MEIOTIC CELL DIVISION

Mr. Binu BabuMBA, M.Sc. (N)Asst. Professor

Mrs. Jincy EaliasM.Sc. (N)Asst. Professor

Most cells of the human body undergo cell division, the process by which cells reproduce themselves. • The two types of cell division—– Somatic cell division (mitotic division or

mitosis)– Reproductive cell division (meiotic division or

meiosis)

Somatic cell division• A cell undergoes a nuclear division called

mitosis • A cytoplasmic division called cytokinesis• This two process produce two genetically

identical cells, each with the same number and kind of chromosomes as the original cell.

• Somatic cell division replaces dead or injured cells and adds new ones during tissue growth.

Reproductive cell division

• It is the mechanism that produces gametes, the cells needed to form the next generation of sexually reproducing organisms.

• This process consists of a special two step

division called meiosis in which the number of chromosomes in the nucleus is reduced by half.

Meiosis is a form of cell division by which gametes, with half the number of chromosomes, are produced.

• Diploid (2n) haploid (n)

• Meiosis is sexual reproduction. It is a two divisions process (meiosis I and meiosis II).

• Sex cells divide to produce gametes (sperm or egg).

• Gametes have half of the chromosomes.

• Occurs only in gonads (testes or ovaries).

Male: spermatogenesisFemale: oogenesis

• Meiosis is similar to mitosis with some chromosomal differences.

Fertilization• The fusion of a sperm and egg to form a zygote.• A zygote is a fertilized egg

n=23egg

sperm n=23

2n=46zygote

Spermatogenesis

2n=46

humansex cell

diploid (2n)

n=23

n=23

meiosis I

n=23

n=23

n=23

n=23

sperm

haploid (n)

meiosis II

Interphase• During interphase the cell replicates its DNA, it also

produces additional organelles and cytosolic components in anticipation of cell division.

• Interphase is a state of high metabolic activity but cell does not grow.

• Interphase consists of three phases– G1:- it is the interval between the mitotic phase and the S

phase– S or synthesis:- DNA replication occurs– G2 is the interval between the S phase and the mitotic

phaseG0:- Cells that remain in G1 for a very long time, perhaps destined never to divide again.

Meiosis I

Cell division that reduces the chromosome number by one-half.

• Four phases:a. Prophase Ib. Metaphase Ic. Anaphase Id. Telophase I

Prophase I

It is the longest and most complex phase (90%) in meiosis. In this phase

• The chromosomes condense.• Synapsis occurs: homologous chromosomes come

together to form a tetrad.

• Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).

Prophase I - Synapsis

Homologous chromosomes

sister chromatids sister chromatidsTetrad

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 autosomesb. 01 pair of sex chromosomes

• Karyotype:- A method of organizing the chromosomes of a cell in relation to number, size, and type.

Crossing Over

• Crossing over (variation) may occur between non sister chromatids at the chiasmata.

• Crossing over: segments of non sister 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

Sex Chromosomes

XX chromosome - female XY chromosome - male

Metaphase I• It is the shortest phase in meiosis

• Tetrads align on the metaphase plate.

Anaphase I

• The members of each homologous pair of chromosomes separate as they are pulled to opposite poles of the cell

• The paired chromatids, held by a centromere, remain together.

Telophase I

• Each pole now has haploid set of chromosomes.• Cytokinesis occurs and two haploid daughter cells

are formed.

Meiosis II

• No interphase II (or very short - no more DNA replication)

• Meiosis II is similar to mitosis

Prophase - II

• During early prophase - II, the chromatin fibers condense and shorten into chromosomes.

• The condensation process may prevent entangling of the long DNA strands as they move during mitosis.

Metaphase - II

• During this phase, the microtubules of the mitotic spindle align the centromeres of the chromatid pairs at the exact center of the mitotic spindle.

• This midpoint region is called the metaphase plate.

Anaphase - II• During this phase, the centromeres split,

separating the two members of each chromatid pair, which move toward opposite poles of the cell.

• Once separated, the chromatids are termed chromosomes.

• As the chromosomes are pulled by the microtubules of the mitotic spindle during anaphase, they appear V-shaped.

Telophase - II

• The final stage of meosis II, telophase II, begins after chromosomal movement stops.

• The identical sets of chromosomes, now at opposite poles of the cell, uncoil and revert to the threadlike chromatin form.

• A nuclear envelope forms around each chromatin mass, nucleoli reappear in the identical nuclei, and the mitotic spindle breaks up.

• After the completion of telophase - II Cytokinesis occurs.• After completion of meiosis – II four

haploid daughter cells produced.• Gametes = sperm or egg

THANK YOU