BIOC0005 - DNA REPLICATION EXAM QUESTIONS AND
ANSWERS WITH COMPLETE SOLUTIONS
DNA replication is...
semiconservative
How was the semiconservative nature of DNA replication demonstrated?
Bacteria grown in 15N to make all DNA heavy, then transferred to 14N medium resulted
in 15N-14N hybrid DNA, showing that the two strands of the double helix separate from
each other during replication.
Where does replication begin in prokaryotes? How does this compare to
eukaryotes?
Origin of replication: OriC in prokaryotes.
Only one origin of replication; DNA replicated from one origin is called a replicon.
Therefore, only one replicon in prokaryotes. Ter regions for termination of replication.
Multiple origins of replication in eukaryotes; multiple replicons on the same chromosome
ultimately merge during replication. Not all origins of replication activate.
What is the replisome?
The machinery that takes place in DNA replication.
How many replication forks are there from a single origin?
2
Explain how DNA replication begins at oriC in E. coli.
,oriC has 3 13mer regions and 5 9mer regions, spanning 245 bp. The tandem array of
13-mer regions are AT rich.
DnaA is a hexamer; it binds to the 5 9mer regions. When bound to ATP, DnaA interacts
with DNA in the 13mer regions which results in strand separation here. SSB prevent
reannealing.
DnaA then recruits DnaB and DnaC complex (helicase and helicase loader).
Helicase is inactive when bound by the helicase loader and only activated when loaded
onto DNA by helicase loader.
DNA polymerase is then recruited and replication can proceed.
What is a consensus sequence?
The most commonly found sequence.
Describe how DNA polymerase begins replication of the DNA.
Primase binds to strand; helicase loader dissociates and helicase is activated. An RNA
primer is synthesised.
3 DNA Poly III holoenzyme involved during replication: one on leading and 2 on lagging
strand. These bind to the RNA primer and move along the strand via the sliding clamp,
synthesising new DNA.
What is the most processive DNA polymerase?
Most efficient is DNA polymerase III.
How is the lagging strand replicated?
, DNA primase (made of RNA) associates with DNA helicase and synthesises a new
primer on the lagging strand.
Cannot replicate continuously as strand goes 3' to 5' but can only replicate 5' to 3'. DNA
pol III replicates DNA from primer until it reaches the end and unbinds from the strand.
More ssDNA is then exposed; DNA pol III binds to new primer and synthesises DNA.
Okazaki fragments are synthesised.
Explain how termination occurs in circular chromosomes.
Replication results in catenanes, which are 2 circular linked pieces of DNA.
Topoisomerase II nicks one DNA double strand and passes the other through the break,
separating the two DNA circles. This is known as decatenation.
What is an issue of replication termination of linear chromosomes?
End replication problem only applies to the lagging strand template.
When Okazaki fragments are repaired, RNA primers are first removed then DNA ligase
synthesises the rest of the strand. However, the end of the strand does not have a 3'
OH therefore DNA is incompletely replicated. Over time, this would lead to a shorter
chromosome.
How can bacteria solve the end replication problem?
Protein priming.
A protein is used (usually has tyrosine) instead of an RNA primer, for the last Okazaki
fragment - this provides the OH group needed for DNA synthesis.
How do eukaryotes solve the end replication problem?
The use of telomeres.
A telomere is a TG rich repeated sequence of DNA found at the 3' end of eukaryotes.
ANSWERS WITH COMPLETE SOLUTIONS
DNA replication is...
semiconservative
How was the semiconservative nature of DNA replication demonstrated?
Bacteria grown in 15N to make all DNA heavy, then transferred to 14N medium resulted
in 15N-14N hybrid DNA, showing that the two strands of the double helix separate from
each other during replication.
Where does replication begin in prokaryotes? How does this compare to
eukaryotes?
Origin of replication: OriC in prokaryotes.
Only one origin of replication; DNA replicated from one origin is called a replicon.
Therefore, only one replicon in prokaryotes. Ter regions for termination of replication.
Multiple origins of replication in eukaryotes; multiple replicons on the same chromosome
ultimately merge during replication. Not all origins of replication activate.
What is the replisome?
The machinery that takes place in DNA replication.
How many replication forks are there from a single origin?
2
Explain how DNA replication begins at oriC in E. coli.
,oriC has 3 13mer regions and 5 9mer regions, spanning 245 bp. The tandem array of
13-mer regions are AT rich.
DnaA is a hexamer; it binds to the 5 9mer regions. When bound to ATP, DnaA interacts
with DNA in the 13mer regions which results in strand separation here. SSB prevent
reannealing.
DnaA then recruits DnaB and DnaC complex (helicase and helicase loader).
Helicase is inactive when bound by the helicase loader and only activated when loaded
onto DNA by helicase loader.
DNA polymerase is then recruited and replication can proceed.
What is a consensus sequence?
The most commonly found sequence.
Describe how DNA polymerase begins replication of the DNA.
Primase binds to strand; helicase loader dissociates and helicase is activated. An RNA
primer is synthesised.
3 DNA Poly III holoenzyme involved during replication: one on leading and 2 on lagging
strand. These bind to the RNA primer and move along the strand via the sliding clamp,
synthesising new DNA.
What is the most processive DNA polymerase?
Most efficient is DNA polymerase III.
How is the lagging strand replicated?
, DNA primase (made of RNA) associates with DNA helicase and synthesises a new
primer on the lagging strand.
Cannot replicate continuously as strand goes 3' to 5' but can only replicate 5' to 3'. DNA
pol III replicates DNA from primer until it reaches the end and unbinds from the strand.
More ssDNA is then exposed; DNA pol III binds to new primer and synthesises DNA.
Okazaki fragments are synthesised.
Explain how termination occurs in circular chromosomes.
Replication results in catenanes, which are 2 circular linked pieces of DNA.
Topoisomerase II nicks one DNA double strand and passes the other through the break,
separating the two DNA circles. This is known as decatenation.
What is an issue of replication termination of linear chromosomes?
End replication problem only applies to the lagging strand template.
When Okazaki fragments are repaired, RNA primers are first removed then DNA ligase
synthesises the rest of the strand. However, the end of the strand does not have a 3'
OH therefore DNA is incompletely replicated. Over time, this would lead to a shorter
chromosome.
How can bacteria solve the end replication problem?
Protein priming.
A protein is used (usually has tyrosine) instead of an RNA primer, for the last Okazaki
fragment - this provides the OH group needed for DNA synthesis.
How do eukaryotes solve the end replication problem?
The use of telomeres.
A telomere is a TG rich repeated sequence of DNA found at the 3' end of eukaryotes.
