DNA replication

Question 1

Describe the T-Loop structure of telomeres.

Answer 1

protruding 3’ overlap (G tail)
loops back on itself forming duplex with another part of telomere sequence and displacing part of original duplex
end recognised by SHELTERIN proteins
protects ends from DNA repair mechanisms.
catalysed by TRFII

Question 2

order of events in DNA replication?

Answer 2

dNTP forms base pair with template 
3'OH makes nucleophilic attack in alpha phosphate 
3'5' phosphodiester forms 
Ppi released 
Ppi hydrolysis

Question 3

what is the ORC

Answer 3

origin receptor complex (hexameric)

multi subunit binding complex that binds origins of replication

Question 4

what is the role of DNA ligase in lag strand synthesis?

Answer 4

seals nicks using energy from ATP hydrolysis in eukaryotes & archaea or by cleaving NAD+ in bacteria.

Question 5

what is the first proofreading mechanism?

Answer 5

correct base has higher affinity for polymerase due to base pairing making it more energetically favourable.
Pre- covalent addition enzyme undergoes conformational change –> tightens grip around active site, more readily occurs with correct base.

Question 6

What can cause lesions in DNA?

Answer 6

Covalent reactions of DNA bases with large HC pyrimidine dimers TT, TC, CC
Caused by UV light.