The key difference between bivalent and synaptonemal complex is that bivalent is an association between male and female homologous chromosomes while the synaptonemal complex is the tripartite protein structure that forms between two homologous chromosomes.
Meiosis is the process of cell division followed by gamete cells. During meiosis, the chromosome number is reduced by half to maintain the chromosome number during sexual reproduction. The male and female chromosomes segregate and then divide into the successive generation. There are two main phases of meiosis: they are the meiosis I and meiosis II. Similar to mitosis, meiosis also goes through the stages of prophase, metaphase, anaphase and telophase.
The chromosomes are obtained from two different gamete cells: the female ovum and the male sperm. Therefore, during the meiosis process, these homologous chromosomes undergo crossing over. During the meiotic prophase, bivalents are formed, and the genetic composition is mixed at points known as the chiasma. Bivalent or tetrad is an association of homologous chromosomes formed during the prophase I of meiosis. The formation of a synaptonemal complex is the first step of the process of forming a bivalent complex. It is the protein structure that forms between two homologous chromosomes during meiosis.
CONTENTS
1. Overview and Key Difference
2. What is Bivalent
3. What is Synaptonemal Complex
4. Similarities Between Bivalent and Synaptonemal Complex
5. Side by Side Comparison – Bivalent vs Synaptonemal Complex in Tabular Form
6. Summary
What is Bivalent?
Bivalent is formed during the meiosis process between homologous chromosomes. Two sets of chromosomes from male and female gamete are involved in meiosis. The bivalent is formed as an association between male and female homologous chromosomes. The bivalent is also referred to as the tetrad. Under normal cell division conditions, each bivalent contains at least one cross over point known as chiasma. The number of chiasma in the bivalent gives an idea about the cross over efficiency of the DNA during meiosis. Formation of a bivalent in meiosis is vital as it allows the segregation of chromosomes during meiosis.
Figure 01: Bivalent
Formation of a bivalent is a complex process and involves the following steps:
- Formation of the synaptonemal complex containing the two homologous chromosomes.
- Pairing of the two homologous chromosomes, which takes place between the leptotene and the pachytene phase of prophase I of meiosis.
- DNA is exchanged at certain points known as chiasma.
- A physical connection is established in the diplotene phase of prophase I of meiosis.
- At the end of the diplotene phase, a bivalent is formed.
The formation of bivalents will ensure that the genetic composition is mixed between the gamete cells. Upon the formation of bivalents, a tension is created, and each chromatid is pulled to the opposite direction. This will allow the bivalents to arrange in the centre of the cell.
What is Synaptonemal Complex?
Synaptonemal complex is the protein structure formed between two homologous chromosomes. And, this structure facilitates homologous chromosome pairing, synapsis and recombination. There are two parallel lateral regions and a central element in the synaptonemal complex. Therefore, it is a tripartite structure which shows a characteristic ladder-like organization. These three components of the synaptonemal complex are made from SC protein-1 (SYCP1), SC protein-2 (SYCP2), and SC protein-3 (SYCP3).
Figure 02: Synaptonemal Complex
Synaptonemal complex connects the two homologous chromosomes along their lengths by the process called synapsis, which promotes recombination between homologous chromosomes. Moreover, the synaptonemal complex is involved in the correct chromosome segregations during the anaphase I of meiosis.
What are the Similarities Between Bivalent and Synaptonemal Complex?
- Bivalent and synaptonemal complex are two structures developed during the meiosis cell division.
- Synaptonemal complex formation is the initial step of forming the bivalent.
- Both are formed during the prophase I of meiosis.
- They are specific nuclear structures.
- They promote genetic recombination.
- Moreover, they allow correct chromosome segregations during anaphase I.
What is the Difference Between Bivalent and Synaptonemal Complex?
Bivalent is an association formed between male and female homologous chromosome pair. On the other hand, the synaptonemal complex is a meiosis-specific proteinaceous tripartite structure. So, this is the key difference between bivalent and synaptonemal complex. Structurally, bivalent is composed of two homologous chromosomes while the synaptonemal complex is a protein structure consisting of three elements.
Moreover, bivalents facilitate genetic mixing between homologous chromosomes while the synaptonemal complex connects homologous chromosomes along their lengths. Therefore, this is the main functional difference between bivalent and synaptonemal complex.
Summary – Bivalent vs Synaptonemal Complex
Bivalent is an association formed between a homologous chromosome pair during the prophase of meiosis. Meanwhile, the synaptonemal complex is a tripartite protein structure formed between two homologous pair during the prophase of meiosis. Thus, bivalent is a chromosome pair while synaptonemal complex is a proteinaceous structure. Therefore, this is the key difference between bivalent and synaptonemal complex. Both structures promote genetic recombination between homologous chromosomes.
Reference:
1. “Synaptonemal Complex.” Wikipedia, Wikimedia Foundation, 19 July 2019, Available here.
2. “Chromosome Bivalent.” Chromosome Bivalent – an Overview | ScienceDirect Topics, Available here.
Image Courtesy:
1. “Bivalent” By internet – (CC BY-SA 2.5) via Commons Wikimedia
2. “Synaptonemal complex.” By Daniel G. Peterson, Mississippi Genome Exploration Laboratory, Mississippi State University, Mississippi State, Mississippi, United States – Hey J: What’s So Hot about Recombination Hotspots? PLoS Biol 2/6/2004: e190. doi:10.1371/journal.pbio.0020190 (CC BY 2.5) via Commons Wikimedia