Pairing of Homologous Chromosomes during Meiosis

In a diploid mammal or human, when both the chromosomes of DNA have the same gene sequences, locus, centromere, and the size of the chromosome, then they are called homologous chromosomes. X and Y are the two homologous chromosomes found in humans and mammals. The male has an XY chromosome and a female has an XX chromosome that is ultimately used for sex determination. A mother transfers the XX chromosome to the offspring and a father transfers the different chromosomes as X and Y chromosomes to the offspring. The chromosomes from the father decide the sex of offspring because the mother passes only the X chromosome to the next generation. When the father transfers the X chromosome, the offspring is a female and when the Y chromosome is passed, it gives rice to male offspring. Now, one chromosome from each parent forms homologous chromosomes in the cell during meiosis and forms haploid daughter cells.

Meiosis cell cycle completely occurs in two parts as meiosis 1 and meiosis 2. Meiosis 1 has further stages of prophase 1, metaphase 1, anaphase 1, and telophase 1. The same stages with little different functions are for meiosis 2 as prophase 2, metaphase 2, anaphase 2, and telophase 2. Meiosis completes with a small resting phase in between meiosis 1 and meiosis 2 and that is known as interkinesis. Before the meiosis cycle starts, the cell needs to be prepared, and this cell enters interphase where G1 Phase (Growth 1), S Phase (Synthesis), and G2 Phase (Growth 2) occur. G1 Phase supports the enzyme and protein synthesis for cell differentiation. S Phase allows the replication of the genetic material to form the identical chromatids, whereas G2 Phase does not take much time to occur and functions as prophase 1.

Prophase 1 of meiosis 1 cell cycle takes the maximum time in comparison to the other stages. In this phase, the recombination, crossing over, and cell segregation occur to ultimately give the paired homologous chromosomes. This results in the divalent chromosome or the tetrads (four chromatids). Prophase 1 further occurs in the four stages as leptotene, zygotene, pachytene, diplotene, and mitotic spindle fiber formation.

Leptotene: This is the very first stage in which all the chromosomes are initially attached to the nuclear envelope. Later, these chromosomes are attached with other chromosomes forming a homologous sister chromosome. These sister chromosomes have the same centromere and are attached with a thin thread-like fiber named cohesion. Leptotene also stands for a Greek word called ‘thin thread’. Recombination also takes place at this stage in which the breaking of homologous chromosomes breaks down later. Leptotene takes place at many stages of prophase 1 and is visible through the electron microscope.

Zygotene: The word “zygotene” is the Greek word for ‘paired threads’. These threads bind the two sister homologous chromosomes more densely and closely. These cohesion threads bind the sister chromatids from many points such that they cannot separate easily, become stable, and do not cause any mutations. This is the stage where preparation of synapsis occurs and ‘synaptonemal complex’ in between the homologous chromosomes.

Pachytene:This word has the Greek meaning of ‘thick thread’. At this stage, every chromosome gets its sister chromosome and sister chromatids. The threads attaching the sister chromatids get loosened without separating the homologous chromosomes. These homologous chromosomes rearrange somehow so that non-sister chromatids come closer for the crossing over. Till now, recombination has been done and homologous chromosomes are prepared for the cross-over process.

Diplotene: The loosely bound sister chromatids move toward the non-sister chromatids and perform the cross-over by forming a chiasma. The chiasma site transfers the small amount of gene sequence from one non-sister chromatid to another. They exchange a good amount of genetic material from mothers and fathers.

Pairing of homologous chromosomes and allowing them to cross over before their segregation is referred to as synapsis. The crossing over is also named genetic recombination. In the crossing over process, the end of the non-sister chromatids is brought together for the exchange of genetic material. Before exchanging, they get connected via protein–RNA complex or synaptonemal complex. This process causes a chiasma of two chromatids that are physically connected in a cross (X) shape.

These chromatids can distinguish between the self and non-self, such that crossing over never occurs in sister chromatids. The exchange of a region of chromatids independently occurs; thus, gene transfer occurs from generation to generation allowing mutations sometimes. And this increases the genetic variations in species.

The sex chromosomes also go through synapsis. Many mutations are studied that occurred due to the genetic exchange.

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