Difference Between Bivalent and Tetrad

Main Difference – Bivalent vs Tetrad

Bivalent and tetrad are two closely related terms used to describe chromosomes in their different stages. Bivalent is the homologous chromosome pair, which consists of two chromosomes. One of the two chromosomes has a maternal origin and the other has a paternal origin. The formation of the homologous pair is observed during meiosis, which produces gametes for the sexual reproduction. Before entering to the meiotic division, the chromatin inside the nucleus is replicated. Thus, each chromosome consists of two sister chromatids. Therefore, when a bivalent is formed, it consists of four sister chromatids together. These four sister chromatids are collectively known as a tetrad. Thus, the main difference between bivalent and tetrad is that bivalent is the group of two homologous chromosomes whereas tetrad is the group of four sister chromatids inside the homologous chromosome pair.

This article explores,

1. What is a Bivalent
– Definition, Formation, Characteristics
2. What is a Tetrad
– Definition, Formation, Characteristics
3. What is the difference between Bivalent and Tetrad

What is a Bivalent

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.

Figure 1: A Bivalent

What is a Tetrad

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. When a cell enters its division phase, the chromatin is more condensed to form chromosomes, which are visible under the microscope as thread-like structures. Then, each chromosome consists of two identical DNA molecules. These types of identical DNA molecules are known as sister chromatids. That means a single, replicated chromosome consists of two sister chromatids. During meiotic division, homologous chromosomes are paired at the prophase 1 of meiosis1. One chromosome in the homologous pair bears a maternal origin while the other chromosome bears a paternal origin. When these two homologous chromosomes are paired together at the prophase 1 of meiosis 1, four sister chromatids can be found all together, grouped in the homologous pair. Chromosomal crossing over occurs within the non-sister chromatids of a homologous chromosome pair, leading to the genetic variation among offspring. These four sister chromatids in the homologous pair are known as tetrad. A tetrad is shown in figure 2.

Figure 2: A tetrad

Definition

Bivalent: A bivalent is the pair of two homologous chromosomes, occurred during the prophase 1 of meiosis 1.

Tetrad: A tetrad is the group of four sister chromatids found within the homologous pair.

Formation

Bivalent: A bivalent occurs during the prophase 1 of meiosis 1.

Tetrad: Each of the two sister chromatids occurs by DNA replication during the S phase of the interphase. The group of four sister chromatids can be observed after pairing of homologous chromosomes together.

Number of Constituents

Bivalent: A bivalent consists of two constituents, the two homologous chromosomes.

Tetrad: A tetrad consists of four components, the four sister chromatids of a homologous chromosome pair.

Conclusion

Bivalent and tetrad are two terms used in describing the homologous chromosome pair. A replicated chromosome consists of two sister chromatids. During prophase 1 of meiosis 1, homologous chromosomes pair together within the nucleus. The two homologous chromosomes are held together within a pair by synaptonemal complexes formed between two chromosomal arms. These two chromosomes in the homologous pair are called as bivalent. Four sister chromatids can be identified within the homologous pair. These four sister chromatids are collectively called as a tetrad. This is the basic difference between bivalent and tetrad.

Reference:
“Bivalent (genetics).” Wikipedia. Wikimedia Foundation, 02 Mar. 2017. Web. 16 Mar. 2017.

Image Courtesy:
“Tetrad” (CC BY-SA 2.5) via Commons Wikimedia
“Bivalent“(CC BY-SA 2.5) via Commons Wikimedia