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LIFE 102 (201)

Replicating DNA, antiparallel elongation, DNA replication complex, proofreading/repairing DNA, telomeres, chromosomes, gene expression

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Life Science
LIFE 102
Louis Bjostad

11 AprilSynthesizing a New DNA StrandEnzymes called DNA polymerases catalyze the elongation of new DNA at a replication forkMost DNA polymerases require a primer and a DNA template strandThe rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cellsEach nucleotide that is added to a growing DNA strand is a nucleotide triphosphatedATP supplies adenine to DNA and is similar to the ATP of energy metabolismThe difference is in their sugars dATP has deoxyribose while ATP has riboseAs each monomer of dATP joins the DNA strand it loses two phosphate groups as a molecule of pyrophosphateAntiparallel ElongationThe antiparallel structure of the double helix two strands oriented in opposite directions affects replicationDNA polymerases add nucleotides only to the free 3 end of a growing strand therefore a new DNA strand can elongate only in the 5 to 3 directionAlong one template strand of DNA the DNA polymerase synthesizes a leading strand continuously moving toward the replication forkTo elongate the other new strand called the lagging strand DNA polymerase must work in the direction away from the replication forkThe lagging strand is synthesized as a series of segments called Okazaki fragments which are joined together by DNA ligaseThe DNA Replication ComplexThe proteins that participate in DNA replication form a large complex a DNA replication machineThe DNA replication machine is probably stationary during the replication processRecent studies support a model in which DNA polymerase molecules reel in parental DNA and extrude newly made daughter DNA molecules
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