Meiosis is a form of cell division in which the chromosome number is reduced to half. Meiosis I and meiosis II are two cycles of nuclear and cell division. Prophase I is one of the stages in the process of meiosis I.
The fourth stage of prophase 1 is diplotene. The Diplotene stage is followed by crossing over in the pachytene stage.
Prophase I of meiosis is divided into five stages-
In this first stage, chromosome condensation occurs. Chromosomes become visible under the light microscope. They appear as a string of beads known as nucleosomes. In this stage, each chromatid lies very close to the other. This stage occurs as a leptotene- zygotene transition and itself is a very short stage.
2. Zygotene -
In this stage, chromosome pairing starts, and this process of association is known as synapsis and leads to the formation of the complex structure known as the synaptonemal complex. Synapsis is a process of meiosis in which homologous chromosomes pair up and remain bound to each other for the exchange of genetic material. This pairing up between homologous chromosomes is known as tetrad or bivalent.
The process of crossing over and the subsequent recombination will go ahead once a tetrad has developed, where a little of the genetic material from the sequences of parental DNA is swapped over to increase gene variation. At this stage, while the chromosomes remain attached as a pair, the chromatid sisters (the two chromatid strands that make up a single chromosome) begin to separate from each other. This, under an electron microscope, makes them even more distinctive. The chiasma is the linking point between two non-sister chromatids.
Desyanpsis begins in the diplotene stage of meiotic prophase I.
(Desyanapsis - It refers to the separation of homologous chromosomes).
The synaptonemal complex dissolves allowing the two homologous chromosomes in a bivalent to pull away from each other to some extent.
(Bivalent: One pair of chromosomes (sister chromatids) in a tetrad is known as bivalent).
However, each bivalent remains to join by one or more chiasmata representing the sites where crossing over has occurred. In oocytes (developing eggs), diplotene can last for months or years, since it is at this stage that the chromosomes are decondensed and engage in RNA synthesis to provide storage materials for the egg. In the extreme, the diplotene chromosomes can become highly active in RNA synthesis and expand to an enormous extent, producing the lampbrush chromosomes found in amphibians and some other organisms.
Chiasmata are essential for homologous chromosomes to be connected to opposite spindle poles (bipolar attachment) and subsequently segregated to the opposite pole.
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Followed by diplotene is diakinesis in which terminalisation of chiasmata takes place. This stage represents a transition to metaphase. Other systemic changes are occurring in this stage in order to prepare for the next step in meiosis I. Dissolution of the nucleolus and the nuclear envelope occurs. This allows the centrioles that contribute to spindle formation free to migrate.
Microtubules in the cell cytoplasm are the predominant building blocks of spindle construction.
Did You Know?
Some important facts about mitosis and meiosis-
Meiosis is the mechanism in which the parent cell splits into four daughter cells, which contain half of the original amount of genetic material, i.e. the haploid daughter cells. Gametes are produced by meiosis.
Meiosis is responsible for the formation of gametes that are responsible for sexual reproduction.
The creation of sex cells or gametes that are responsible for sexual reproduction is the duty of Meiosis.
It activates genetic information and deactivates sporophytic information for the production of sex cells.
The constant number of chromosomes is maintained by halving the same. This is significant since, after fertilization, the chromosome number doubles.
Independent maternal and paternal chromosome assortment takes place in this process. The chromosomes and the characteristics regulated by them are thus reshuffled.
Due to abnormalities in cell division by meiosis, the genetic mutation takes place. By natural selection, the mutations that are advantageous are passed on.
A new combination of traits and variations is produced by crossing over.