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Lecture 8

Lecture 8 IS - Origins of Variation word.doc

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Department
Biology
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Biology 1001A
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Dr.Mike

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Lecture 8 IS- Origins of Variation Mechanisms that Promote Inheritance of Sameness - Mechanism of proofreading - Mechanism of mßismatch repair Mechanisms that Promote Inheritance of Difference - mutations in DNA may change protein which may change function, which is important for the evolutionary process - Transposition 1. Mechanism of proofreading and likely result of proofreading defects - DNA polymerase rarely make mistakes and their mistakes are usually base- pair mismatches and these errors are usually corrected - proofreading mechanism depends on the ability of DNA polymerases to back up and remove mispaired nucleotides from a DNA strand - the correct pairs allow the hydrogen bonds that stablize the DNA to form - if nucleotide is mismatch, DNA polymerase cannot continue to add nucleotides - If the nucleotide is mismatched, DNA polymerase reverses using built-in de- oxyribonuclease (3’-5’ exonuclease) to remove the incorrect nucleotide - then it keeps going - If proofreading defects there is still mismatch repair 2. Mechanism of mismatch repair - Second round of correction after proofreading - correct base pairs fit together and their dimensions separate the sugar- phosphate backbone chains at a constant distance - mismatched pairs will be too big or too small and so cannot form the hydro- gen bonds and so distort the structure of the DNA helix - provides a recognition site for the enzymes catalyzing mismatched pairs - repair enzymes move along the double helix for distortions and if they en- counter one, they remove a portion of the new chain - they fill in the gap using DNA polymerase - DNA ligase then comes and seals the nucleotide chain into a continuous DNA molecule - this mechanism is also used for correction of alterations in DNA caused by chemicals, and radiation - people with Xeoderma pigmentosum have faulty repair mechanisms and so can develop skin cancer easily - if errors remain in DNA, it is a primary source of mutation - will get carried on in replicated copies - may alter the property of the protein encoded - may alter how organism functions 3. Differences among insertion sequences, transposons and retrotransposons - Transposition: - occur at a low frequency Lecture 8 IS- Origins of Variation - does a cut and past process where the transposable elements leaves it original location and transposes to a new location - or a copy of a transposable element transposes to a new location leaving the original transposable element behind - for most, transposition starts with contact between the transposable element and the target site - makes genetic changes eg, making mutations by transposing into genes and knocking out their functions - Insertion sequences ( a type of transposable elements in bacteria) - simplest - relatively small and contain only genes for their transposition, the gene for transposase (enzyme that catalyzes some of the recombina- tion reactions for inserting or removing the TE from the DNA) - has a short inverted repeat sequence (same DNA sequence running in opposite directions) at each ends of an IS - allows transposase to identify ends of the TE when it catalyzes transposition - Transposon (other type of transposable element in bacteria) - has central region that may include genes that code for antibiotic re
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