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Lec 06 Genomic Replication.docx

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Biology 1001A

Lecture 06: Genomic Replication Purine and pyrimidine base-pairing in DNA/RNA - Purines: fused rings of C and N; adenine and guanine - Pyrimidines: single C ring; thymine and cytosine - Chargaff’s rules: o Number of pyrimidines = number of purines o # adenosine = # thymine; # cytosine = # guanine Outcome of the classic Messelson and Stalh experiment 15 - Used nonradioactive “heavy” N isotopes to tag potential DNA (in order to be able to distinguish parent DNA from newly synthesized ones) 15 - Used E. coli bacteria with an entire DNA strand labelled with N - Discovered that new polynucleotide chains assembled on original chains as they unwind - Semiconservative replication! Direction of movement of DNA polymerase on the template strand - 5’ end: exposed phosphate group attached to 5’ C of sugar - 3’ end: exposed hydroxyl group attached to 3’ C of sugar - DNA strands run in antiparallel (opposite directions) - DNA polymerase assembles chains in 5’ > 3’ direction (“reads” template strand from 3’ > 5’) Terms: - Semi-conservative: new polynucleotide chains assemble on original chains as they unwind - Semi-discontinuous: one of the strands in replication is replicated discontinuously because DNA polymerase can only add to the 3’ end of DNA - Leading strand: new DNA strand synthesized in the direction of DNA unwinding - Lagging strand: strand synthesized discontinuously in the opposite direction General action of proteins - Figure 12.15 (Volume 1: page 272) Basic structure of double-stranded DNA - Double helical structure made up of 2 antiparallel backbones - Antiparallel: each strand has a distinct 3’ end and 5’ end o Confers “polarity” on DNA backbones (gives it direction) - A strand is made up of nucleotides linked together with phosphodiester bonds o Nucleotide = 5C sugar (deoxyribose) + phosphate group + 1 of 4 nitrogenous bases (A,G,T,C) Com
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