Meiosis specialises in contributing to sexual reproduction in eukaryotic organisms: plants, animals, and fungi. Unlike mitosis, it brings about a reduction in the number of chromosomes by half to create gametes-sperm and eggs-genetically unique cells. Meiosis I is a topic of the chapter Cell Cycle and Cell Division in Biology.
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Meiosis I is a reductional cell division that halves the number of chromosomes. It changes a diploid to a haploid state so that gametes can then combine during reproduction without excessive chromosome numbers. The reduction is accomplished in two successive cell divisions: meiosis I and meiosis II.
Whereas mitosis produces two identical daughter cells, meiosis I produce two cells that differ from each other. The parent cell after prophase I, in which homologous chromosomes pair up, and anaphase I, during which the paired chromosomes separate. This way, by allowing for more genetic diversity through crossing over, all the resulting gametes will have a unique combination of genetic material, being thus a driver for evolutionary adaptation.
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Meiosis I is studied under Prophase I, Metaphase I, Anaphase I and Telophase I.
It is the longest phase of meiosis I.
It is divided into five substages: leptotene, zygotene, pachytene, diplotene and diakinesis.
The chromatin begins to condense to form chromosomes.
Chromosomes appear as thin and long threads.
By the end of this phase, chromosomes become visible under a microscope.
The pairing of the homologous chromosomes initiates in this phase.
The pairing of homologous chromosomes is called synapsis.
The synapsed homologous chromosomes appear in the form of bivalent chromosomes or tetrad of chromatids.
In the tetrad, two similar chromatids of the same chromosome are called sister chromatids and those of two homologous chromosomes are termed non-sister chromatids.
A filamentous ladder-like nucleoproteins complex, called a synaptonemal complex appears between the homologous chromosomes. It holds the homologous chromosomes together.
The exchange of parts between non-sister chromatids occurs during this phase.
It is called the crossing over.
Crossing over occurs through breakage and reunion of chromatid segments.
Breakage is called nicking. It is assisted by an enzyme endonuclease.
Reunion is termed annealing. It is aided by an enzyme ligase.
This phase involves pulling away the synapsed homologous chromosomes.
The point of attachment of the homologous chromosomes where crossing over occurs is called chiasma.
Homologous chromosomes remain attached only at chiasma.
There can be more than one chiasmata.
It marks the terminalization of chiasma.
The nuclear membrane and nucleolus degenerate.
Chromosome recondenses and tetrad moves to the metaphase plate.
Spindle fibres begin to form.
When the diakinesis of prophase-I is completed than cell enters into metaphase-I.
During this phase, bivalents arrange themselves on the metaphase plate.
Hence, a fully formed spindle and equatorial alignment of the chromosomes are seen during this phase.
The alignment of homologous chromosomes is independent of each other.
This is responsible for generating genetic variability.
The homologous chromosomes of each bivalent separate from each other.
The separated homologous chromosomes move to opposite poles
Therefore, in this phase, the chromosomes separate and not the chromatids.
So, each chromosome will still have two sister chromatids.
Hence, anaphase I involve a reduction in the number of chromosomes.
Two daughter nuclei are formed but the chromosome number is half the chromosome number of the mother cell.
This phase is not necessarily complete wholly.
The spindle disappears, but new nuclear envelopes need not form before the onset of meiosis II.
It may or may not follow the telophase I.
When it occurs, it forms the dyad of cells.
Following cytokinesis I, the cells enter interkinesis.
It is also known as intermeiotic interphase.
During this phase, there is no duplication or replication of DNA as the chromosomes are already duplicated.
Meiosis I is essential in providing ways in which genetic diversity can take place. Through crossing over processes during prophase I and independent assortment that takes place in metaphase I leads to specific sets of genetic materials that form the gametes. Such genetic diversity is quite important for the survival as well as adaptation of species.
Since it allows for greater variability in the offspring; this way, they could become better adapted to changing environmental conditions. Meiosis I also provides for the decreased number of chromosomes from diploid to haploid so that when such gametes fuse during fertilisation. The correct number of chromosomes characteristic of the species is present in the zygote. Thus, meiosis I is recognised as a process of great importance in maintaining genetic integrity and therefore in promoting evolutionary success among sexually reproducing organisms.
Also Read:
Cell Cycle | Meiosis II |
Difference Between Mitosis and Meiosis | Difference Between Karyokinesis and Cytokinesis |
Eukaryotic Cells | Prokaryotic Cells |
Meiosis I is a division where homologous chromosomes are paired and crossed over in such a way that they are separated into two daughter cells with half the number of the original chromosomes. On the other hand, Meiosis II is similar to mitosis but it deals with haploid cells; its end products are four genetically different daughter cells.
Stages of prophase I include leptotene, zygotene, pachytene, diplotene and diakinesis.
Crossing over, which occurs in prophase I of meiosis, involves the exchange of DNA segments between homologous chromosomes. New combinations are now formed due to crossing over on the chromosomes. This results in more genetic variation in the gametes and offspring.
Meiosis is called reductional division since meiosis reduces by half the number of chromosomes. In meiosis I, homologous chromosomes separate. This results in two daughter cells with half the number of chromosomes of the parent cell.
Homologous chromosomes pair up during meiosis I, facilitating crossing over and ensuring that each resulting gamete receives a unique combination of genetic material from both parents. This process promotes genetic diversity among offspring, which is crucial for adaptation and evolutionary success.
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