- Introduction : Meiosis is a special kind of cell division that occur in germ cells of sexually reproducing organisms, where it reduces the chromosome number by half to generate four haploid (only one copy of each chromosome) daughter cells (gametes – sperm and egg) from a diploid (two copies of each chromosome) parent cell.
- In Meiosis, cell undergoes one round of DNA replication and two consecutive rounds of nuclear division.
- In sexual reproduction, formation of offspring requires union of two cells (gametes – sperm and egg) with haploid (n) chromosomes to yield a embryo or zygote with diploid (2n) chromosomes.
- The word “Meiosis” was coined in 1905 from the Greek work meaning “reduction”.
- Meiosis cell division ensures a haploid phase in the life cycle of a sexually reproducing diploid organism. Later, fertilization between two haploid gametes restores the diploid phase in zygote, as observed in majority of cells of a diploid organism.
- In absence of a reduction division or meiosis, during gamete formation of sexually reproducing organism, the number of chromosome sets will keep doubling with every future round of fertilization.
- Hence for sexual reproduction to occur properly, a round of reduction division is crucial, where chromosome number is reduced by half.
- Meiosis shares many aspects with mitosis, such as stages of cell divisions (G1-S-G2), molecular mechanism for organizing and separating chromosomes.
- Meiosis differs from mitosis as it exhibits crossing over at prophase I between two non-sister chromatids of homologous chromosomes (similar but nonidentical chromosome pairs an organism receives one each from its two parents).
- Meiosis can be divided into two: meiosis I and Meiosis II
- The cell prepares for division during the G1 and G2 phases as observed in mitosis. In S phase, the entire chromosome sets of the cell is completely replicated to give rise to two identical full sets of chromosomes.
- The replicated sister chromatids are held at centromere.
- The centrioles are also duplicated.
- The duplicated DNA / chromosomes begin to condense.
- Each chromosome is composed to two sister chromatids with identical genetic information.
- The homologous chromosomes align along the entire length of chromosome. For e.g.: Both copies of chromosome 1 are together and similarly for chromosome 2 and so on.
- In mitosis there is no pairing of homologous chromosomes is observed.
- The tight pairing of the homologous chromosomes is called synapsis.
- The synaptonemal complex ( a lattice of proteins) helps in exchanging chromosomal segments between homologous non-sister chromatids by holding them together—a process termed as recombination or crossing over.
- Crossing over results in mixed or recombinant chromosome at the point of exchange with varied genetic information when compared to parental chromosomes.
- Spindle fibers emerge from centrosomes.
- Homologous chromosomes randomly assemble at metaphase plate with the help of microtubules.
- The difference of this arrangement from mitosis is that each chromosome attaches to microtubules from just one pole of the spindle and it’s the both homologous pairs but not individual chromosomes align at the metaphase plate for separation.
- The movement of homologous chromosome is completely random, and can end up at either pole after separation. It is pulled towards the pole to which it is directly attached through microtubules .
- This event of random (or independent) assortment of homologous chromosomes at the metaphase plate is another event which contributes to genetic variation into the gametes along with the crossing over mechanism.
The microtubules pull the linked chromosomes towards the pole ( one homologous chromosome to one pole and other homolog ends up at opposite pole). However, the sister chromatids remain tightly bound together at the centromere.
Telophase I and cytokinesis:
- Complete chromosome set reach at each pole.
- Remember at metaphase I, homologous chromosomes were pulled to the opposite poles. So, at each pole only haploid set (n) of duplicated chromosomes are present.
- A membrane forms around each set of chromosomes to create two new nuclei.
- The two separate daughter cells each with a haploid set of chromosomes within a nucleus are formed after cytokinesis.
- Meiosis I result in forming two haploid daughter cells.
- Meiosis II is much simpler process when compared to meiosis I.
- Meiosis II is very much similar to mitosis and it can be considered as “mitosis of haploid cells”.
- The duplicated haploid daughter cells (with attached sister chromosomes) formed at the end of meiosis I, enter Meiosis II.
- These duplicated chromosomes undergo all the steps similar to mitosis to yield four haploid daughter cells.
- In humans, meiosis in germ or sex cells results in four sperm or egg cells. All the four sperm cells formed by meiosis are functional but in case of oogenesis, only one cell becomes functional egg and other end up as polar body.
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