Meiosis is a crucial process in sexual reproduction, ensuring genetic diversity by producing haploid gametes. It consists of two successive divisions: meiosis I and meiosis II. Let's delve into the key stages and features of meiosis.
**1. Prophase I:**
Meiosis begins with the replication of DNA, resulting in homologous chromosomes, each composed of two sister chromatids. During prophase I, homologous chromosomes pair up in a process called synapsis, forming tetrads. Crossing over occurs, where chromatids exchange genetic material, enhancing genetic diversity.
**2. Metaphase I:**
Tetrads align at the cell's equator, with spindle fibers attaching to each homologous chromosome. Unlike in mitosis, the separation at this stage is of homologous chromosomes, not sister chromatids.
**3. Anaphase I:**
Homologous chromosomes are pulled apart and move to opposite poles of the cell. Unlike mitosis, the sister chromatids remain attached at this point.
**4. Telophase I:**
The separated homologous chromosomes reach the cell poles, and the cell undergoes cytokinesis, resulting in two daughter cells, each with half the original chromosome number. These cells are haploid, containing one set of chromosomes.
**5. Interkinesis:**
This brief stage between meiosis I and II involves little to no DNA replication. Chromosomes may temporarily decondense, and the cell prepares for the second division.
**6. Prophase II:**
In this stage, a new spindle apparatus forms in each haploid cell. Chromosomes, composed of two sister chromatids, become visible again.
**7. Metaphase II:**
Chromosomes align individually along the equator of each haploid cell, similar to metaphase in mitosis.
**8. Anaphase II:**
The centromeres split, and sister chromatids are pulled to opposite poles of the cells.
**9. Telophase II:**
Chromatids reach the cell poles, and nuclear envelopes re-form around the separated chromatids. Cytokinesis occurs, resulting in a total of four haploid daughter cells, each genetically distinct.
**10. Genetic Variation:**
Meiosis contributes significantly to genetic diversity. Crossing over in prophase I and the random assortment of chromosomes during metaphase I generate unique combinations of genetic material in gametes.
**11. Significance:**
Meiosis ensures the stability of an organism's chromosome number across generations and promotes genetic diversity, which is crucial for evolutionary processes. The resulting gametes are ready for fertilization, ultimately leading to the formation of a genetically diverse offspring.
In summary, meiosis is a complex and precisely regulated process that reduces chromosome number, shuffles genetic material, and ensures the creation of genetically diverse gametes, crucial for sexual reproduction and the perpetuation of species.
3. MEIOSIS is a double division which occurs in a diploid cell and give rise to
four haploid cells , each having half the number of chromosomes as
compared to the parent cell.
The term meiosis is coined by Farmer and Moore in 1905.
Meiosis consist of two division
Meiosis I
Meiosis II
The transition period between M-phase I (meiosis I) and M-
phase(meiosis II) is short and without DNA replication. It is called
interkinesis.
Interphase occurs prior to Meiosis I . It is similar to interphase of
Mitosis except that S-phase is prolonged. DNA replication occurs during S-
phase. A distint G2 phase is either short or absent. At this time each
chromosome comes to have two chromatids.
4. Chromosome replication occurs once but meiosis has two
M-phases each with its own Karyokinesis and Cytokinesis.
As a result chromosome number is halved.
Meiosis I is called heterotypic or reductional division.
During this division the number of chromosome is reduced
to half.
Meiosis II is known as homotypic or equational
division, because the chromosome number remain the
same as produced after the end of the first division.
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6. Meiosis I consist of karyokinesis and cytokinesis.
Karyokinesis of meiosis I is studied under four stages-
Prophase I :
it is divided into five sub-phases :
i. Leptonene
ii. Zygotene
iii. Pachytene
iv. Diplotene
v. Diakinesis
Metaphase I
Anaphase I
Telophase I
7. Meiosis I
Prophase I: it is more complicated and prolonged
as compared to the similar stage of mitosis.
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15. Metaphase I
•A colourless bipolar spindle apparatus appears in the region of
degenerated nucleus. The spindle apparatus has maximum diameter in
the middle region which is called equator.
•The bivalents arrange themselves on the equator of the bipolar spindle.
Since there are two centromeres in each bivalent, the centromeres of all
the bivalent produce a double metaphasic plate.
•The distribution of bivalents is at random so that individual paternal and
maternal chromosome can face either of the two poles of the spindle.
•Each chromosomes gets attached to the spindle pole of its side by means
of a chromosome fibre or tactile fibril which aries in the region of the
centromere.
(This is in contrast to the development of two tactile fibril from the same
centromere in mitosis)
20. Interkinesis or
Intrameiotic Interphase
It is metabolic stage between telophase of meiosis I and
prophase of meiosis II. Chromosomes are elongated but
chromatin reticulum is not formed. Protein and RNA
synthesis may occur. Centrosomes or centriole pairs
undergo replication in animal cells. However there is no
DNA synthesis. It is important for bringing true haploidy in
daughter cells.
