Meiotic cell division

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