Meiosis‌ ‌I‌ ‌Stages‌ ‌And‌ ‌Processes‌

Meiosis is a special type of cell division, in which the genetic material from one cell is divided into four cells. Meiosis produces gametes or spores containing one set of haploids. The gametes combine at the time of fertilization to supply a diploid set of chromosomes. This leads to the fusion of gametes with many unique combinations of maternally and paternally derived chromosomes among the haploid complement.

Meiosis is the main reason behind variation. Crossover, which occurs in meiosis, leads to genetic exchange between members of every homologous pair of chromosomes. This leads to the production of offspring that always differ from either parent.

We divide Meiosis into Meiosis 1 or reductional division and meiosis 2 or equational division. Meiosis 1 has five substages:

• Prophase 1
• Metaphase 1
• Anaphase 1
• Telophase 1

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Figure 1 - Meiosis I Stages And Processes Explained

Prophase 1:

The chromatin condenses and coils into visible chromosomes. Each member of a homologous pair of chromosomes undergoes synapsis. Synapsis is that process where the pairing of homologous chromosomes occurs. This is often where crossover happens.

Prophase 1 is further divided into:

• Leptotene: The chromosomes condense, and it becomes visible under the microscope. During this stage, the homology search is initiated, which is essential for the pairing process of homologs.
• Zygotene: The chromosomes further shorten and thicken at this stage. The homology search is complete and a process called chromosomal synapsis is initiated. Synaptonemal complex is formed between the paired homologous chromosomes. We term these paired homologs as bivalents.
• Pachytene: During this stage, the homologous pair of non-sister chromatids undergo crossing-over at recombination sites.
• Diplotene: Each pair separates within the tetrad of non-sister chromatids. But some areas remain in touch where the chromatids intertwine. We call such areas chiasmata, which represents some extent where the non-sister chromatids have undergone genetic exchange through the crossing-over process. The results of the crossover are visible only when the duplicated chromosome separates.
• Diakinesis: This is often the last stage. The chromosome pulls apart, and non-sister chromatids remain loosely associated via chiasmata. The process of termination begins in the late diplotene stage and is completed in diakinesis. In this phase, the complete breakdown of the nucleolus and the nucleolus membrane occurs. During this process, the two centromeres of the tetrad get attached to the newly developed spindle fibers. These spindle fibers are nothing but cytoplasmic substances that are produced at the time of cellular division. The spindle fibers assist in the separation of chromatids during their movement towards the poles. The metaphase plate of every cell shelters the centromeres of the tetrad.

Metaphase 1:

The chromosomes are now in a shortened form and are considerably thick as well. The tetrad is arranged on the central metaphasic plate and spindle fibers now attach these chromatids and are ready to pull the pair towards opposite poles.

Anaphase 1:

Each pole pulls half the tetrad towards it. A series of dyads (one-half of every tetrad) equivalent to the haploid number is present at each pole. If crossover had not happened, each dyad at each pole would have contained paternal or maternal chromatids. But the exchange produces mixed chromatids of paternal and maternal origin.

Telophase 1:

Nuclear membranes form around the dyads. This phase is brief. The cell moves on meiosis 2.

Meiosis 2 also has prophase, metaphase, anaphase, and telophase. In prophase 2, each dyad containing a pair of sister chromatids is attached by a standard centromere. In Metaphase 2, the centromeres are placed on the metaphase plate. In Anaphase 2, the sister dyads are pulled towards the other poles.

The number of binary groups is adequate for the number of haploids. Telophase 2 is where one member of every pair is present at each pole. So now, each chromosome is a monad. Following telophase 2, cytokinesis occurs.

So, four haploid gametes are formed at the end of one meiotic event. The haploid state is achieved after the completion of the Meiosis 2

Significance of meiosis:

• Meiosis results in the formation of sex cells or gametes that are important for sexual reproduction.
• It activates the genetic information for sexual reproduction and deactivates the sporophyte formation.
• It maintains the constant number of chromosomes by halving because the chromosome number doubles after fertilization.
• Independent assortment, a law of genetics is fulfilled because of meiosis.
• Meiosis is the main reason behind traits and variation.