Download - Meiotic cell division
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MEIOTIC CELL DIVISION
Mr. Binu BabuMBA, M.Sc. (N)Asst. Professor
Mrs. Jincy EaliasM.Sc. (N)Asst. Professor
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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)
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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.
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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.
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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).
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• 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.
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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
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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
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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.
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Meiosis I
Cell division that reduces the chromosome number by one-half.
• Four phases:a. Prophase Ib. Metaphase Ic. Anaphase Id. Telophase I
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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).
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Prophase I - Synapsis
Homologous chromosomes
sister chromatids sister chromatidsTetrad
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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.
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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.
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Crossing Over - variation
nonsister chromatids
chiasmata: site of crossing over
variation
Tetrad
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Sex Chromosomes
XX chromosome - female XY chromosome - male
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Metaphase I• It is the shortest phase in meiosis
• Tetrads align on the metaphase plate.
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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.
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Telophase I
• Each pole now has haploid set of chromosomes.• Cytokinesis occurs and two haploid daughter cells
are formed.
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Meiosis II
• No interphase II (or very short - no more DNA replication)
• Meiosis II is similar to mitosis
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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.
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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.
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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.
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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.
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• After the completion of telophase - II Cytokinesis occurs.• After completion of meiosis – II four
haploid daughter cells produced.• Gametes = sperm or egg
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