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18 Cards in this Set
- Front
- Back
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Which topoisomerases relax supercoils and which induce them? |
Types I and II respectively |
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What is the mechanism of type I topoisomerases? |
-Topoisomerase cleaves one strand of DNA by means of a tyrosine residue attacking the phosphoryl group. -The strand rotates around the other strand Cleaved strand religated -Works by altering the linking (Lk) number |
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How many molecules of ATP are needed for action of topoisomerase I? |
Zero - the process is energetically favourable |
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Describe the structure of topoisomerase II |
Dimeric, large cavity at the bottom, DNA segment binding sites and ATP binding sites |
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Describe the mechanism of topoisomerase II |
-Topoisomerase binds G (gate) segment of DNA, each strand being adjacent to a tyrosine residue -Complex loosely binds T (transported) segment of DNA -ATP binds to ATP binding sites and domains with those sites come together, trapping the T segment -This conformational change causes the cleavage of the G strand by the tyrosine residues -T segment passes through G segment into the large cavity -Ligation of the G segment leads to the release of the T segment through gate at the bottom of the enzyme -Hydrolysis of ATP to ADP and orthophosphate separates the ATP binding domains - Lk number reduced by two |
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What is DNA gyrase? |
Bacterial type II topoisomerase |
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Name three antibiotics that act upon DNA gyrase |
Novobiocin, Nalidixic acid, Ciprofloxacin |
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How does the anti-cancer drug camptothecin work? |
Stabilises the complex of enzyme and DNA to prevent ligation |
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What controls the initiation of DNA replication? |
Cell cycle dependant cyclins and cyclin dependant kinases |
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What is the role of Proliferating Cell Nuclear Antigen (PCNA) in DNA replication? |
Clamps the DNA polymerase to the DNA |
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What is the role of Replication Protein A (RPA) in DNA synthesis? |
Maintaining the DNA in the right conformation for DNA synthesis |
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What is the problem caused by the linear DNA strands in eukaryotes? |
Ends would shorten due to binding of primers |
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How do cells prevent DNA shortening with each replication? |
Telomeres |
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What are telomeres? |
TTAGGG repeats in humans maintained by telomerases |
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Where are telomerases active? |
Germ and stem cells - not adult body cells |
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What role do telomerases have in tumour growth? |
Cancer cells reactivate telomerases allowing uncontrolled replication |
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Where does proofreading occur? |
Polymerase |
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Name four other repairing mechanisms |
Mismatch excision repair Nucleotide excision repair Homologous recombination Non-homologous end joining
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