Difference Between Synapsis and Crossing Over

Main Difference – Synapsis vs Crossing Over

Synapsis and crossing over are two events that occur during the chromosome segregation in meiosis 1. Meiosis occurs during the production of gametes in order to achieve the sexual reproduction of organisms. Both synapsis and crossing over are important in exerting genetic variation among the individuals by allowing the exchange of genetic material between homologous chromosomes. The main difference between synapsis and crossing over is that synapsis is the pairing of homologous chromosomes during the prophase 1 of the meiosis 1 whereas crossing over is the exchange of the genetic material during synapsis.

This article looks at,

1. What is Synapsis
       – Definition, Mechanism, Function, Characteristics
2. What is Crossing Over
      – Definition, Mechanism, Function, Characteristics
3. What is the difference between Synapsis and Crossing Over

Difference Between Synapsis and Crossing Over - Comparison Summary

What is Synapsis

The pairing of the two homologous chromosomes during the meiosis is known as synapsis. The synapsis occurs at prophase 1 of meiosis 1. It allows the two homologous chromosomes to segregate at the anaphase 1 of meiosis 1.

Mechanism of Synapsis

During synapsis, the ends of the individual chromosomes are attached to the nuclear envelope first. These chromosomal end-membrane complexes are migrated until they find the other homologue to pair with the assistance of the extranuclear cytoskeleton. Then, the intervening regions of the two chromosomes are connected by synaptonemal complexes, which consist of RNA and proteins. A synaptonemal complex is shown in figure 1.

Difference Between Synapsis and Crossing Over

Figure 1: Synaptonemal Complex (SC)
(A) Lateral view: homologous chromosomes (light blue rods) aligned together by the meshwork of transverse (black lines) and longitudinal (dark blue rods) filaments. (B) Tomato SCs at the top and two tomato SCs (Crossing over occurs at the marked sites.)

Autosomes form two synaptonemal complexes at the two ends of the chromosome. But, sex chromosomes form a single synaptonemal complex at one end of each chromosome.

Function of Synapsis

The major role of synapsis is the recognition of the two homologues by pairing, in order to undergo a successful synapsis. During synapsis, genetic variation is allowed in two ways. First is the independent orientation of the pairs of the homologous chromosomes in the cell equator. This is called the law of independent assortment, which allows the segregation of maternal and paternal chromosomes in a random nature. Secondly, the chromosomal cross-over at chiasmata of non-sister chromatids allows the genetic recombination of chromosomes to occur, resulting in new combinations of alleles in the inherited chromosomes. DNA repair occurs in response to DNA damage, especially double-strand breaks during synapsis.

What is Crossing Over

The exchange of DNA pieces between non-sister chromatids during the synapsis is known as crossing over. Thus, chromosomal crossing over occurs during the prophase 1 of meiosis 1 in the sexual reproduction.

Mechanism of Crossing Over

Crossing over occurs when breaking down of similar DNA regions occurs within the homologous chromosome pair, leading to the genetic recombination. Non-sister chromatids in this matching DNA regions intertwine first, breaking apart the similar DNA regions. This similar DNA regions are exchanged through crossing over. The chiasma is an X like region, where the two non-sister chromatids are joined together during crossing over. The formation of the chiasma stabilizes the bivalents or the chromosomes during their segregation at the metaphase 1. The process of crossing over is shown in figure 2.

Main Difference - Synapsis vs Crossing Over

Figure 2: Crossing Over

Synthesis-dependent strand annealing (SDSA) is another type of recombination that occurs during synapsis, which allows the exchange of information, but not the physical exchange of DNA pieces.

Function of Crossing Over

Crossing over increases the genetic variation within an offspring. The genetic variation exerted by crossing over provides a defense against the process called chromosome killer. The repeated crossing over allows the genes in that regions to concentrate independently. The concentration of beneficial genes would be an advantage towards the species.

Difference Between Synapsis and Crossing Over

Definition

Synapsis: The pairing of the homologous chromosomes during the prophase 1 of the meiosis 1 is known as the synapsis.

Crossing Over: The exchange of the genetic material during synapsis is known as the crossing over.

Correspondence

Synapsis: Synapsis occurs at first.

Crossing Over: Synapsis is followed by crossing over.

Correlation

Synapsis: Synapsis always occurs.

Crossing Over: Crossing over may occur sometimes.

Role

Synapsis: Synapsis ensures the proper segregation of homologous chromosomes and allows the recombination through crossing over.

Crossing Over: Crossing over allows the variations of alleles in a population through genetic recombination.

Movement

Synapsis: Two homologous chromosomes come together during synapsis.

Crossing Over: DNA pieces of the non-sister chromatids are exchanged during crossing over.

Conclusion

Synapsis and crossing over occurs during the prophase 1 of meiosis 1, which is involved in the sexual reproduction of organisms. Synapsis is the pairing of homologous chromosomes in a cell. The two chromosomes are held together by synaptonemal complexes, which are formed by a collection of RNA with proteins. Chiasma is the position where the two non-sister chromatids are held together during synapsis. It is an X-shaped DNA region, which allows the double-strand break of both non-sister chromatids, exchanging the genetic material between the two non-sister chromatids. This type of exchanges in genetic material are known as recombination. Genetic variation is made up among the offspring by recombination. The main difference between synapsis and crossing over is their correspondence along with the function.

Reference:
1. “Synapsis.” Wikipedia. Wikimedia Foundation, 18 Feb. 2017. Web. 15 Mar. 2017.
2. “Chromosomal crossover.” Wikipedia. Wikimedia Foundation, 13 Mar. 2017. Web. 15 Mar. 2017.

Image Courtesy:
1. “Synaptonemal complex” By Images and legend courtesy of Daniel G. Peterson, Mississippi Genome Exploration Laboratory, Mississippi State University, Mississippi State, Mississippi, United States. (CC BY 2.5) via Commons Wikimedia
2. “Cross-over” (Public Domain) via Commons Wikimedia

About the Author: Lakna

Lakna, a graduate in Molecular Biology & Biochemistry, is a Molecular Biologist and has a broad and keen interest in the discovery of nature related things

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