GEN 3022 1st Edition Lecture 29 Outline of Last Lecture I Nature of mutations a Characteristics of mutations b Gene expression II Types of mutations a Missense mutations b Germ line mutation c Conditional mutation d Position effect e Silent mutation f Neutral mutation g Frameshift mutation h Nonsense mutation III Mutation rate a Ames test IV DNA repair a DNA polymerase b Non homologous end joining c Removal of segment of DNA d Base excision repair e DNA mismatch repair Outline of Current 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 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 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 Current Lecture I Genetic recombination involves chromosomes breaking and rejoining to form new combinations Three main types are homologous recombination site specific recombination and transposition II Homologous recombination crossing over occurs in meiosis I and occasionally during mitosis Found in all species but the cells of any given species may have more than one molecular mechanism for homologous recombination a Mechanism a pair of homologous chromosomes align followed by a breakage at analogous locations and exchange of corresponding segments b Sister chromatid exchange crossing over between sister chromatids These chromatids are genetically identical to each other so this exchange does not produce a new combination of alleles c Holliday model involves alignment of homologous chromosomes nicking at identical locations strand invasion and finally branch migration d Newer studies modified initiation phase of recombination in which one DNA helix is nicked or both strands are broken III Site specific recombination occurs when small segments of DNA called transposons move to multiple positions within the host s chromosomal DNA IV Gene conversion when one of the alleles is converted to the other allele caused by genetic recombination a Mechanisms DNA mismatch repair and DNA gap repair synthesis V Transposition involves the integration of small segments of DNA into the chromosome Can occur at many different locations within the genome a Transposable elements small mobile segments of DNA involved in transposition Also known as jumping genes i DNA sequences organized in several different ways Inverted repeats are identical to the original sequence but run in the opposite direction Direct repeats are exact copies of the original sequence Insertion element is the simplest type of transposable element Long terminal repeats are very long sequences few hundred nucleotides in length ii Mutation and evolution over the past few decades researchers have found that transposable elements probably occur in the genomes of all species iii Debate of evolution either transposable elements exist just because they can like parasites or they exist because they offer an advantage to the organism like antibiotic resistance The first theory is called the selfish DNA theory iv Proliferation of transposable elements in mammals long interspersed elements are usually 1 000 to 10 000 bp long and found in a few thousand to several hundred thousand copies making up almost 17 of the human genome Short interspersed elements are less than 500 bp in length and are present in about 1 million copies in the human genome b McClintock studies identified transposable elements in her studies of corn in the 1950s c Transposition pathways many transposable elements have been found in bacteria fungi plant and animal cells i Simple transposition called cut and paste because the transposon is excised and moved to another location This mechanism is found widely in bacteria and eukaryotes and can increase copy number if the transposition occurs after the replication fork has passed through the transposition element resulting in two copies of the transposition element in one genome ii Retrotransposition transposable elements move via an RNA intermediate reverse transcriptase and integrase The types of elements moved are called retroelements retrotransposons or retroposons Very common but only in eukaryotes d Transposase enzyme that catalyzes the removal of a transposable element and its reinsertion at another location by recognizing the inverted repeats at the ends of a transposable element and bringing them close together