What is a Tetrad — Definition, Formation, Characteristics 3. What is the difference between Bivalent and Tetrad. A pair of chromosomes that is associated in a homologous manner during the prophase 1 of meiosis 1 is known as a bivalent. Each chromosome in the homologous pair contains two identical sister chromatids produced during the replication. The two homologous chromosomes are held together physically by the formation of synaptonemal complexes.
The synaptonemal complexes are formed during the leptotene stage of the prophase 1. DNA double-strand breaks can occur during the leptotene stage of prophase 1. These double-strand breaks are repaired by a process called crossing over , which is one of the most important events, achieving genetic variation during the meiotic division. The site where the crossing over occurs is known as chiasma. Thus, the physical exchange of DNA segments occurs through the chiasma. Leptotene stage is followed by the pachytene stage.
Both leptotene and pachytene are two substages found in prophase 1 of meiosis 1. The formation of the synaptonemal complexes and homologous recombination can be observed throughout leptotene and pachytene stages. The four parts of the homologous chromosome pair can be visible under the microscope with the disintegration of the nuclear envelope during diakinesis stage, which is one of the later substages of prophase 1.
The formation of the synaptonemal complexes provides a support to hold the two homologous chromosomes together throughout the prophase 1 of meiosis 1. It also allows the alignment of homologous chromosome pairs on the cell equator for a proper segregation of homologous pairs during meiosis 1.
A bivalent is shown in figure 1. The four sister chromatids are collectively called as the tetrad. Before entering the cell division, the chromatin inside the nucleus is replicated with the aid of DNA polymerases.
This DNA replication occurs during the S phase of the interphase. Each bivalent consists of 4 chromatids and 2 centromeres and is also called as tetrad. Here two centromeres, therefore 2 chromosomes. Each chromosome with 2 chromatids each. In total 4 chromatids, therefore also called as tetrad. The process of crossing over takes place during this bi valent stage.
Crossing over is the exchange of segments between non-sister chromatids of homologous chromosomes during pachytene stage of prophase I of meiosis I. This process is the major reason for genetic variation between individuals in a population. Tags: centromeres chromatids crossing over meiosis I pachytene stage Tetrad and bivalent. So each bivalent appears as 'tetrad' i.
Homologous chromosomes exchange parts in a process called crossing as the first prophase stage of meiosis continues. For this, homologous pairing and appearance of bivalent is important. Crossing over can take place when bivalent is in tetrad stage. What is the difference between a homologous chromosome and a tetrad? Mandira P. Jul 20, Explanation: Although both are very similar, the difference between the two is the pairing. Hope this helps :.
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