Lecture 03 Flashcards
What mechanisms are used in DNA repair?
Proofreading and post-replication repair
In what direction does DNA polymerase synthesize DNA?
5’-to-3’
Which strand is synthesized continuously? Which is synthesized in segments?
Leading strand - continuous
Lagging strand - segments
What are the mechanisms behind proofreading of DNA?
Enzyme tightens around active site to test if correct base is in place. If incorrect base added, 3’ to 5’ exonuclease clips off unpaired residue at 3’ primer terminus
DNA primase
Synthesizes RNA primer that primes DNA synthesis
DNA helicase
Unwinds DNA via conformational change
Mismatch repair
Detects and removes errors missed by proofreading by detecting distortion in strand. MutS protein binds to mismatch, MutL scans for nicks and triggers degradation of nicked strand
DNA topoisomerase
Enzyme that breaks phosphodiester bond to change superhelicity - relieves supercoiling of DNA
Type I Topoisomerase
Creates transient single strand break in DNA; DNA on either side of nick rotates freely. Rapid sealing does not require additional energy.
Type II Topoisomerase
Makes a transient double-stranded break in DNA; requires ATP to break double stranded helix, allow second strand to pass through, and reseal break.
Replication origin
A-T rich regions where sequence attracts initiator proteins to pry open DNA
Which type of chromatin is late replicating?
Heterochromatin
What are the requirements for a sequence to be an origin of replication (ORI)?
- Must have binding site for origin recognition complex
- Must have A-T rich stretch for easy unwinding
- Must have binding site for proteins that attract ORC
Telomerase
Replenishes special termination sequences by elongating parent strand 5’ to 3’ using RNA template on the enzyme
T-loops
Structures that protect ends of DNA and distinguishes healthy from broken strands
