Chapter 19 RECOMBINATION AND TRANSPOSITION AT THE MOLECULAR LEVEL. Ref: Genetics (Analysis & Principles), 5 th ed by Robert Brooker, Chapter 19 1

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1 hapter 19 REOMBINATION AND TRANSPOSITION AT THE MOLEULAR LEVEL Ref: Genetics (Analysis & Principles), 5 th ed by Robert Brooker, hapter 19 1

2 INTRODUTION Genetic recombination involves chromosomes breaking and rejoining to form new combinations There are three main types 1. Homologous recombination Occurs between DNA segments that are homologous An essential feature of all species 2. Site-specific recombination : 교과서를읽어볼것! Occurs when non-homologous DNA segments are recombined at specific sites (Phage DNA integration into E.coli chromosome; V(D)J recombination of Igs) 3. Transposition : 교과서를읽어볼것! Occurs when small segments of DNA called transposons move within the host s chromosomal DNA opyright The McGraw-Hill ompanies, Inc. Permission required for reproduction or display

3 17.1 HOMOLOGOUS REOMBINATION As described in hapters 3 and 5, crossing over occurs in meiosis I and occasionally during mitosis There, crossing over involved exchange of DNA between non-sister chromatids of homologous chromosomes rossing over that occurs between sister chromatids is called sister chromatid exchange (SE) Sister chromatids are genetically identical to each other Therefore, SE does not produce a new combination of alleles opyright The McGraw-Hill ompanies, Inc. Permission required for reproduction or display

4 Therefore, SE is not considered a form of recombination Also termed nonparental genotype Figure 19.1 Two types of homologous recombination in eukaryotes

5 The Holliday Model for Homologous The first model of homologous recombination was deduced from the outcome of fungal genetic crosses As was discussed in hapter 6, geneticists have learned a great deal from the analysis of fungal asci An ascus contains the products of a single meiosis A cross of two haploid fungi that differ at a single gene should yield an ascus containing an equal proportion of each genotype Refer back to Figure 6.11 Recombination opyright The McGraw-Hill ompanies, Inc. Permission required for reproduction or display

6 Figure 6.11 Equal proportion of each allele

7 As early as 1934, H. Zickler noticed that some asci contained unequal proportions of the spores E.g., octads with six orange spores and two white spores These unusual asci occurred at too high a rate to be explained by new mutations Zickler used the term gene conversion to describe this phenomenon When gene conversion occurs, one allele is converted to the allele on the homologous chromosome Based on studies on gene conversion, Robin Holliday in 1964 proposed a model for homologous recombination This model is shown in Figure 19.2 Later in the chapter, more recent models will be discussed opyright The McGraw-Hill ompanies, Inc. Permission required for reproduction or display

8 Alignment of homologous chromosomes Nicking Figure 19.2 Holliday model for homologous recombination

9 Strand invasion apable of migrating in a lateral direction Branch migration Figure 19.2

10 The only difference between the two chromosomes Markers exchanged Isomerization Resolution Figure 19.2

11 The only difference between the two chromosomes Isomerization Resolution Figure 17.4 opyright The McGraw-Hill ompanies, Inc. Permission required for reproduction or display

12 The Holliday Model for Homologous Recombination The Holliday model can account for the general properties of recombinant chromosomes in meiosis Molecular research has supported the central tenets( 원칙, 교의 ) of the Holliday model A particularly convincing piece of evidence came from electron micrographs of recombination structures Refer to Figure 19.4b The structure has been called a chi (c) form Its shape is similar to the Greek letter c opyright The McGraw-Hill ompanies, Inc. Permission required for reproduction or display

13 More Recent and Refined Models for Homologous 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 one DNA helix to incur a single nick or a break in both strands Either of these is enough to initiate recombination A single nick is favored in the model proposed by Matthew Meselson and harles Radding A double-stranded break is favored in the model proposed by Jack Szostak, Terry Orr-Weaver, Rodney Rothstein and Franklin Stahl opyright The McGraw-Hill ompanies, Inc. Permission required for reproduction or display

14 Double-stranded break in one chromosome initiates recombination Displacement loop Figure 19.3 A simplified version of the double-stranded break model

15 For simplicity, this illustration does not include the formation of heteroduplex DNA Figure 19.3 A simplified version of the double-stranded break model

16 19-22

17 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 doublestrand breaks requires at least 10 different proteins One of them, Spo11, is instrumental in actually breaking the DNA The role and the interactions among the other proteins is not well understood Nevertheless, the double-strand break created by cellular enzymes can initiate homologous recombination based on the model described in Figure

18 Supplement Holliday model of homologous recombination Ref: Pierce genetics, h 12

19 Holliday model of homologous recombination Supplement

20 Holliday model of homologous recombination Supplement

21 Holliday model of homologous recombination Supplement

22 Supplement

23 auses of Gene onversion As discussed earlier, genetic 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 Gene conversion can occur in one of two ways 1. DNA mismatch repair Refer to Figure DNA gap repair synthesis : we discuss this Refer to Figure 19.5

24 opyright The McGraw-Hill ompanies, Inc. Permission required for reproduction or display G Branch migration G G G G A G G T G Branch migration travels over a region that has a minor sequence difference. G G G T 3 5 Heteroduplexes 3 5 These base differences are responsible for the different expression of the alleles G A G G 5 DNA mismatch repair yields 4 possible combinations. Figure 19.4 Gene conversion by mismatch DNA repair G G G G G G G G Gene conversion G G G G G A G G T No gene conversion G G A T G G G G G No gene conversion G G A T A T G G G G Gene conversion Only the two recombining strands are shown. There are 2 other strands that do not change.

25 Figure 19.5 Gene conversion by gap repair synthesis

26 reation of unusual octads with six orange spores and two white spores via gene conversion Gene onversion