DNA replication and Repair Flashcards
what are the features of DNA that help the process of replication?
- H bonding B/W base pair allows for accurate replication
- 2 antiparallel strands are templates for each other
different models of DNA replication
semi- conservative
conservative
dispersive
DNA helical structure is stable and H bonds must be brocken for strands to unwind
torsional stress in the long, winded molecule needs to be overcome
What does semi conservative mean?
newly synthesised DNA Molecules contain one of the old strand and one new strand
one strand is used as template for synthesis of new strand
Dispersive replication
DNA is produced in parts from new and old strand as mixture
conservative replication
whole new DNA mol produced
which experiment proved semi conservative model of replication
Meselson and stahl experiment proved watson and cricks hypothesis
describe meselson and Stahls experiment
Bacteria were grown in source of N15(heavier isotope than N14)
N15 was incorporated and taken up by bases of DNA
Cells were collected after each division and DNA was extracted.DNA Centrifuged and density was measured
DNA containing N15 was heavier than N14 if newly synthesised DNA was semiconservative it would have intermediate density as one strand is parental and one new
when they centrifuged they found that DNA sat in middle of test tube
phases of DNA replication
- initiation (different in bacteria and eukaryote)
- elongation
- termination
explain initiation in bacteria
- in bacteria DNA replication starts on the circular genome on DNA at a point called origin. proteins called initiation complex
- proteins recognise DNA sequence at the origin and bind to it starting to unwind and open the helix
elongation- - this forms replication bubble that allows access to proteins that synthesise new strands. they move away from origin as they make new DNA replication fork is produced
- replication happens in both directions- bidirectional
replication in eukaryotes
eukaryotes have multiple replication origins
replication origins will start to appear as bubbles which come together
explain how new nucleotides are incorporated and DNA is elongated
for DNA mol to increase in length and be replicated a new base must be incorporated at end of DNA molecule
* the base is nucleotide triphosphate which is attacked by OH group of ribose forming phosphodiester bond with the release of 2 phosphate groups called pyrophosphate
Role of DNA polymerase
- enzyme joins DNA strands together
- it can only add nucleotides at 3’ end of deoxyribose therefore DNA is made from 5end to 3’ direction
- enzyme responsible for elongation of DNA
- There are several types of DNA polymerase
- DNA polymerase must have a template and can only extend on 3’ end
- they require primer- small pieces of DNA
Role of DNA Primase
- Primase is an enzyme that synthesises short RNA sequences called primase. these primase act as starting point for DNA synthesis
- primase functions by synthesising short RNA sequences that are complementary to a single stranded piece of DNA, which serves as its template
what enzyme produces RNA Primers?
what are these used for
RNA primer is produced by DNA Primase
* DNA polymerase uses this to continue this primer and create a new DNA
role of DNA ligase
ligase enzyme joins Okazaki fragments together making one strand
enzymes involved in DNA replication
- DNA plymerase
- DNA helicase
- dna ligase
- topioisomerase
- single stranded DNA binding proteins
Role of DNA Helicase
Converts double stranded DNA into single stranded so it can be accessed by polymerase and primase
opens DNA helix and unwinds it
single stranded DNA binding proteins
binds to single stranded DNA when double helix is opened up
* protect the single stranded DNA generated during DNA replication preventing it being attacked by nuclease or becoming tangled
topioisomerase
when DNA is opened up it generates lots of supercoils and torsion stress ahead of it
proteins that deal with supercoiling of DNA are topoisomerase
what is the processivity/ rate of DNA polymerase like
what can be used to improve the processivity
DNA polymerase is a low processivity enzyme this is ability of DNA polymerase to carryout continuous DNA synthesis without frequently dissociating
processivity factor or clamp protein
what are clamp proteins
multimeric proteins with a ring shape
When DNA polymerase connects with these its processivity is increased
what does fidelity of DNA polymerase means
fidelity refers to DNA polymerases ability to accurately replicate a template
what is fidelity of DNA polymerase like
it accurately replicates
error rate= 1 in 10to the power of 10
what factors increase accuracy of DNA Polymerase
- Base pairing- correct base pairing at the active site of polymerase enzyme
- proof reading-polymerase enzyme has 3>5 exonuclease activity meaning it checks the base pairing as it goes
incorrect base pairing activates a second catalytic site which would remove the incorrect base pair
uses of DNA polymerase outside the cell
- used in sanger sequencing to sequence long streches of DNA
- used in PCR to amplify pieces of DNA, Diagnostic, forensic and research
Drugs that inhibit DNA replication
- antibacterial
- antimicrobial (folate antagonist)
- antiviral(HIV)
*Herpes simplex(acycloviv)
What kind of damages can occur to DNA?
- replication errors(incorrect base inserted), deletion, mutations
- oxidative damage by free radicals
- spontaneous alteration in DNA
- alkylatind agents(product of cellular respiration that binds to DNA)
DNA can be damaged by external sources name some damages
- uv light
- pollution
- carcinogens
- radiotherapy(x ray)
- chemotherapy
what kind of damages can occur?
- bases can become oxidised, alkylated, deaminated
- bases can become dimerised
- DNA backbone can break- single/ double stranded break
- strands can become crosslinked
How does cell deal with errors
direct reversal
nucleotide excision repair
base excision repair
mismatch repair
recombinational repair
non homologous end joining
Direct revearsal
for example uv light
