Topic 7.1: DNA Structure and Replication Flashcards
What is the purpose of helicase?
Helicase unwinds and separates the double-stranded DNA by breaking the hydrogen bonds between base pairs
This occurs at specific regions (origins of replication), creating a replication fork of two strands running in antiparallel directions
What is the purpose of DNA gyrase?
DNA gyrase reduces the torsional strain created by the unwinding of DNA by helicase
It does this by relaxing positive supercoils (via negative supercoiling) that would otherwise form during the unwinding of DNA
What is the purpose of Single Stranded Binding (SSB) Proteins?
SSB proteins bind to the DNA strands after they have been separated and prevent the strands from re-annealing
These proteins also help to prevent the single stranded DNA from being digested by nucleases
SSB proteins will be dislodged from the strand when a new complementary strand is synthesised by DNA polymerase III
What is the purpose of DNA Primase?
DNA primase generates a short RNA primer (~10–15 nucleotides) on each of the template strands
The RNA primer provides an initiation point for DNA polymerase III, which can extend a nucleotide chain but not start one
What is the purpose of DNA polymerase III?
Free nucleotides align opposite their complementary base partners (A = T ; G = C)
DNA pol III attaches to the 3’-end of the primer and covalently joins the free nucleotides together in a 5’ → 3’ direction
As DNA strands are antiparallel, DNA pol III moves in opposite directions on the two strands
On the leading strand, DNA pol III is moving towards the replication fork and can synthesise continuously
On the lagging strand, DNA pol III is moving away from the replication fork and synthesises in pieces (Okazaki fragments)
What is the purpose of the DNA polymerase I?
As the lagging strand is synthesised in a series of short fragments, it has multiple RNA primers along its length
DNA pol I removes the RNA primers from the lagging strand and replaces them with DNA nucleotides
What is the purpose of the DNA ligase?
DNA ligase joins the Okazaki fragments together to form a continuous strand
It does this by covalently joining the sugar-phosphate backbones together with a phosphodiester bond
What is the first step of DNA replication?
Helicase unwinds and separates the double-stranded DNA by breaking the hydrogen bonds between base pairs
After Helicase unwinds and separates the double-stranded DNA by breaking the hydrogen bonds between base pairs, what is the next step of DNA replication?
DNA gyrase reduces the torsional strain created by the unwinding of DNA by helicase
After DNA gyrase reduces the torsional strain created by the unwinding of DNA by helicase, what is the next step of DNA replication?
SSB proteins bind to the DNA strands after they have been separated and prevent the strands from re-annealing
After SSB proteins bind to the DNA strands after they have been separated and prevent the strands from re-annealing, what is the next step of DNA replication?
DNA primase generates a short RNA primer (~10–15 nucleotides) on each of the template strands
After DNA primase generates a short RNA primer (~10–15 nucleotides) on each of the template strands, what is the next step of DNA replication?
DNA pol III attaches to the 3’-end of the primer and covalently joins the free nucleotides together in a 5’ → 3’ direction
After DNA pol III attaches to the 3’-end of the primer and covalently joins the free nucleotides together in a 5’ → 3’ direction, what is the next step of DNA replication?
DNA pol I removes the RNA primers from the lagging strand and replaces them with DNA nucleotides
After DNA pol I removes the RNA primers from the lagging strand and replaces them with DNA nucleotides, what is the last step of DNA replication?
DNA ligase joins the Okazaki fragments together to form a continuous strand
