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Biology IB HL - 7.1 DNA Structure Part 2

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This deck covers key concepts related to DNA structure and the mechanisms of DNA replication, including the roles of various enzymes and proteins involved in the process.

How many bonds are formed between A and T, G and C?

Adenine and thymine paired via two hydrogen bonds, whereas guanine and cytosine paired via three hydrogen bonds
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Key Terms

Term
Definition
How many bonds are formed between A and T, G and C?
Adenine and thymine paired via two hydrogen bonds, whereas guanine and cytosine paired via three hydrogen bonds
What does the DNA structure suggest about the 2 mechanisms for DNA replication?
Replication occurs via complementary base pairing (adenine pairs with thymine, guanine pairs with cytosine) Replication is bi-directional (proceeds in...
What type of process is DNA replication?
DNA replication is a semi-conservative process that is carried out by a complex system of enzymes
What is helicase's role in DNA replication?
Helicase unwinds and separates the double-stranded DNA by breaking the hydrogen bonds between base pairs
Where does helicase unwind DNA?
This occurs at specific regions (origins of replication), creating a replication fork of two strands running in antiparallel directions
What is the role of DNA gyrase?
DNA gyrase reduces the torsional strain created by the unwinding of DNA by helicase

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TermDefinition
How many bonds are formed between A and T, G and C?
Adenine and thymine paired via two hydrogen bonds, whereas guanine and cytosine paired via three hydrogen bonds
What does the DNA structure suggest about the 2 mechanisms for DNA replication?
Replication occurs via complementary base pairing (adenine pairs with thymine, guanine pairs with cytosine) Replication is bi-directional (proceeds in opposite directions on the two strands) due to the antiparallel nature of the strands
What type of process is DNA replication?
DNA replication is a semi-conservative process that is carried out by a complex system of enzymes
What is helicase's role in DNA replication?
Helicase unwinds and separates the double-stranded DNA by breaking the hydrogen bonds between base pairs
Where does helicase unwind DNA?
This occurs at specific regions (origins of replication), creating a replication fork of two strands running in antiparallel directions
What is the role of DNA gyrase?
DNA gyrase reduces the torsional strain created by the unwinding of DNA by helicase
How does DNA gyrase reduce the torsional strain?
It does this by relaxing positive supercoils (via negative supercoiling) that would otherwise form during the unwinding of DNA
What 7 proteins are involved in DNA replication?
helicase; DNA gyrase; SSB Proteins (single-stranded binding proteins); DNA primase; DNA polymerase III; DNA polymerase I; DNA Ligase
What is the role of SSB's in DNA replication?
SSB proteins bind to the DNA strands after they have been separated and prevent the strands from re-annealing
Apart from separating the DNA strands, what is the role of SSB's?
These proteins also help to prevent the single stranded DNA from being digested by nucleases
What happens to SSBs once DNA replication is finished?
SSB proteins will be dislodged from the strand when a new complementary strand is synthesised by DNA polymerase III
What is the role of DNA primase?
DNA primase generates a short RNA primer (~10–15 nucleotides) on each of the template strands
What is the role of RNA primer?
The RNA primer provides an initiation point for DNA polymerase III, which can extend a nucleotide chain but not start one
How is the new DNA strand created?
Free nucleotides align opposite their complementary base partners (A = T ; G = C)
What is the role of DNA Polymerase III?
DNA pol III attaches to the 3’-end of the primer and covalently joins the free nucleotides together in a 5’ → 3’ direction
In what ways does DNA Polymerase III move?
As DNA strands are antiparallel, DNA pol III moves in opposite directions on the two strands
How does DNA P III work on the leading strand 3' to 5'?
On the leading strand, DNA pol III is moving towards the replication fork and can synthesise continuously
How does DNA P III work on the lagging strand 5' to 3'?
On the lagging strand, DNA pol III is moving away from the replication fork and synthesises in pieces (Okazaki fragments)
What is the role of DNA polymerase I?
DNA pol I removes the RNA primers from the lagging strand and replaces them with DNA nucleotides
Which strand has more RNA primers?
As the lagging strand is synthesised in a series of short fragments, it has multiple RNA primers along its length
What is the role of DNA ligase?
DNA ligase joins the Okazaki fragments together to form a continuous strand
How does DNA ligase join the Okazaki fragments?
It does this by covalently joining the sugar-phosphate backbones together with a phosphodiester bond
Can DNA Polymerase simply initiate replication?
NO | DNA polymerase cannot initiate replication, it can only add new nucleotides to an existing strand
What must first happen for DNA replication to occur?
For DNA replication to occur, an RNA primer must first be synthesised to provide an attachment point for DNA polymerase
What does DNA polymerase do to initiate replication?
DNA polymerase adds nucleotides to the 3’ end of a primer, extending the new chain in a 5’ → 3’ direction