Zygotene

Zygotene is the second stage of the first prophase of meiosis, during which homologous chromosomes form bivalents. At this stage, pairing of homologous chromosomes and their synapsis occurs. Homologous regions of chromosomes begin to closely approach each other and form complexes called bivalents. The bivalent consists of two synapsed homologous chromosomes connected in the region of chiasmata - areas of chromosome crossover. During zygotene, a large number of such chiasmata are formed. Synapsis of chromosomes occurs with the participation of proteins of the synaptonemal complex. Thus, as a result of zygotene, homologous chromosomes are paired together into bivalents, which is of great importance for subsequent crossing over and correct chromosome segregation in the first division of meiosis.

Zygotene is the second stage of the first prophase of meiosis, during which an important stage of preparation for chromosome splitting occurs. At this stage, homologous chromosomes begin to form bivalents, which is a key event of meiosis.

During zygotene, pairs of chromosomes, called homologues, begin to move closer together and line up opposite each other. This results in a structure called a synaptoneme, which holds homologous chromosomes together. Synaptonema is formed thanks to special proteins called synaptonemal complexes.

The formation of bivalents at the zygotene stage is of great importance for meiosis, since it ensures the accurate distribution of genetic information between daughter cells. Each bivalent consists of two chromosomes, each of which is inherited from different parents. During the process of meiosis, bivalents are separated into individual chromosomes, which are then distributed among daughter cells.

However, the zygotene stage can also pose a risk of errors in the distribution of genetic information, which can lead to genetic abnormalities and diseases. For example, misalignment of homologous chromosomes or breaks in the synaptonema can lead to errors in chromosome distribution, which can cause syndromes associated with abnormal numbers of chromosomes (eg, Down syndrome).

Thus, zygotene is an important stage of meiosis, which ensures the precise distribution of genetic information between daughter cells. It also emphasizes the importance of proper functioning of meiosis for human health.

Zygotene is the second stage of prophase I. Due to conjugation, sister chromatids are connected by their centromeres, forming a morphologically homogeneous syndesmosomal hiatus. The bivalent is formed due to the intersection of their centromeres. Variability in the relative position of the mother and daughter chromatids gives rise to many forms of zygotene.

Zygotene is preceded by the first stage, pachynema, when the unraveling of sister monochromatid loops is completed and daughter chromatids are formed. The end of this stage is indicated by the appearance of the nuclear envelope.

During interkinesis, mitotic DNA replication continues. “Parental” daughter chromatids contain only one copy of the maternal chromosome (1n), and sister daughters contain two. With the help of the mRNA and protein system, interkinesis reorganizes the prokaryotic genome from the beginning of the cell cycle - from the G1 state to metaphase.