Chapter 13Chapter 13

Meiosis

Fig. 13-1

I. Inheritance of Genes

• Genes = segments of DNA

• Genes are passed by gametes (sperm and eggs)

• Each gene has a specific location (locus) on a chromosome

II. Asexual vs Sexual Reproduction

• Asexual reproduction = one parent, genetically identical offspring, mitosis

• Sexual reproduction = two parents, offspring have combinations of genes

Fig. 13-2

(a) Hydra (b) Redwoods

Parent

Bud

0.5 mm

III. Human Chromosomes

• Human somatic cells (body cells) have 23 pairs of chromosomes

• Karyotype = an ordered display of the pairs of chroms from a cell

• Homologous chromosomes = The two chroms in each pair (homologs)

• Chroms are same length and carry genes controlling the same traits

Fig. 13-3a

APPLICATION

Fig. 13-3b

TECHNIQUE

Pair of homologousreplicated chromosomes

Centromere

Sisterchromatids

Metaphasechromosome

5 µm

• Sex chromosomes are X and Y

• Human females have a pair of X chroms (XX)

• Human males have one X and one Y (XY)

• Autosomes = 22 pairs of chroms that do not determine sex

• Each pair of homologous chroms has one chrom from each parent

• Diploid cell (2n) has two sets of chromosomes

• For humans, the diploid number is 46 (2n = 46)

• In a cell in which DNA synthesis has occurred, each chromosome is replicated (two identical sister chromatids)

Fig. 13-4

Key

Maternal set ofchromosomes (n = 3)

Paternal set ofchromosomes (n = 3)

2n = 6

Centromere

Two sister chromatidsof one replicatedchromosome

Two nonsisterchromatids ina homologous pair

Pair of homologouschromosomes(one from each set)

• Gamete contains a single set of chroms, and is haploid (n) (monoploid)

• Humans, haploid number is 23 (n = 23)

• Each set of 23 = 22 autosomes + 1 sex chrom

• In an unfertilized egg (ovum), the sex chrom is X

• In a sperm cell, the sex chrom may be either X or Y

• Fertilization = union of gametes

• Zygote = fertilized egg and has one set of chroms from each parent (2n)

• Produces somatic cells by mitosis and develops into an adult

IV. Chromosomes in Human Life Cycle

• At maturity, ovaries and testes produce haploid (n) gametes

• Gametes are only types of human cells produced by meiosis (23 chroms, 1 from each pair)

Fig. 13-5Key

Haploid (n)Diploid (2n)

Haploid gametes (n = 23)

Egg (n)

Sperm (n)

MEIOSIS FERTILIZATION

Ovary Testis

Diploidzygote(2n = 46)

Mitosis anddevelopment

Multicellular diploidadults (2n = 46)

V. Stages of Meiosis

• First cell division (meiosis I), homologous chroms separate

• Meiosis I = two haploid daughter cells with replicated chrom; reductional division

• Second cell division (meiosis II), sister chromatids separate

• Meiosis II = four haploid daughter cells with unreplicated chroms; equational division

Fig. 13-7-1Interphase

Homologous pair of chromosomesin diploid parent cell

Chromosomesreplicate

Homologous pair of replicated chromosomes

Sisterchromatids Diploid cell with

replicated chromosomes

Fig. 13-7-2Interphase

Homologous pair of chromosomesin diploid parent cell

Chromosomesreplicate

Homologous pair of replicated chromosomes

Sisterchromatids Diploid cell with

replicated chromosomes

Meiosis I

Homologouschromosomesseparate

1

Haploid cells withreplicated chromosomes

Fig. 13-7-3Interphase

Homologous pair of chromosomesin diploid parent cell

Chromosomesreplicate

Homologous pair of replicated chromosomes

Sisterchromatids Diploid cell with

replicated chromosomes

Meiosis I

Homologouschromosomesseparate

1

Haploid cells withreplicated chromosomes

Meiosis II

2 Sister chromatidsseparate

Haploid cells with unreplicated chromosomes

Fig. 13-8

Prophase I Metaphase I Anaphase ITelophase I and

CytokinesisProphase II Metaphase II Anaphase II Telophase II and

Cytokinesis

Centrosome(with centriole pair)

Sisterchromatids Chiasmata

Spindle

Homologouschromosomes

Fragmentsof nuclearenvelope

Centromere(with kinetochore)

Metaphaseplate

Microtubuleattached tokinetochore

Sister chromatidsremain attached

Homologouschromosomesseparate

Cleavagefurrow

Sister chromatidsseparate Haploid daughter cells

forming

Prophase I

• Occupies more than 90% of time needed for meiosis

• Chromosomes condense

• Synapsis = homologous chroms loosely pair up, aligned gene by gene

VI. Meiosis I

• Crossing over = nonsister chromatids exchange DNA segments

• Each pair of chromosomes forms a tetrad, a group of four chromatids

• Usually has one or more chiasmata, X-shaped regions where crossing over occurred

Metaphase I

• Tetrads line up at the metaphase plate, one chrom facing each pole

• Microtubules attached to the kinetochore of each chromosome of each tetrad

Fig. 13-8b

Prophase I Metaphase I

Centrosome(with centriole pair)

Sisterchromatids Chiasmata

Spindle

Centromere(with kinetochore)

Metaphaseplate

Homologouschromosomes

Fragmentsof nuclearenvelope

Microtubuleattached tokinetochore

Anaphase I

• Pairs of homologous chroms separate, sister chromatids remain attached

Telophase I and Cytokinesis

• Each half of the cell has a haploid set of chroms; each chrom still consists of two sister chromatids

