chromosomes to thehaploid number
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Stages of Meiosis I
Meiosis I is divided into prophase I, metaphase I,anaphase I, and telophase I
Prophase I is subdivided into five stages: leptotene,
zygotene, pachytene, diplotene, and diakinesis
Pairing ( synapsis ) and recombination of homologs takeplace in prophase I
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Homologous chromosomes align and the synaptonemalcomplex forms between non-sister chromatids
Synapsis
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Pachytene
Chromosome condensation continues in pachytene
Paired homologs are called tetrads, due to the four
visible chromatids
Recombination nodules can be seen at intervals in
the synaptonemal complex
These are aggregates of enzymes and proteins
needed for crossing over between homologs 20
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Telophase I and Cytokinesis
In telophase I the nuclear membranes reform aroundthe separated haploid sets of chromosomes
Cytokinesis follows telophase I and divides thecytoplasm to create two haploid cells
Meiosis I is called the reductional division becausethe ploidy of the daughter cells is halved comparedto the original diploid parent cell
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Meiosis II
Meiosis II divides each haploid daughter cell intotwo haploid cells, by separating sister chromatidsfrom one another
The process is similar to mitosis in a haploid cell
Four genetically distinct haploid cells areproduced, each carrying one chromosome of ahomologous pair
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The Mechanistic Basis of Mendelian Ratios
Separation of homologs and sister chromatid inmeiosis constitutes the mechanical basis of Mendelslaws
For example, in an organism that is genotype Aa , thehomologs bearing A and a separate from oneanother during anaphase I
At the end of meiosis, two gametes have the A alleleand two have a ; this generates the 1:1 ratio predicted
by the law of segregation 24
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Independent Assortment
Independent assortment of alleles is illustrated bybehavior of two pairs of homologs during meiosis
For an organism with genotype AaBb , two equallylikely arrangements of paired homologs can occur
One yields gametes AB and ab , whereas the otherproduces gametes Ab and aB ; these four gametecombinations occur with equal likelihood
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Segregation in Single-Celled Diploids
The yeast, Saccharomyces cerevisiae can live as either haploid
or diploid, and can reproduce either sexually or asexually.Sexual reproduction requires union of haploid cells of oppositemating types (called a and a ) yielding a diploid cell
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3.3 The Chromosome Theory of Heredity
Sutton and Boveri observed that chromosome behavior in celldivision mirrors hereditary transmission of genes
TH Morgans lab members found numerous variants of
Drosophila melanogaster , and analyzed these in controlledcrosses to test Mendels rules
He concluded from his results that genes were carried on
chromosomes
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3.4 Chromosomal and Genetic Sex Determination
Sex determination is the biological process giving rise to thedistinctive male and female characteristics.
In most species, this differentiation process is triggered bygenetic factors, often borne on chromosomes that are visiblydistinct.
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Mammalian Sex Determination
In placental mammals, maleness is triggered by the presence ofthe SRY gene on the Y chromosome. Thus, individuals that areXY (normal), XXY, or XYY will be male; while individuals thatare XX (normal), XO, or XXX will be female.
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Diversity of Sex Determination
A different system, the Z/W system , is used by birds, somereptiles, some fish, butterflies, and moths
In Z/W systems, females are the heterogametic sex, with two
different sex chromosomes ( ZW ) and males are thehomogametic sex, possessing two identical sex chromosomes(ZZ )
Sex chromosomes of the platypus consist of 5 pairs of sexchromosomes with 5 XY pairs in males and 5 XX pairs infemales
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The duck-billed platypus has weird sex chromosomes
photo -Stefan Kraft
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X/Autosome ratio systems
In some species (e.g. Drosophila), the X/A ratio orX/autosome ratio determines sex based on the number of Xchromosomes relative to sets of autosomes - Males have anX/A ratio of 0.5 and females have a ratio of 1.0
Thus in flies, males have X0, XYY, or XY (normal) whereasfemales are XXY or XX (normal)
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3.5 Sex-Linked Traits Follow Distinct Patterns
In X-linked recessive inheritance, females homozygous for the recessive
allele or males hemizygous for it display the recessive phenotypeIn X-linked dominant traits, heterozygous females and males hemizygousfor the dominant allele express the dominant phenotype
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Y-Linked Inheritance
Y-linked traits are transmitted father-to-son
Fewer than 50 genes have been found on the Y chromosome
Males have only one Y chromosome, but they are not
hemizygous for all Y-linked genes, as most of the genes on theY are still present in two copies in males
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Pseudoautosomal Regions
Two small regions of homology, thepseudoautosomal regions (PAR1and PAR2), exist between the Xand Y chromosomes, enabling themto pair as homologs in meiosis.
Crossing-over occurs between Xand Y within these regions
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3 6 D C i E li D f
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3.6 Dosage Compensation Equalizes Dosage ofSex-Linked Genes
In organisms with sex chromosomes, there is typicallya sex imbalance between the copy number of genes onthe sex chromosomes
Any mechanism that compensates for the difference innumber of copies of genes between males and femalesis called dosage compensation
There are several different mechanisms of dosagecompensation
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X-Chromosome Inactivation in Placental Mammals
Early in mammalian development, one of two X chromosomesin each female somatic cell is randomly inactivated. Theinactive X chromosome is visible cells, as a condensed Barrbody , first described by Murray Barr (1949)
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Female Mammals Are Mosaics
Once X inactivation has occurred in a cell, it is permanent inall the descendants of that cell
Female mammals are mosaics of two types of cells; one
expresses the maternal X and the other the paternal X
Since X inactivation is random, alleles of both chromosomesare expressed approximately equally over the wholeorganism
This type of mosaicism is distinct from genetic mosaicism41
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Calico and Tortoiseshell Cats Are Visibly Mosaic
In cats, the X chromosome
carries a gene influencing coatcolor
One allele specifies a black color;the other an orange color
X inactivation in heterozygous
females leads to a pattern oforange and black patches that isunique to each individual
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Mechanism of X Inactivation
Random X inactivation requires an X-linked genecalled Xist ( X-inactivation-specific-transcript )
The gene produces large RNA molecules that spreadout and cover the chromosome to be inactivated
Xist can only silence the chromosome from which it istranscribed (i.e., it acts in cis )
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