DNA Replication Flashcards
Briefly describe the structural features of DNA replication
Formation of phosphodiester bonds between 3OH + 5’ Phosphate group
Nucleotides added to 3’ OH end of chain so synthesised 5’-3’
Describe initiation of DNA replication
Initiating protein binds to origin of replication (ori)
Multiple ori along eukaryotic chromosome, each bubbles along axis of chromosome (Prokaryotes have 1 ori, Euk have 10,000 in humans)
What direction does replication occur in?
Both directions from Ori > Bi-directional
Describe the process of DNA replication
DNA helicase/Initiating proteins open double helix, primer added to 3’OH end
When nucleotides added, DNA polymerase I removes RNA, replaces with DNA
Missing bonds filled by DNA ligase joining 5’P-3’OH
What is a primer?
Enzyme - Synthesises RNA primer
what is DNA ligase?
Enzyme - Joins gaps in DNA backbone
Which DNA polymerases are found in E.Coli?
DNA poly III - Responsible for replication
DNA poly I - Responsible for repair/removing primers
Which polymerases are found in eukaryotes?
DNA poly alpha, delta, epsilon - In chromosome replication
How do prokaryotes divide?
By binary fission using chromosome replication and cell growth, forms 2 new cells with identical DNA
Describe end-replication in eukaryotes?
Eukaryotes have telomeres which repeat sequences at end of chromosomes
Describe telomerase
Uses RNA template to complete ends of chromosomes
What is the function of DNA helicase?
Breaks H-Bonds btw CBP>ATP needed to move along DNA and to form 2 template strands
How is a double helix stabilised?
Single strand binding proteins bind single stranded DNA (ssDNA) made by helicases
Straighten/stabilise ssDNA working together with DNA polymerase
What is a polymerisation reaction?
A condensation reaction
Why do extra phosphate groups carry energy?
For forming covalent bonds
What are free nucleotides in the nucleus called?
Deoxynucleoside triosphosphate
What is the function of DNA polymerase I?
To proof read the strands
If error occurs, translocation of enzyme to next position along template is inhibited, so opportunity for correction
What happens to incorrectly paired nucleotides during proofreading?
Nucleotides removed through exonuclease activity
Cell has other DNA repair mechanisms
What are topoisomerases?
Prevent DNA tangling during replication - Reversible nucleases that break phosphodiester bonds
Describe type I topoisomerases
Make nicks in one DNA strand, relieve supercoiling
Describe type II topoisomerases
Make nicks in both strands, relieve supercoiling/untangle linked DNA helices
What happens in the absence of topoisomerase?
DNA can’t rapidly rotate and torsional stress builds up
What is DNA supercoiling caused by?
Torsional stress
