Recombination
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Genetic recombination involves the breaking and rejoining of DNA to form new combinations of genes. It occurs primarily during meiosis through several types of recombination, including homologous recombination where DNA exchanges occur between similar DNA molecules. This increases genetic diversity and allows for traits to be mixed. Recombination benefits populations by generating variety among offspring and allowing deleterious genes to be removed without losing the entire chromosome. It has applications in cloning, mapping genes, and making transgenic organisms.
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Recombination
- 2. Recombination Aum Sri Sai Ram Amit Kumar Sahoo I MSc BIOSCIENCES 15151
- 3. What Is Recombination? Recombination is a process by which pieces of DNA are broken and recombined to produce new combinations of alleles. This recombination process creates genetic diversity at the level of genes that reflects differences in the DNA sequences of different organisms. In eukaryotic cells, which are cells with a nucleus and organelles, recombination typically occurs during meiosis. During the alignment, the arms of the chromosomes can overlap and temporarily fuse, causing a crossover. Crossovers result in recombination and the exchange of genetic material between the maternal and paternal chromosomes. As a result, offspring can have different combinations of genes than their parents.
- 4. Genetic recombination Genetic recombination is the production of offspring with combinations of traits that differ from those found in either parent. During meiosis in eukaryotes, genetic recombination involves the pairing of homologous chromosomes. This may be followed by information exchange between the chromosomes. Recombination may also occur during mitosis in eukaryotes where it ordinarily involves the two sister chromosomes formed after chromosomal replication.
- 5. Types of recombination A+ B+ C+ A- B- C- A+ B+ C+A- B- C- Homologous or general A B C D E F A B CD E F Site-specific att att l att att l integrase Replicative recombination, transposition A B C A B C transposase l E. coli
- 6. Homologous recombination Homologous recombination is a type of genetic recombination in which nucleotide sequences are exchanged between two similar or identical molecules of DNA. It is most widely used by cells to accurately repair harmful breaks that occur on both strands of DNA, known as double-strand breaks. These new combinations of DNA represent genetic variation in offspring, which in turn enables populations to adapt during the course of evolution. Homologous recombination is also used in horizontal gene transfer to exchange genetic material between different strains and species of bacteria and viruses.
- 7. Two Types of Homologous Recombination. Double-strand break repair (DSBR) pathway (sometimes called the double Holliday junction model) . Synthesis-dependent strand annealing (SDSA) pathway.
- 8. Holliday junction - corresponding strands of two aligned homologous DNA duplexes are nicked and the nicked strands cross over to pair with the nearly complementary strands of the homologous duplex after which the nicks are sealed.
- 9. Formation of Holliday Junction Branch migration-4 strands exchange base pairing partners
- 10. Resolution of Holliday Junction occurs in 2 ways 1.The cleavage of the strands that did not cross over exchanges the ends of the original duplexes to form, after nick sealing, traditional recombinant DNA 2.The cleavage of strands that crossed over exchanges a pair of homologous single-stranded segments.
- 11. X-ray structure of the cross over event. Electron micrographs of intermediates in the homologous recombination of two plasmids.
- 12. The complex of Rec B, Rec C and Rec D proteins recognizes the ends of a double stranded break, and travels along the DNA until reaching the closest Chi site. Rec D cleaves the backbone of one strand And dissociates from the complex. Rec BC continue to unwind the DNA beyond the chi site creating a stretch of single Stranded DNA.
- 14. Site-specific recombination Site-specific recombination, is a type of genetic recombination in which DNA strand exchange takes place between segments possessing only a limited degree of sequence homology. Site-specific recombinases (SSRs) perform rearrangements of DNA segments by recognizing and binding to short DNA sequences (sites).
- 16. Replicative recombination It is a type of recombination which generates a new copy of a segment of DNA. Many transposable elements use a process of replicative recombination to generate a new copy of the transposable element at a new location. It is seen for some transposable elements, shown as red rectangles, again using a specific enzyme, in this case encoded by the transposable element. Step 1 - Replicative Integration Step 2 - Resolution
- 18. Benefits of recombination Greater variety in offspring: Generates new combinations of alleles Negative selection can remove deleterious alleles from a population without removing the entire chromosome carrying that allele Essential to the physical process of meiosis, and hence sexual reproduction Yeast and Drosophila mutants that block pairing are also defective in recombination, and vice versa!!!!
- 19. Application Homologous recombination employs zinc finger nucleases to boost genomic integration and shows their usefulness in efficient genome modification. Site-specific recombination systems are potent genome modifiers. DNA transposition-based strategies provide means to generate single-copy insertions. DNA Cloning Using In Vitro Site-Specific Recombination Used to map genes on chromosomes - recombination frequency proportional to distance between genes. Making transgenic cells and organisms.
- 20. References Molecular Biology of the Cell, 4th edition Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, andPeter Walter. New York: Garland Science; 2002.ISBN-10: 0-8153-3218-1ISBN- 10: 0-8153-4072-9 Lodish, H. (2008). Molecular Cell Biology. W. H. Freeman The Mechanism of Conservative Site-Specific Recombination Annual Review of Genetics Vol. 22: 77-105 (Volume publication date December 1988) DOI: 10.1146/annurev.ge.22.120188.000453 N L Craig
Editor's Notes
- Meiosis is a form of cell division that produces gametes, or egg and sperm cells. During the first phase of meiosis, the homologous pairs of maternal and paternal chromosomes align. Genes that are located farther apart on the same chromosome have a greater likelihood of undergoing recombination, which means they have a greater recombination frequency.
- Both the RecD and RecB subunits are helicases, i.e., energy-dependent molecular motors that unwind DNA (or RNA in the case of other proteins). The RecB subunit in addition has a nuclease function.[5] Finally, RecBCD enzyme (perhaps the RecC subunit) recognizes a specific sequence in DNA, 5'-GCTGGTGG-3', known as Chi(sometimes designated with the Greek letter χ). RecBCD is unusual amongst helicases because it has two helicases that travel with different rates[6] and because it can recognize and be altered by the Chi DNA sequence.[7][8] RecBCD avidly binds an end of linear double-stranded (ds) DNA. The RecD helicase travels on the strand with a 5' end at which the enzyme initiates unwinding, and RecB on the strand with a 3' end. RecB is slower than RecD, so that a single-stranded (ss) DNA loop accumulates ahead of RecB (Figure 2). This produces DNA structures with two ss tails (a shorter 3’ ended tail and a longer 5’ ended tail) and one ss loop (on the 3' ended strand) observed by electron microscopy.[9] The ss tails can anneal to produce a second ss loop complementary to the first one; such twin-loop structures were initially referred to as “rabbit ears.”