Principles of Genetics & Population

Genetics-AGB 121

2012-13

Dept. Of Animal Genetics and Breeding

Veterinary College

Hebbal, Bangalore -24

Meiosis & Gametogenesis

Meiosis

• The form of cell division by which gametes,

with half the number of chromosomes, are

produced.

• Diploid (2n) haploid (n)

• Meiosis is sexual reproduction.

• Two divisions (meiosis I and meiosis II).

Interphase I

• Similar to mitosis interphase.

• Chromosomes replicate (S phase).

• Each duplicated chromosome consist of two

identical sister chromatids attached at their

centromeres.

• Centriole pairs also replicate.

Interphase I

• Nucleus and nucleolus visible.

nuclear

membrane

nucleolus

cell membrane

chromatin

Meiosis I (four phases)

• Cell division that reduces the chromosome

number by one-half.

• four phases:

a. prophase I

b. metaphase I

c. anaphase I

d. telophase I

Meiosis I (four phases)

Prophase I

• Longest and most complex phase (90%).

• Chromosomes condense.

• Synapsis occurs: homologous chromosomes

come together to form a tetrad.

• Tetrad is two chromosomes or four

chromatids (sister and nonsister chromatids).

Leptotene,Zygotene,Pachytene,Diplotene,Diakinesis

Prophase I - Synapsis

Homologous chromosomes

sister chromatids sister chromatids Tetrad

Homologous Chromosomes

• Pair of chromosomes (maternal and paternal) that are

similar in shape and size.

• Homologous pairs (tetrads) carry genes controlling

the same inherited traits.

• Each locus (position of a gene) is in the same

position on homologues.

• Humans have 23 pairs of homologous chromosomes.

a. 22 pairs of autosomes

b. 01 pair of sex chromosomes

Homologous Chromosomes

Paternal Maternal

eye color

locus

eye color

locus

hair color

locus hair color

locus

Crossing

Over

• Crossing over (variation) may occur between

nonsister chromatids at the chiasmata.

• Crossing over: segments of nonsister

chromatids break and reattach to the other

chromatid.

• Chiasmata (chiasma) are the sites of crossing

over.

Crossing Over - variation

nonsister chromatids

chiasmata: site

of crossing over variation

Tetrad

Anaphase 1: Homologous pairs

separate with sister chromatids

remaining together.

Metaphase 1: Homologous

chromosomes align at the equatorial

plate.

Recombination occurs here

Prophase II: DNA does not replicate.

Metaphase II

Chromosomes align at the equatorial plate.

Second division of meiosis

Centromeres divide and sister

chromatids migrate separately to

each pole.

Cell division is complete.

Four haploid daughter cells

are obtained

Second division of meiosis

•One parent cell produces four daughter cells.

•Daughter cells have half the number of chromosomes

found in the original parent cell and with crossing over, are

genetically different.

Comparison of Mitosis & Meiosis

Spermatogenesis-

•starts at puberty and

continues throughout the

reproductive life of the

animals.

basal membrane of the

seminiferous tubules

Type A and Type B

spermatagonia by

Mitosis

Primary and Secondary

spermatocyte by

Meiosis

Spermatids mature to form

spermatazoa

In bull approximate total time is 48 days from

primary spermatocyte to ejaculation

Occurs within the seminiferous tubules of

testes

Spermatogenesis

2n=46

human

sex cell

diploid (2n)

n=23

n=23

meiosis I

n=23

n=23

n=23

n=23

sperm

haploid (n)

meiosis II

Oogenesis

It starts in germinal epithelium of female

gonads (ovaries) in diploid primordial cells

called oogonia.

Fertilization

• The fusion of a sperm and egg to form a zygote.

• A zygote is a fertilized egg

n=23

egg

sperm

n=23

2n=46

zygote

Fertilization and formation of various tissues