Homologous Recombination Lecture 9 PDF

Homologous recombination CHAPTER 19.6 19.6 Homologous Recombination Homologous recombination involves crossing over between identical or homologous regions of chromosomes It occurs in meiosis I and occasionally during mitosis Involves the alignment of a pair of homologous c...

Homologous recombination CHAPTER 19.6 19.6 Homologous Recombination Homologous recombination involves crossing over between identical or homologous regions of chromosomes It occurs in meiosis I and occasionally during mitosis Involves the alignment of a pair of homologous chromosomes, followed by breakage at analogous locations and exchange of corresponding segments Crossing over that occurs between sister chromatids is called sister chromatid exchange (SCE) ◦ Sister chromatids are genetically identical to each other ◦ SCE does not produce a new combination of alleles Crossing over that occurs between homologous chromosomes may result in genetic recombination, producing new combinations of alleles 2 (Figure 19.22) Two types of homologous recombination in eukaryotes (a) Sister chromatid exchange (b) Recombination between homologous chromosomes during meiosis Access the text alternative for slide images. 3 The Holliday Model 1 The Holliday model, proposed by H. Zickler, can account for the general properties of homologous recombination during meiosis Deduced from genetic crosses in fungi ◦ A particularly convincing piece of evidence came from electron micrographs of recombination structures ◦ The structure has been called a chi (χ) form Zickler used the term gene conversion to describe the phenomenon in which one allele is converted to the allele on the homologous chromosome 4 (Figure 19.23) The Holliday model for homologous recombination 1 5 The Holliday Model 2 1. See Figure 19.23. At the beginning of the process, two homologous chromatids are aligned with each other. According to the model, a break or nick occurs at identical sites in one strand of each of the two homologous chromatids. 2. The strands invade the opposite helices and base-pair with the complementary strands. This event is followed by a covalent linkage to create a Holliday junction. 6 The Holliday Model 3 3. The Holliday junction can migrate in a lateral direction. A DNA strand in one helix is swapped for a DNA strand in the other helix, a process called branch migration. The swapping of the DNA strands during branch migration may produce a heteroduplex, a region in the double-stranded DNA that contains one or more base-pair mismatches. 4. The last event in the recombination process is called resolution because it involves the breakage and rejoining of two DNA strands to create two separate chromosomes 7  (b) Micrograph of a Holiday junction From: H. Potter and D. Dressler, “DNA Recombination: In Vivo and In Vitro Studies,” Cold Spring Harb Symp Quant Biol 1979.43: 969-985, © Cold Spring Harbor Laboratory Press. Image provided by Huntington Potter, Ph.D. Figure 19.24a (1) Access the text alternative for slide images. (Figure 19.23) The Holliday model for homologous recombination 3 (a) The Holliday model for homologous recombination Access the text alternative for slide images. 10 More Recent Models Have Refined the Steps of Recombination More detailed studies of genetic recombination have led to a refinement of the Holliday model In particular, more recent models have modified the initiation phase of recombination ◦ Two nicks in the same location on two strands is unlikely ◦ Rather, it is more likely for a DNA helix to incur a break in both strands of one chromatid ◦ A double-strand break model was proposed by Jack Szostak, Terry Orr-Weaver, Rodney Rothstein and Franklin Stahl (See Figure 19.24) ◦ Requires DNA gap repair synthesis 11 (Figure 19.24) A simplified version of the double-strand break model 1 Access the text alternative for slide images. 12 (Figure 19.24) A simplified version of the double-strand break model 2 Access the text alternative for slide images. 13 Various Proteins Facilitate Homologous Recombination 1 Molecular studies in two different yeast species suggest that double-strand breaks initiate the homologous recombination that occurs in meiosis In other words, double-strand breaks create sites where a crossover will occur In Saccharomyces cerevisiae, formation of DNA double-strand breaks requires at least 10 different proteins ◦ One of them, Spo11, is instrumental in actually breaking the DNA 14 Various Proteins Facilitate Homologous Recombination 2 Homologous recombination is found in all species ◦ The cells of any given species may have more than one molecular mechanism for homologous recombination The enzymology of homologous recombination is best understood in E. coli ◦ Table 19.8 summarizes some of the proteins involved ◦ Note: The term Rec indicates that the proteins function in recombination 15 (Table 19.8) E. coli Proteins that play a role in Homologous Recombination Protein Description RecBCD A complex of three proteins that tracks along the DNA and recognizes double-strand breaks. The complex partially degrades the double- stranded regions to generate single stranded regions that can participate in strand invasion. RecBCD is also involved in loading RecA onto single stranded DNA. In addition, RecBCD can create single-strand breaks that are used to initiate homologous recombination. Single-strand binding protein Coats broken ends of chromosomes and prevents excessive strand degradation. RecA Binds to single-stranded DNA and promotes strand invasion, which enables homologous strands to find each other. It also promotes the displacement of the complementary strand to generate a D-loop. RuvABC This protein complex binds to Holliday junctions. RuvAB promotes branch migration. RuvC is an endonuclease that cuts the crossed or uncrossed strands to resolve Holliday junctions into separate chromosomes. RecG Protein that can also promote branch migration of Holliday junctions. 16 Causes of Gene Conversion Homologous recombination can cause two different alleles to become identical alleles ◦ This process, whereby one of the alleles is converted to the other, has been termed gene conversion ◦ The converted allele is close to the crossover site Gene conversion can occur in one of two ways 1. DNA mismatch repair ◦ Refer to Figure 19.25 2. DNA gap repair synthesis ◦ Refer to Figure 19.26 17 (Figure 19.25) Gene conversion by DNA mismatch repair 1 Access the text alternative for slide images. 18 (Figure 19.25) Gene conversion by DNA mismatch repair 2 Access the text alternative for slide images. 19 Gap Repair Synthesis 1 Gene conversion by gap repair synthesis according to the double-strand break model The top chromosome carries the recessive b allele, and the bottom chromosome carries the dominant B allele Figure 19.26 Access the text alternative for slide images. 20 Gap Repair Synthesis 2 Figure 19.26 Both chromosomes carry the B allele. Access the text alternative for slide images. 21