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Lecture

BIOB11H3 Lecture Notes - Guanine, Chromosome, Radiography


Department
Biological Sciences
Course Code
BIOB11H3
Professor
Aarti Ashok

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Lecture 21: DNA Replication and Repair
x Figure A: anti parallel strand that compose a DNA double helix
x Bases pointing towards interior, specific fashion: a with t and c with g
x Thank Watson and Crick in the 1950
x Wrote paper, scientific and concise writing
x Last sentence of paper: proposed that
x ^WvE_-Æ ^µPZ_E
x Parent DNA strand comes apart and sequence of each strand is used to construct
complementary strand (light blue) Æ new strand
x Parent strand is template for creation of two new strands
x One parent strand and one new strand
x Semi-conservative model = proposed by W and C (parent DNA only half is conserved in each
daughter DNA molecule)
x Envisioned but NOT proven by them or anyone else
x 3 models realistically proposed, each have their own level of merit
x Conservative, semi-conservative, dispersive
Conservative
x Parent DNA will come apart and be copied, 1st generation parent strands come together and
daughter strands stay together NOT CONSERVATIVE
x Two parent strands coming back together, two new strands created
x Second round of DNA replication, come apart be copied and will come back together
x Conserves each of the parent strand together
Semi-Conservative
x Parent DNA come apart, copied, paired with new DNA strand
Dispersive replication
x Little pieces/segments of each strand copied
x Hybrid daughter DNA molecule, new pieces joined with parental DNAs
x That process would have continued and have hybrid molecules about a half and half ratio of old
parent DNA with new DNA
x No distinguishing between them until Figure D
Meselson-Stahl
x Bacteria with medium of N15 t ammonium chloride provided to bacteria with labelled...sole
source of N for bacteria
x Nitrogeneous bases would be N15 labelled
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x After several generations of growing, switch them to medium with N14 (lighter, can separate
molecules synthesized in the heavier and lighter isotope)
x Either allow them to go one around of replication or 2 rounds of replication
x Make extract of DNA from bacterial culture, separate them by density using cesium chloride
gradient (like glucose gradient t using different substances)
x Distinguish in tube after centrifugation whether certain molecules were more dense (sink
further to the tube) or less dense (more at the top)
x What happened to DNA molecule during X rounds of replication
x First round
o Conservative: would have parent DNA heavier, migrating pretty low AND new strand
synthesized would be lighter, staying at the top
o Semi-}vÀ]ÀWZµPZu}oµoZí}(ZUZ(}Ç}µ[ZÀv
intermediate density band
o Dispersive: intermediate band...cutting and pasting...end up with intermediate
o hdv[]]vPµ]ZÁvu]-conservative and dispersive (identical in the tube)
x Second round
o Conservative: all of new strand made are in the lighter isotope (N14 labelled), ¾ of your
N band migrating at low density (1/4 migrating at the heavy density)
o Semi conservative: two bands Æ intermediate represents the two hybrid molecule (one
heavy and one light) AND light band represents the middle two Æ two strands of N14
labelled DNA
o Dispersive: approximately the same except things are getting lighter, are shifting
towards N 14 light isotope BUT no difference between the 4 daughter DNAs created
o Now you can distinguish between models
x SEMI CONSERVATIVE MODEL WAS HELD TRUE Æ pattern that was seen
o True in bacteria AND eukaryotes
x Eukaryotic genomes: linear chromosomes = multiple ori (100-1000)*
x Start point of replication is the ori (origin)
x Prokaryotes has circular chromosome Æ single point of original, replication goes in either
direction (bi directional replication fork)
x Eukaryotes have multiple origins, each segments have its own origin t replicon
x Each is its own replicon, each origin has a bubble structure because of forks going out
bidirectional
x Figure G t ][]]}voEreplication occurring
x Type II topoisomerase Æ DNA gyrase in terms of DNA replication, relieves the positive
supercoiling ahead of where DNA is being unwound Æ overwinding
x Type 2 enzyme deals with 2 strands of DNA, clip one of the two strand, pass red NDA through
ZPvvZ]voZP]vZoµUoo}ÁP}(}vEvZ[
above to get through it to relieve supercoiling
x We need DNA polymerase for replication (a family of enzymes)
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