ch03 slides v02

8/10/2019 Ch03 Slides v02

1/44

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

8/10/2019 Ch03 Slides v02

2/44

Copyright 2012 Pearson Education Inc.Genetics Analysis: An Integrated Approach

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)

2

8/10/2019 Ch03 Slides v02

3/44


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

3

8/10/2019 Ch03 Slides v02

4/44


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

4

8/10/2019 Ch03 Slides v02

5/44


5

Stages of the Cell CycleCell division is regulated by control of the cell cycle , a cycle ofDNA replication and division common to all eukaryotes

8/10/2019 Ch03 Slides v02

6/44


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

6

8/10/2019 Ch03 Slides v02

7/44Copyright 2012 Pearson Education Inc.Genetics Analysis: An Integrated Approach

7

8/10/2019 Ch03 Slides v02


8

8/10/2019 Ch03 Slides v02


9

Mitosis separates replicatedcopies of sister chromatidsinto identical nuclei, formingtwo genetically identicaldaughter cells

Mitosis Produces Identical Daughter Cells

8/10/2019 Ch03 Slides v02


10

Cell Cycle CheckpointsCell cycle checkpoints are monitored by proteininteractions for readiness to progress to the next stage

8/10/2019 Ch03 Slides v02


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

11

8/10/2019 Ch03 Slides v02


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

12

8/10/2019 Ch03 Slides v02


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

13

8/10/2019 Ch03 Slides v02

14/44

8/10/2019 Ch03 Slides v02


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

15

8/10/2019 Ch03 Slides v02


16

8/10/2019 Ch03 Slides v02


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

17

8/10/2019 Ch03 Slides v02


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

18

8/10/2019 Ch03 Slides v02


19

Homologous chromosomes align and the synaptonemalcomplex forms between non-sister chromatids

Synapsis

8/10/2019 Ch03 Slides v02


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

8/10/2019 Ch03 Slides v02


21

8/10/2019 Ch03 Slides v02


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

22

8/10/2019 Ch03 Slides v02


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

23

8/10/2019 Ch03 Slides v02


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

8/10/2019 Ch03 Slides v02


25

8/10/2019 Ch03 Slides v02

26/44


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

26

8/10/2019 Ch03 Slides v02

27/44


27

8/10/2019 Ch03 Slides v02

28/44


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

28

8/10/2019 Ch03 Slides v02

29/44


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

29

8/10/2019 Ch03 Slides v02

30/44


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.

30

8/10/2019 Ch03 Slides v02

31/44


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.

31

8/10/2019 Ch03 Slides v02

32/44


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

32

8/10/2019 Ch03 Slides v02

33/44


The duck-billed platypus has weird sex chromosomes

photo -Stefan Kraft

8/10/2019 Ch03 Slides v02

34/44


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)

34

8/10/2019 Ch03 Slides v02

35/44


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

35

8/10/2019 Ch03 Slides v02

36/44


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

36

8/10/2019 Ch03 Slides v02

37/44


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

37

3 6 D C i E li D f

8/10/2019 Ch03 Slides v02

38/44


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

38

8/10/2019 Ch03 Slides v02

39/44


39

8/10/2019 Ch03 Slides v02

40/44


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)

40

8/10/2019 Ch03 Slides v02

41/44


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

8/10/2019 Ch03 Slides v02

42/44


42

8/10/2019 Ch03 Slides v02

43/44


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

43

8/10/2019 Ch03 Slides v02

44/44

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 )

ch03 slides v02

Documents