• Cytokinesis forms two haploid daughter cells

• No chrom replication occurs between meiosis I and meiosis II

Fig. 13-8c

Anaphase ITelophase I and

Cytokinesis

Sister chromatidsremain attached

Homologouschromosomesseparate

Cleavagefurrow

Prophase II

• Spindle apparatus forms

• Chroms (composed of two chromatids) move toward the metaphase plate

VII. Meiosis II

Metaphase II

• Sister chromatids are arranged at the metaphase plate

• Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical

• Kinetochores attach to microtubules

Fig. 13-8e

Prophase II Metaphase II

Anaphase II

• Sister chromatids separate

• Sister chromatids of each chrom now move as two newly individual chroms

Telophase II and Cytokinesis

• Nuclei form, and the chroms begin decondensing

• Cytokinesis makes four daughter cells, each with a haploid set of unreplicated chroms

• Daughter cells are genetically distinct from the others and from the parent cell

Fig. 13-8f

Anaphase IITelephase II and

Cytokinesis

Sister chromatidsseparate Haploid daughter cells

forming

• Meiosis - Andersen (9 min)

• Mitosis / Meiosis Simulation - Andersen (12 min)

VIII. Comparison of Mitosis and Meiosis

• Mitosis keeps the number of chromosomes the same, producing cells that are genetically identical to the parent

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

• Mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis

Fig. 13-9a

MITOSIS MEIOSIS

MEIOSIS I

Prophase I

Chiasma

Chromosomereplication

Homologouschromosomepair

Chromosomereplication

2n = 6

Parent cell

Prophase

Replicated chromosome

Metaphase Metaphase I

Anaphase ITelophase I

Haploid n = 3

Daughter cells ofmeiosis I

MEIOSIS II

Daughter cells of meiosis II

nnnn

2n2n

Daughter cellsof mitosis

AnaphaseTelophase

Fig. 13-9b

SUMMARY

MeiosisMitosisProperty

DNAreplication

Number ofdivisions

Occurs during interphase beforemitosis begins

One, including prophase, metaphase,anaphase, and telophase

Synapsis ofhomologouschromosomes

Does not occur

Number ofdaughter cellsand geneticcomposition

Two, each diploid (2n) and geneticallyidentical to the parent cell

Role in theanimal body

Enables multicellular adult to arise fromzygote; produces cells for growth, repair,and, in some species, asexual reproduction

Occurs during interphase before meiosis I begins

Two, each including prophase, metaphase, anaphase, andtelophase

Occurs during prophase I along with crossing overbetween nonsister chromatids; resulting chiasmatahold pairs together due to sister chromatid cohesion

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

• Three events are unique to meiosis (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

Cell Cycle / Mitosis / Meiosis - Andersen (14 min)

IX. Genetic Variation in Offspring

• Mvmt of chroms during meiosis and fert gives variation

• Mutations (changes in DNA) are original source of genetic diversity

• Mutations create different versions of genes (alleles, A or a)

• Reshuffling of alleles during sexual reproduction produces genetic variation

A. Independent Assortment of Chromosomes

• Homologous chroms orient randomly at metaphase I of meiosis

• Each pair of chroms sorts maternal and paternal homologues into daughter cells independently of the other pairs

• Number of combinations possible is 2n, where n is the haploid number

• Humans (n = 23), there are more than 8 million (223) possible combinations

Fig. 13-11-1

Possibility 1 Possibility 2

Two equally probablearrangements ofchromosomes at

metaphase I

Fig. 13-11-2

Possibility 1 Possibility 2

Two equally probablearrangements ofchromosomes at

metaphase I

Metaphase II

Fig. 13-11-3

Possibility 1 Possibility 2

Two equally probablearrangements ofchromosomes at

metaphase I

Metaphase II

Daughtercells

Combination 1 Combination 2 Combination 3 Combination 4

B. Crossing Over

• Produces recombinant chromosomes (combine genes inherited from each parent)

• Begins in prophase I, as homologous chroms pair up gene by gene

• Homologous portions of two nonsister chromatids trade places

• Variations come from combining DNA from two parents into a single chromosome

Fig. 13-12-1Prophase Iof meiosis

Pair ofhomologs

Nonsisterchromatidsheld togetherduring synapsis

Fig. 13-12-2Prophase Iof meiosis

Pair ofhomologs

Nonsisterchromatidsheld togetherduring synapsis

Chiasma

CentromereTEM

Fig. 13-12-3Prophase Iof meiosis

Pair ofhomologs

Nonsisterchromatidsheld togetherduring synapsis

Chiasma

Centromere

Anaphase I

TEM

Fig. 13-12-4Prophase Iof meiosis

Pair ofhomologs

Nonsisterchromatidsheld togetherduring synapsis

Chiasma

Centromere

Anaphase I

Anaphase II

TEM

Fig. 13-12-5Prophase Iof meiosis

Pair ofhomologs

Nonsisterchromatidsheld togetherduring synapsis

Chiasma

Centromere

Anaphase I

Anaphase II

Daughtercells

Recombinant chromosomes

TEM

C. Random Fertilization

• Variation arises because any sperm can fuse with any ovum (unfertilized egg)

• Fusion of two gametes (each with 8.4 million possible combinations from ind assort) produces a zygote with about 70 trillion diploid combinations

• Crossing over adds even more variation

X. Evolution and Genetic Variation

• 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

You should now be able to:

1. Distinguish between the following terms: somatic cell and gamete; autosome and sex chromosomes; haploid and diploid

2. Describe the events that characterize each phase of meiosis

3. Describe three events that occur during meiosis I but not mitosis

4. Name and explain the three events that contribute to genetic variation in sexually reproducing organisms

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