dna replication 1 Flashcards
what enzyme is required in bacterial cells for DNA synthesis
DNA polymerase III
which direction do bacterial polymerases synthesise
5’ —-> 3’
how do we know if a gene is important
simple knock-outs will be lethal, meaning the replication cannot occur
what is a quick-stop mutant
where the inactivated gene will stop replication immediately
what is a slow-stop mutant
the current round of replication will finish but a new one cannot start
what does plectonemically coiled mean
where the dna strands are wrapped around eachother
what do we use to unwind the dna duplex and what is the process called
helicase
atp hydrolysis
how does helicase work
a conformational change occurs in the helicase and these pull on the DNA strand and separate it
what direction do helicases move in
3’
what can unwinding the duplex cause and what does this lead to
it can cause torsional strain
supercoiling
what will the writhe be when the DNA is in its relaxed form
0
if the linking number is bigger than the relaxed form, what does this mean
there is positive supercoling
if the linking number is smaller than in the relaxed form, what does this mean?
there is negative supercoiling
what does writhe do
it releases tension from supercoiling
if the dna is wound to the right, what does it make the DNA and heat does this mean for the type of winding
negative
underwound
if the dna is wound to the left, what does it make the DNA and heat does this mean for the type of winding
positive
overwound
what will the supercoiling value be for purified cellular dna
-0.06
what will the supercoiling value be for purified cellular DNA and why is this
-0.06
makes it easier to separate when replicating
when will dna in eukaryotes be underwound and what does it form
around histones
a nucleosome
when will positive supercoiling occur and why does this happen
during helicase bases unwinding as it causes overwinding elsewhere a the linking number wont be able to release the stress without breaking bonds
when will positive supercoiling occur and why does this happen
( 2 occasions )
-during helicase bases unwinding as it causes overwinding elsewhere the linking number won’t be able to release the stress without breaking bonds
-long linear DNA as it causes local overwinding
what is the problem with positive supercoiling
it needs to be removed for replication to continue
what enzymes remove supercoils and what do the two types do
topoisomerases
- type 1 cleaves 1 backbone strand
-type 2 cleaved both backbone strands
how does type 1 work and how does it affect the linking number
-the DNA backbone transfers to the enzyme via tyrosine residue and then transferred back when it is unwound
- increases the Lk by 1
what does type 1 do
stops the dna from becoming too unwound
