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Lecture 6

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Biology 1001A
Nicholas Hudson

9/18/2013 6:40:00 PM Lecture 6: Genomic Replication Independent Study Outcomes 1. purine and pyrimidine base-pairing in DNA/RNA 2. outcome of the classic Meselson and Stahl experiment 3. direction of movement of DNA polymerase on the template strand 4. meaning of semi-conservative, semi-discontinuous, leading and lagging strand 5. general action of proteins in Fig. 12.15. Lecture Outcomes In multiple choice questions, identify the basic structure of double-stranded DNA anti-parallel, held together by H bonds, distinct 3’ and 5’ end confers polarity on DNA backbones, 3’ has free hydroxyl (OH), 5’ has free phosphate components necessary for DNA synthesis Helicase: unwinds DNA helix Single-stranded binding proteins: stabilize single-stranded DNA and prevent the two strands at replication fork from reforming double stranded DNA. Topoisomerase: avoids twisting of DNA ahead of replication fork (in circular DNA) by cutting the DNA, turning the DNA on one side of the break in the direction opposite to that of the twisting force, and rejoining the two strands. Primase: assembles RNA primers in the 5’ to 3’ direction to initiate a new DNA strand. DNA polymerase III: main replication enzyme in E.coli; extends the RNA primer by adding DNA nucleotides to it. DNA polymerase I: E.coli enzyme that uses its 5’ to 3’ exonuclease activity to remove the RNA of the previously synthesized Okazaki fragment, and uses its 5’ to 3’ polymerization activity to replace the RNA nucleotides with DNA nucleotides. Sliding clamp: tethers DNA polymerase III to the DNA template, making replication more efficient. DNA ligase: seals nick between adjacent bases after RNA primers replaced with DNA direction of elongation of a given DNA strand 5’ to 3’ structure of a replication bubble - Structure resulting from bi-directional DNA replication from a given origin. Two forks, travelling in opposite directions, create a bubble. - Each of the replication forks move away from the ori as DNA replication proceeds, with the events at each fork mirroring those in the other. - Movement of the two forks in opposite directions form each origin extends the replication bubbles until the forks eventually meet along the chromosomes to produce fully replicated DNA molecules. relationship between replicated DNA and metaphase chromosomes In mitosis, ploidy doesn’t change (always diploid, 2n). BUT, C value doubles after S phase until end of anaphase. Stage----------Chromosome#----------DNA# G1------------------16----------------16 (2C) G2------------------16----------------32 (4C) Mitosis:Pro/Meta/Ana16----------------32 (4C) Mitosis:Telo/End-----16----------------16 (2C) why chromosomes shorten at each replication basically, chromosomes shorten at each replication b/c DNA polymerase III only extends 5’ to 3’ and requires an RNA primer. AT the end of a eukaryote’s linear chromosome, RNA primer is removed. Since there is no primer and polymerase can’t extend 3’ to 5’, DNA polymerase can’t replicate the 3’ ends of the chromosomes. So, there is always a short segment of unreplicated DNA at the ends of each chromosome. SO after successive replications, chromosome shsorten. New DNA synthesis on 3’ to 5’ template must be started with an RNA primer. When primer removed, a gap will be left in its place at the 5’ end of the new DNA strand. Everywhere else on
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