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Thwin_Grace_Block5_MM10_DNAReplication (Elongation (DNA polymerase III…
Thwin_Grace_Block5_MM10_DNAReplication
DNA Structure/Replication
each double helix is made of two very long DNA molecules
each strand goes in an opposite direction-- 3' to 5' or 5' to 3'
adenine matches with thymine and guanine matches with cytosine
A & T form double bonds
these are easier to break apart
C & G form triple bonds
these are harder to break apart
each chromosome is made up of two coiled DNA strands around many core histones
DNA replication is semiconservative
each sister chromatid has a strand derived from the parent chromsome and one derived from the previous strand
replication is bidirectional
complementary strands will go in 3' direction in opposite directions
thus leading strand are moving in opposite directions on two complementary DNA strands
telomeres
non-coding regions of DNA located at the ends of eukaryotic chromosomes
telomerase (enzyme)
increases the length of telomeres at the ends of DNA
allows more error and mistaken deletion of genetic material without affecting the necessary DNA
Termination
leading strand
5' to 3'
continuous replication, only needs a single DNA pol III
if RNA pol blocks leading strand, lagging strand may become ahead of the leading strand
nature doesn't like this so there are ways to stop it
lagging strand
3' to 5'
leading and lagging strands should terminate simultaneously
if one strand is stalled, the other should be too
Tus proteins stall DNA pol by binding to ter (terminator) sites
in prokaryotes there are 10 different ter sites
a tus-ter complex is formed, blocking DnaB from expanding the replication fork
ter sites are uni-directionally permissive to helicase
Initiation
9mer site
4 repeats of a 9-nucleotide section of DNA
site at which DnaA bonds to
13mer site
three repeats of a 13-nucleotide section of DNA
rich in A/T because the bonds can break more easily
DnaA
initiator protein that binds to the origin of replication (ori/9mer)
binding causes 13mer to oscillate between denatured state and reannealed state
forms an unstable replication bubble
single-stranded DNA binding protein (SSB)
stabilizes the replication bubble by binding to the single strands,
this makes the strands unable to reanneal
causes DnaA to fall off
DnaB (helicase)
unzips DNA at replication forks at ends of replciation bubble
binds to replication forks and catalyzes separation of DNA strands
initiatioon complex
is complete when DnaB binds to the replciation forks
Elongation
replication elongation = synthesis of new daughter strands
Gyrase/topoisomerase
moves ahead of the replication fork to release buildup and causes supercoils in the DNA as
it unwinds
primase
adds the short complementary RNA primer sequences to single-stranded DNA
DNA polymerase II
detects and corrects DNA replication errors
DNA polymerase I
removes RNA primers and replaces them with DNA
DNA polymerase III
responsible for elongating DNA from the RNA primers
unable to unwind the DNA double helix and denature the complementary template strands
cannot initiate daughter strand elongation
grows the new strands only in the 5’ to 3’ direction
ligase
joins together okazaki fragments after DNA pol I has removed primer
okazaki fragments are the fragments of DNA formed by DNA pol III that were made in the 3' direction, but need to be arrranged in the 5' direction
necessary for lagging strand