BIOL 112 Lecture Notes - Lecture 23: X-Ray Crystallography, Primase, Dna Replication

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21 Jul 2016
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o!Chargaff's analysis also showed that the ratio of A:T is 1:1, same for C:G (so
those are always paired)
!X-ray crystallography (diffraction etc, cool stuff)
o!Showed that DNA consists of two strands, double helix, antiparallel (so each
end has a 5' and a 3' end)
o!Phosphates probably on the outside
o!Watson and Crick put it all together for a good model of DNA
o!Minor groove: when backbones are close together; major groove: when
they're far apart
o!DNA strands are the reverse complement to each other (opposite directions,
and A-T, C-G)
o!The reason A-T and C-G are paired: natural affinity of bases
!!A-T, form hydrogen bonds when they lie together in a plane with the
backbone on the outside
!!C-G, form three hydrogen bonds (?) so these hydrogen bonds
determines base pairing
!!Note: A, G are purines (large); T-C, pyrimidines (small bases), so no
bulges; distance between two strands always the same
o!RNA, another nucleotide, can also form complementary strands, with U
instead of T
!Key to DNA: sequence of nucleotides not constrained by the structure; can
accommodate any sequence
o!Each strand has the same information as the other strand - so in a sense it's
already replicated
o!But how does it actually replicate (to make children etc)
4.2DNA replication
!Original possibilities considered:
o!Semi-conservative, two strands separate, new strand for each
o!Conservative, two strands separate, make copies, those copied strands join
!Experiment to determine what actually occurs (Meselson and Stahl):
o!To distinguish between new and old DNA - N14, light DNA; N15, heavy DNA
o!Use a centrifuge to separate them
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