Meiosis

Principles of Genetics & Population Genetics-AGB 121 2012-13

Meiosis & Gametogenesis Dept. Of Animal Genetics and Breeding Veterinary College Hebbal, Bangalore -24

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.

chromatin

nuclear membrane

cell membrane nucleolus

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

Tetrad

sister chromatids

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

eye color locus

eye color locus

hair color locus

hair color locus

Paternal

Maternal

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

Tetrad

variation

Metaphase 1: Homologous chromosomes align at the equatorial plate.

Anaphase 1: Homologous pairs separate with sister chromatids remaining together.

Recombination occurs here

Second division of meiosis Prophase II: DNA does not replicate.

Metaphase II Chromosomes align at the equatorial plate.

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-

Occurs within the seminiferous tubules of testes

•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

Spermatogenesis n=23 human sex cell

sperm

n=23

n=23

2n=46 haploid (n)

n=23 diploid (2n)

n=23

n=23 meiosis I

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