DNA replication Flashcards
Describe the Hershey and Chase (1952) experiments proving DNA is the genetic material
Viruses were grown in 1 of 2 isotopic mediums to radioactively label a specific viral component.
Describe what happened in the 2 isotopic mediums (Hershey & Chase)
2 mediums: 1) grown in 35 S radio-labelled proteins no radioactivity enters cell S is present in proteins and not in DNA
2) grown in 12 P
radio-labelled DNA
radioactivity enters cell
P is present in DNA and not in proteins
Once the viruses had infected the bacterium (E.coli), how were they separated?
(Hershey & Chase)
Via centrifugation - larger bacteria formed a solid pellet, while the viruses remained in the supernatant
What did Hershey and Chase find was the genetic material of a bacterium?
DNA was the genetic material because DNA was transferred to the bacteria
List the 3 methods of replication
- Semiconservative
- Conservative
- Dispersive
Describe the semiconservative method of replication
2 identical copies of DNA produced - 1 old and 1 new strand
Describe the conservative method of replication
2 duplexes containing 2 old strands and the other containing 2 new strands
Describe the dispersive method of replication
2 duplexes containing strands of a mixture of old and new
What method of DNA replication did Meselson & Stahl aim to prove?
Semiconservative
Describe the method and findings of Meselson & Stahl’s experiment
- bacteria grown in 15 N medium and 14 N medium
- Samples taken at 0, 20 and 40 mins
- A centrifuge was used to separate DNA labelled with isotopes of different densities
- They found a pattern supporting the semi conservative method
Replication is initiated at an ____ of replication that gives 2 replication ____
origin
forks
Do prokaryotes or eukaryotes have a larger genome?
eukaryote - it has multiple forks
Where is the replication fork and what occurs there?
The branch point in the replication eye at which DNA synthesis occurs
Replication forks are almost always unidirectional/bidirectional
bidirectional
Rerji Okazaki elucidated the ____-________ method of replication
semi-discontinuous
There are 3/4/5 copies of a 9bp sequence that bind ____ proteins
4
DnaA
What is the process called when all binding sites are occupied and recruit more DnaA proteins?
DnaA barrel formation
What happens after DnaA barrel formation?
An A-T rich area of DNA is exposed via torsional stress
DnaB initiates the formation of the pre-______ complex
priming
DnaB/DnaA helicase breaks ___ bonds between base pairs
DnaB
Hydrogen
Which proteins stop the strands base pairing and protect DNA from attack by free radicals + nuclease enzymes?
SSBs - single stranded binding proteins
Elongation in prokaryotes:
DnaG (primase) binds near helicase and synthesises the ___ primer on the ______ strand.
SSBs stabilise the _______ strand
DNA ______ III holoenzyme clamps to the ________ strand and synthesises DNA
RNA leading lagging polymerase leading
What enzyme carries out DNA synthesis?
DNA polymerase - 5’-3’
The lagging strand is generated by the synthesis of ______ fragments, in the same/opposite direction to the movement of the replication fork
Okazaki
opposite
DNA polymerisation is carried out by which enzyme?
DNA polymerase
Why does DNA polymerisation occur in the 3’-5’ direction?
most favourable energetics
What are Okazaki fragments?
Short sequences of DNA nucleotides, synthesized discontinuously (3’-5’) and linked together by the DNA ligase to create the lagging strand during DNA replication.
In Okazaki fragments which strand is replicated continuously in the 5’ - 3’ direction?
leading
Parallel replicated DNA is called a _____
replisome
Why is the helix opened before the head of the fork?
To stop over-winding
What enzymes alleviate the topological problem of DNA replication?
Topoisomerases
Describe the action of Type I topoisomerase
Introduces a break in one strand to pass the other strand through then seals the break
Describe the action of Type II topoisomerase
Breaks both strands and passes a double helix through the gap before resealing the break
The breaks in DNA are _____ attached to enzymes so they don’t lose the ends
covalently
In prokaryotes, the two replicons must meet ___ degrees away from the origin of replication
180
Describe how the regulatory mechanism works in the termination of replication
It ensures the replicon meets at a specific point if one gets there first it will wait for the other to arrive before signalling replication completion
If the replicon meets a transcription _____ then it will wait and not overtake
bubble
Once the 2 daughter cells are generated they are _____, and unlinked by __________
catenated
topoisomerases
Eukaryote replication forks are faster/slower than prokaryotes
slower
Eukaryote replication forks move at ___ bp/s
50
What is required to initiate DNA replication in eukaryotes?
protein licensing factor complex
Why can telomeres not be replicated semi-discontinuously?
There is no DNA to elongate once the RNA primer is removed from the 5’ end of the lagging strand - potential loss of genetic material
What is the non-coding repeat sequences in chromosomes?
TTAGGG - 3’ end overhangs 5’ end
What can repressing telomerase activity in somatic cells lead to?
- gradual loss of DNA
- shortening of chromosomes
- aging
