DNA synthesis Flashcards
What does it mean that ‘DNA replication is semi-conservative’?
How long does a cell take to divide in the mammalian cell cycle?
16 - 24 hours.
What are replication origins?
The replication origin is a particular sequence in a genome at which replication is initiated.
What recognises replication origins? What happens when it is recognised?
- Replication origins are recognised by an initiation complex.
- At the origins the DNA unwinds to form a replication bubble and allow access to the replication machinery.
How long does it take for a cell to divide in a bacterial cell cycle?
20 - 30 minutes.
In what phase of the cells cycle does DNA synthesis take place?
It takes place in the S phase.
How does DNA synthesis take place?
- The initiator protein (DNA helicase) binds to the replication origins separating the two DNA strands by breaking the hydrogen bonds. This process required ATP.
- Since the two strand want to naturally reform the bonds, proteins called single-stranded binding protein (SSB) attaches to the single-strands and stabilise them.
- At each replication origin two replication forks are formed. There forks move in opposite directions away from the replication origin, replicating DNA as they move.
- DNA Primase synthesises a RNA primer (a short RNA sequence) to bind to the daughter strand.
- For the leading strand only a primer is only needed to start replication. It is continuously synthesised.
- For the lagging strand new primers are continuously needed for polymerisation to keep going. It is disconenuously synthesised.
- DNA polymerase can only synthesis a daughter strand in the 5’ to 3’ direction (so the parent strand has to run in the 3’ to 5’ direction).
- So it catalyses the addition of nucleotides to the 3’ end of a growing DNA strand, using one of the parental strand as a template.
- DNA polymerase adds dNTP (basically one nucleotide) by phosphodiester bonds to the 3’ end of each primer to start a new Okazaki fragment and this will continue to elongate until the next primer is reached.
- All the Okazaki fragments are linked together by the enzyme DNA ligase.
- When DNA helicase unwinds the DNA strands it creates positive supercoils.
- DNA Topoisomerase II relieves the stress and tension created form the positive supercoils by adding negative supercoils.
What does DNA helicase do?
It uses ATP to break the hydrogen bonds between the two parent DNA strands.
What does DNA primase do?
It is a type of RNA polymerase that generates RNA primers so that DNA polymerase knows where to start replicating.
What does DNA topoisomerase or DNA Gyrase do?
It gets rid of the coils in the DNA that are caused when DNA helicase is unwinding the strands. This reduces the tension in the strands.
It adds negetive supercoils to the positivly supercoiled DNA hilciase,.
What do the single-stranded binding proteins (SSB) do?
They stabilise the single stranded DNA and prevent it from reannealing together.
What does replicative DNA polymerase do?
It copies the parental strand.
What does repair DNA polymerasae do?
It repairs the fragments and it takes out the RNA so it will be a full
DNA strand.
What does exonuclease do?
It is an enzyme that removes all RNA primers from the original stands and replaces them with appropriate bases.
What does DNA ligase do?
It joins together the Okazaki fragments.
What are the DNA polymerases present in bacterial cells? What are their functions?
What are the DNA polymerases present in eukaryotic cells? What are their functions?
