Main Difference – Recombination vs Crossing Over
Recombination and crossing over are two correlating processes, which lead to genetic variations among the offspring. Both events occur during the prophase 1 of meiosis 1 in eukaryotes. The pairing of homologous chromosomes during prophase 1 lets the crossing over to occur and crossing over between non-sister chromatids, in turn, lets the recombination to occur. Crossing over occurs at points called chiasma, which are created between non-sister chromatids. Chiasma lets the exchange of DNA segments between non-sister chromatids. This exchange of DNA segments produces new combinations of alleles among the offspring, which is identified as genetic recombination. The main difference between recombination and crossing over is that recombination is the production of different combinations of alleles in the offspring whereas crossing over is the exchange of genetic material between non-sister chromatids, the event which produces recombination.
This article contains,
1. What is Recombination
– Definition, Process, Function
2. What is Crossing Over
– Definition, Process, Function
3. What is the difference between Recombination and Crossing Over
What is Recombination
The production of offspring with different combinations of traits compared to their parents is known as recombination in genetics. Genetic recombination is often a natural process. Eukaryotic genetic recombination occurs during prophase 1 of meiosis 1. Meiosis is the process of producing gametes for the sexual reproduction. The variations of genes in the gametes lead to the production of genetically varied offspring.
Eukaryotic genetic recombination occurs through homologous chromosome pairing, followed by the exchange of genetic information between non-sister chromatids. The homologous chromosome pairing is known as synapsis. The exchange of genetic information can occur by either physical transfer or non-physical transfer. The physical transfer of genetic information occurs through the exchange of chromosome segments between non-sister chromatids. On the other hand, sections of genetic material in one chromosome can be copied to another chromosome without exchanging the parts of chromosomes physically. This copying of genetic information occurs through synthesis-dependent strand annealing (SDSA), which allows the exchange of information, but not the physical exchange of DNA pieces. The double Holliday junction (DHJ) pathway is another model of copying genetic information, leading to the non-physical transfer of genetic information. Both SDSA and DHJ pathways are initiated by a gap or double-strand break, followed by the invasion of strands to start the copying down of genetic information. Thus, both SDSA and DHJ pathways are considered as repair mechanisms. The copying down of information can be either non-crossover (NCO) or crossover (CO) types of the flanking regions. During NCO type, a repair of the broken strand occurs, only one chromosome, which holds the double-strand break is transferred with the new information. During CO type, both chromosomes are transferred with new genetic information. The SDSA and DHJ models are described in figure 1.
Figure 1: Homologous Recombination
During mitosis, the exchange of genetic material can occur between sister chromatids after the DNA replication is completed at the interphase. But, new allele combinations are not produced since the exchange occurs between identical DNA molecules, which are produced by the replication.
Recombinases are the class of enzymes which catalyze the genetic recombination. The recombinase, RecA is found in E. coli. In bacteria, recombination occurs through mitosis and the transfer of genetic material between their organisms. In archaea, RadA is found as the recombinase enzyme, which is an ortholog of RecA. In yeast, RAD51 is found as a recombinase and DMC1 is found as a specific meiotic recombinase.
What is Crossing Over
The exchange of DNA segments between non-sister chromatids during the synapsis is known as the crossing over. The crossing over occurs during the prophase 1 of meiosis 1. It facilitates the genetic recombination by exchanging the genetic information and producing new combinations of alleles.
Synapsis of a homologous chromosome pair is achieved by the formation of two synaptonemal complexes between the two p arms and q arms of each chromosome. This tight holding of the two homologous chromosomes allows the exchange of genetic information between the two non-sister chromatids. The non-sister chromatids contain matching DNA regions, which can be exchanged through chiasmata regions. 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 until their segregation at the metaphase 1.
Crossing over is initiated by the breaking down of similar DNA regions that occur within the homologous chromosome pair. Double-strand breaks can be introduced to the DNA molecule either by Spo11 protein or DNA damaging agents. Then, the 5’ ends of DNA edges are digested by exonucleases. This digestion introduces 3’ overhangs into the DNA edges of the DNA strands. The single-stranded 3’ overhangs are coated by recombinases, Dmc 1 and Rad51, producing nucleoprotein filaments. The invasion of this 3’ overhang into the non-sister chromatid is catalyzed by recombinases. This invaded 3’ overhang primes the DNA synthesis, using the non-sister chromatid’s DNA strand as the template. The resulting structure is known as the cross-strand exchange or the Holliday junction. This Holliday junction is pulled along the chiasma by recombinases.
Figure 2: A Holliday junction
Difference Between Recombination and Crossing Over
Definition
Recombination: The production of an offspring which contains different combinations of traits compared to their parents is known as recombination.
Crossing Over: The exchange of DNA segments between non-sister chromatids during the synapsis is known as crossing over.
Correspondence
Recombination: Crossing over leads to genetic recombination.
Crossing over: Synapsis leads to the crossing over.
Function
Recombination: Recombination produces genetic variation among the offspring. It also works as a repair mechanism for double-strand breaks during meiosis.
Crossing Over: Crossing over exerts to the genetic recombination between chromosomes.
Conclusion
Recombination and crossing over are two closely related events that occur during synapsis. During synapsis, homologous chromosomes are tightly held by the synaptonemal complexes. This tight holding allows the chromosomal cross over to occur between non-sister chromatids. The point where the crossing over occurs is known as the chiasma. The four strands structure where the physical exchange of genetic material occurs is known as the Holliday junction. The exchange of genetic material can occur non-physically by the copying down of DNA segments into a second chromosome. The exchange of genetic material leads to the variations of the alleles among the offspring. The formation of different combination of alleles among the offspring is known as the recombination. Recombination also works as a repair mechanism to correct the double-strand breaks. This the main difference between recombination and crossing over.
Reference:
1. “Genetic recombination.” Wikipedia. Wikimedia Foundation, 14 Mar. 2017. Web. 16 Mar. 2017.
2. “Chromosomal crossover.” Wikipedia. Wikimedia Foundation, 13 Mar. 2017. Web. 16 Mar. 2017.
Image Courtesy:
1. “Homologous Recombination” By Harris Bernstein, Carol Bernstein and Richard E. Michod – Chapter 19 in DNA Repair. Inna Kruman editor. InTech Open Publisher. DOI: 10.5772/25117 ( CC BY 3.0) via Commons Wikimedia
2. “Mao-4armjunction-schematic” By Chengde Mao – Mao, Chengde (December 2004). “The Emergence of Complexity: Lessons from DNA”. PLoS Biology 2 (12): 2036-2038. DOI:10.1371/journal.pbio.0020431. ISSN 1544-9173. (CC BY 2.5) via Commons Wikimedia
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