GEN 3022 1st Edition Lecture 30 Outline of Last Lecture I Genetic recombination II Homologous recombination a Mechanism b Sister chromatid exchange c Holliday model d Newer studies III Site specific recombination IV Gene conversion a Mechanism V Transposition a Transposable elements i DNA sequences ii Mutation and evolution iii Debate of evolution iv Proliferation of transposable elements b McClintock studies c Transposition pathways i Simple transposition ii Retrotransposition d Transposase Outline of Current Lecture I Gene conversion a Example I b Gap repair synthesis vs DNA mismatch repair II Transposition a Cut and paste mechanism b Reverse transcriptase III Homologous Recombination a Holliday model b Recombinant and nonrecombinant chromosomes c Heteroduplex DNA These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Current Lecture I II III Gene conversion occurs as a result of the resolution phase of homologous recombination in which base pair mismatches are repaired by one of two mechanisms a Example I A gene exists in two alleles B and b The B and b alleles exhibit single base pair differences at four different non adjacent sites If gene conversion changed the b allele into the B allele which mechanism would you favor to explain the conversion mismatch repair or gap repair synthesis b Gap repair synthesis vs DNA mismatch repair gap repair synthesis eliminates one of the two alleles by deleting a section of one of the chromosomes and filling in the gap with information from the other chromosome converting to the other allele DNA mismatch repair gives a 50 50 chance of gene conversion per base pair that is repaired That means that for 4 base pairs to result in complete gene conversion the odds would be 1 2 4 or 1 16 In this case gap repair synthesis is the more likely mechanism of repair Transposition the process of moving small segments of DNA transposable elements from one location to another a Cut and paste mechanism this mechanism is also called simple transposition in which a transposon is simply excised using transposase and inserted into a new location on the DNA strand The transposase activity is responsible for the formation of direct repeats in the target strand The enzyme first makes two staggered nicks in the target DNA strand The top strand of both nicks is degraded and the transposon is inserted Then the DNA is repaired using complementary bases which results in a direct repeat on either side of the transposon b Reverse transcriptase this enzyme is responsible for the relocation of retrotransposons The DNA of the retrotransposon is first transcribed into RNA then the reverse transcriptase converts it back to DNA This enzyme was discovered by David Baltimore and Howard Temin in 1975 If this enzyme was prevalent in our bodies then RNA would be converted back to DNA which would have detrimental effects on our genome Homologous recombination the process in which two homolgouschromosomes exchange genetic information via crossover that results in recombinant or nonrecombinant chromosomes a Holliday model this model explains the mechanism of homologous recombination The first step is the nicking of the two homologous chromosomes in analogous sites Then the DNA to the right of the nicks invades the opposite chromosome in the step called strand invasion Once the crossover region is established the Holliday junction that was formed from strand invasion migrates down the double stranded chromosomes in the step called branch migration This creates a heteroduplex region Any mismatches in this region are resolved through DNA mismatch repair or gap repair synthesis The two chromosomes are then separated from each other resulting in recombinant or nonrecombinant chromosomes b Recombinant and nonrecombinant chromosomes If the two breaks during the resolution phase occur in the crossed DNA strands nonrecombinant chromosomes result If the two breaks occur in the crossed DNA strands nonrecombinant chromosomes result If one break occurs in the crossed and the other in the uncrossed strands two recombinant chromosomes result c Heteroduplex DNA stretch of double stranded DNA in which one of the strands is from the homologous chromosome This can result in some base pair mismatches but does not have to i Formation of heteroduplex DNA heteroduplex DNA is formed from the branch migration phase of homologous recombination The length of the heteroduplex depends on the extent of the migration ii Recombinant products the chromosomes are referred to as recombinant or nonrecombinant based on the segregation of markers on either side of the region that underwent crossing over If there are base mismatches they can be repaired by DNA mismatch repair or gap repair synthesis iii Determination of recombinance a chromosome is considered recombinant if reparation of the mismatches in the heteroduplex affect the phenotype of the offspring gene conversion If the reparation of the mismatches do not affect the phenotype they are considered nonrecombinant