DNA Replication Flashcards
(42 cards)
How many base pairs per turn DNA
10
And 0.34nm btw bases, so 3.4nm per helical turn
Semiconservative
Watson crock model
After one round of dna replication, each dna contains 1 parental and one newly synthesized strand
Modes of dna replication
1) theta replication
2) rolling circle replication
3) linear chromosome replication
Theta replication
Can be either bidirectional or unidirectional
Double stranded dna unwinds at the replication origin, producing single stranded templates for synthesis
- a replication bubble forms, usually with replication fork at each end
- the forks proceed around the circle
- products = two circular DNA mlc
Rolling circle replication
Specialized form of replication that occurs in the f factor of E. coli and some viruses
Only one origin of replication
Unidirectional
Replication is initiated by break in one of the nucleotide strands
- dna synth this begins at the 3 prime end of broken strand
The inner strand is used as a template
Cleavage releases a single-stranded linear DNA and a double stranded circular DNA
The linear guy can circularize and serve as a template for synthesis
Linear chromosome replication
Mode of dna replication in eukaryotes
Occurs in the linear chromosomes of eukaryotic cells
Is bidirectional
Each chromosome contains numerous origins of replication
At each origin, DNA unwinds, producing a replication bubble
- dna synthesis takes place on both strands at each end of the bubble as the replication fork proceeds outward
- eventually the forks from adjacent bubbles run into eachother and the segments of dna fuse
Produces 2 linear identical dna mlcs
What does dna replication require
- dna dependent dna polymerase
- four deoxyribonucleoside triphosphates (dNTPs)
- a single stranded template of DNA to be copied
- an RNA primer
What does RNA primer provide
A 3’ end to initiate dna synthesis by dna polymerase
What direction is dna always synthesized
5’ to 3’
It is always complementary and antiparallel to the parent strand
dNTP
Deoxyribonucleoside triphosphates
(ANTP, gNTP, cNTP and tNTP)
Have the base and sugar (deoxynucleoside) as well as three phosphates
Two phosphates are cleaved off when the chain is growing
What interactions take place to synthesize the new strand
The 3’OH group of last nucleotide on the new strand being made reacts with the 5’ phosphate group of the incoming dNTP
Two phosphates are cleaved off
A phosphodiester bod forms between the two nucleotides
What are Okazaki fragments
They are the short fragments formed when the lagging strand is synthesized
What are longer, Okazaki fragments in prokaryotes or eukaryotes
Prokaryotes
(1-2kbases)
In eaukatyotes, they are shorter (just 0.1-0.2 kbases
Where does dna replication start in all three modes
An origin of replication
oriC
Replication origin in bacteria
215 base pairs in length
Contains 4 DnaA binding sites
AT rich
What kind of proteins bind to the oriC
So obviously this is talking about prokaryotic dna replication
DnA proteins- initiator proteins that bind to oriC and cause a short stretch of DNA to unwind
DnaA
For prokaryotic dna replication
Binds to oriC
Can bind to double stranded dna
Causes stretch of dna to unwind
This allows helicase and other ssbs to attach to single stranded dna
Helicase in prokaryotes
Only binds to single stranded dna
Disrupts hydrogen bonding btw complementary base pairs
Unwinds dna in 5’ to 3’ direction
Travels on the lagging strand only in prokaryotes, ahead of replication machinery
SSBs
Single stranded binding proteins
Unwound single stranded dna is coated with them
They keep dna single stranded and avoid hairpins
DNA gyrase
To Pom erase that makes double stranded breaks in the dna to relieve the torsinal strain that builds up from the activity of the dna helicase
Primase
Aka rna polymerase
Synthesizes a short rna primer (10-12 nucleotides)
That provides the 3’ OH end for DNA synthesis
- binds to the helicase to initiate synthesis
Where is primase required on leading and lagging strands
On leading strand, it’s required only at 5’ end of newly synthesized strand
On lagging strand, a new primer must be generated at the beginning of each okazaki fragment
DNA polymerase projaryotes
Elongates new polynucleotide strand by catalyzing DNA polymerization
Requires a primer
In e. Coli there are 5 different dna polymerases
DNA polymerase III
Main dna polymerase
Has 5’ to 3’ polymerase activity
Has 3’ to 5’ exonuclease activity
This means it adds a dNTP to the 3’ end but can also back up to remove a nucleotide that has been misincorporated
