11-4 Meiosis

P275

Homologous chromosomes

Pair of chromosomes that one comes from the male parent and the corresponding one from the female parent

Homologous ≠ identical

same genes in same loci

same length

same shape

may not the same alleles for one gene

Homologous chromosomes

Alleles

Diploid & Haploid Cells

• two copies of each chromosome

• homologous pairs• body cells

• one copy of each chromosome

• no homologous pairs• gametes (sex cells)

Sperm

Egg

Egg

Sperm

Zygote

Sperm?meiosis

Egg

fertilisation

The purpose of meiosis is to produce gametes. A (haploid) gamete (sex cell, sperm or egg cell) has half the number of chromosomes compared to a (diploid) somatic cell (body cell). A male and a female gamete may then fuse to form a zygote, which will have the same number of chromosomes as a somatic cell.

Human chromosomes

Diploid cells: 23 pairs of homologous chromosomes

1st-22nd pairs: autosomes

23rd pair: sex chromosome

Human chromosomes

1-22 pairs : autosomes, 23 pair : sex chormosome

Karyotype (the type of nucleus)

• The number and appearance of the chromosomes in an organism

Karyotyping

• Arrange the chromosomes in pairs according to their size and structure

• Collect cells• Culture cells• Stop cell division at metaphase• Staining chromosome• Pair (Length, Shape, Position of

centromeres, Bands) • Diagnosis (number, gender,

abnormality)

8

4

3

1 : x,y

Fruit fly’s somatic cells How many pairs

of sex

chromosomes ？

How many pairs of

autosomes？

How many pairs of

homologous

chromosomes？

How many

chromosoems

A B

C D

What is the correct gamete produced by the germ cell

germ cell

Meiosis Overview

reduction division number of chromosomes per cell halvedseparation of homologous chromosomes 1 diploid 4 genetically different haploid DNA copied once, cell divide twice

Sperms are produced by … meiosis

The chromosomes in the nucleus

are copied

Four haploid daughter cells

Diploid parent cell

Eggs are also produced by meiosis

Haploid

Diploid parent cell

Phases of meiosis

interphase (DNA replication before meiosis I)

PROPHASE I

METAPHASE I

ANAPHASE I

TELOPHASE I

PROPHASE I

METAPHASE I

ANAPHASE I

TELOPHASE I

MEIOSIS IMEIOSIS I

PROPHASE II

METAPHASE II

ANAPHASE II

TELOPHASE II

PROPHASE II

METAPHASE II

ANAPHASE II

TELOPHASE II

MEIOSIS IIMEIOSIS II

no interphase (no DNA replication

before meiosis II)

Meiosis I

(reduction division)

Interphase I Prophase I Metaphase I Anaphase I

Cells undergo a round of DNA replication, forming duplicate Chromosomes.

Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.

Spindle fibers attach to the chromosomes.

The fibers pull the homologous chromosomes toward the opposite ends of the cell.

Telophase I

Prophase 1

1. Supercoil of chromosomes

2. Spindle form3. Synapsis

(homologous pair to form bivalents)

4. Crossing over5. Nuclear envelope

breaks down

• The key to the process of meiosis is the Synapsis in Prophase I and the splitting of the homologues in Anaphase I.

• One event that can take place during meiosis is crossing over.

• Crossing over is the exchange of genetic material between two homologous chromosomes which have paired up during Prophase I.

• The result of crossing over is genetic recombination.

Synapsis & Crossing over

Metaphase 1

1. Spindle network complete

2. Bivalents line up on equator

3. Centromere attached to microtubules

Anaphase 1

1. Centromeres do not split

2. Microtubules contract to separate homologous pairs

Telophase 1

1. Chromosomes uncoil

2. Spindle breaks down

3. Nuclear envelope reform

Meiosis II

(similar to mitosis)

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

Prophase II Metaphase II Anaphase II Telophase II

The chromosomes line up in a similar way to the metaphase stage of mitosis.

The sister chromatids separate and move toward opposite ends of the cell.

Meiosis II results in four haploid (N) daughter cells.

There is no DNA

replication

Prophase 2

1. Supercoil of chromosomes

2. Spindle form

3. Nuclear envelope breaks down

Metaphase 2

1. Chromosomes move to equator

2. Spindle network complete

3. Centromere attach to spindle

Anaphase 2

1. Centromere split

2. Microtubules contract and sister chromatids separate

Telophase 2

1. Chromosomes uncoil

2. Spindle breaks down

3. Nuclear envelope reforms

Cytokinesis 2

1. 4 haploid cells

2. 1 or many undergo differentiation into gametes

Which steps in meiosis are important in

producing genetic variety?

1. Crossing over in prophase 1

• Homologous chromosomes exchange nonsister chromatids

chiasmata

2. Independent assortment in metaphase 1

• Bivalents line up on equator in a random way

Possibilities of Crossing over

e.g. 46 chromosomes, 23 pairs2n = 46; n = 232n = 223 = ~ 8 million possible combinations!

3. Random fertilization

Gamete formation in animals

• Male gametes (sperms) are formed through spermatogenesis

• Female gametes (egg) are formed through oogenesis

Growth Meiosis I Meiosis II

spermatids (n)

secondary spermatocytes

(n)

primary spermatocyte

(2n)

spermato-gonium (2n )

sperm (n)

a) Spermatogenesis

Growth Meiosis I Meiosis II

ovum (n)

primary oocyte (2n)

oogonium (2n) secondary

oocyte (n)

polar body (n)

polar bodies (n)

b) Oogenesis

Comparing mitosis and meiosis

• Mitosis: 2 genetically identical diploid cells• Meiosis: 4 genetically different haploid cells