Reproduction and growth are two of the most defining characteristics of a living organism. Be it a unicellular bacterium or a complex multicellular human being, the cells of all organisms undergo division, giving rise to two daughter cells. The eukaryotic cell cycle is an ordered and tightly regulated series of events that cause duplication of a cell’s genome and its division into two daughter cells. In mitosis, a parent cell divides and results in two daughter cells. Each daughter cell receives the same number of chromosomes as were present in the parent cell. However, sexually-reproducing organisms undergo a second division, known as meiosis, in which the chromosomal number in the daughter cells is reduced to half of what was originally present in the parent cells.
Meiosis is described as a reductional division, referring to the reduction in the chromosomal number of the daughter cells.
Meiosis occurs to produce the sex cells aka gametes (i.e., eggs and sperms).
This special form of cell division occurs in diploid organisms, leading to the formation of haploid gametes.
When these haploid gametes fuse during fertilization, the original ploidy of the organism is restored.
The meiotic process comprises two stages or phases, namely, Meiosis I and Meiosis II.
While meiosis I produce two daughter cells, meiosis II terminates with the formation of four daughter cells, which are the gametes (i.e., the sperms or the eggs) of a sexually reproducing organism.
Each gamete consists of half the original amount of chromosomes that were in the parent cell, at the beginning of meiosis I.
Meiosis I and II proceed through 4 stages each, these stages are named the same as in mitosis, albeit they carry the roman numerals I or II, signifying the phase of meiosis.
Meiosis I is often referred to as reduction division, while meiosis II is called equational division.
Meiosis I is characterised by the processes of synapsis, recombination and desynapsis.
During meiosis, I, the parent cell (2n), containing homologous sets of chromosomes divides into two daughter cells. By the end of meiosis I, each daughter cell consists of a haploid set of chromosomes.
Meiosis I comprises four stages: prophase I, metaphase I, anaphase I and finally, telophase I.
Before meiosis begins, the chromosomes undergo replication, similar to mitosis. The meiotic interphase includes the G1, S and G2 phases, just like mitosis.
The following stages define meiosis I:
Prophase I further comprises the following sub-stages:
Leptotene: The replicated chromosomes condense into long threads and become visible.
Zygotene
Chromosomes condense, and homologous chromosomes pair up.
Synapsis, i.e., the formation of the synaptonemal complex between two homologous chromosomes.
The synapsed pair of homologous chromosomes are now referred to as tetrads or bivalents.
Pachytene
Non-sister chromatids of the two homologous chromosomes undergo crossing over, at the synapse, i.e., the homologues exchange corresponding parts of their DNA.
Crossing over leads to the recombination of the DNA.
Diplotene: Characterised by desynapsis of the two homologues, i.e., the dissolution of the synaptonemal complex. However, each bivalent remains joined by chiasmata.
Diakinesis: Condensation of chromosomes, detachment from nuclear membrane, and the disintegration of the nuclear membrane takes place.
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Characterised by lining up of the homologous pairs of chromosomes at the equator or the metaphasic plate.
Spindle fibres from the polar centrosomes form and attach to the centromeres -one microtubule from each pole attaches to one centromere of each homologous pair.
The homologous pairs of chromosomes are separated from each other.
Remember, only the homologous pairs are separated, and not the chromatids (compare this to the anaphase of mitosis). The centromere doesn't split, unlike mitosis.
Meiosis 1 is referred to as reductional division, manifested during anaphase I. This is because the original number of chromosomes (2n) has now been reduced to exactly half (n).
Chromosomes reach the poles at their ends.
At each pole, a new nuclear envelope starts to form, surrounding each set of chromosomes (haploid set).
Reformation of chromatin via decondensation of the chromosomes takes place.
Cytokinesis occurs, marking the completion of two daughter cells.
Each daughter cell now has exactly half the original amount of chromosomes that were present in the parent cell. Hence, each daughter cell contains a haploid set of DNA (n)
Marks the end of meiosis I.
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One of the defining significance of meiosis is that this division leads to the generation of genetically variable cells. The event of crossing over is a manifestation of inheritance of variation by the offspring.
The reduction division that occurs during meiosis ensures that the ploidy of the sexually-reproducing organism is maintained.
In humans and other diploid organisms, the parent cell (2n) undergoes meiosis, eventually resulting in four haploid gametes (n).
The fusion of the male (n) and female gametes (n) during fertilization restores the original number of chromosomes (2n).
Meiosis also represents the molecular basis of the law of segregation and the law of independent assortment. This is manifested in the formation of gametes and the random positioning of the homologous pairs of chromosomes during metaphase I.
Meiosis I is a cell division process that occurs in sexually reproducing organisms, resulting in a reductional division of chromosomes. The distinguishing processes in meiosis I are synapsis and recombination of homologous chromosomes, which results in genetic variation among sexually reproducing organisms. Meiosis I comprises prophase I, metaphase I, anaphase I and telophase I. At the end of meiosis I, each of the two daughter cells thus formed carries exactly half the original number of chromosomes.
Q1. What is the difference between a bivalent and a tetrad?
Ans: The term bivalent refers to the pair of homologous chromosomes as a whole, during synapsis. Tetrad refers to all the four sister chromatids within a bivalent.
Q2. What exactly is the synaptonemal complex?
Ans: The synaptonemal complex is a proteinaceous structure that mediates the joining of the homologous chromosomes during zygotene.
Q3. Does crossing over occur in all organisms?
Ans: No, some organisms such as Drosophila (male) and Bombyx mori (female) do not undergo crossing over during meiosis I.
Q4. At which stage of meiosis 1 are the oocytes arrested?
Ans: The oocytes are arrested at the diplotene stage of meiosis I. The process is resumed just before ovulation, by the action of the luteinizing hormone.
Q5. How is it that the sister chromatids are prevented from separating from each other during the anaphase of meiosis I?
Ans: This is mediated via cohesin known as Rec8, specifically expressed only during meiosis I, which inhibits the separation of the sister chromatids of each homologue.
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