DNA Replication and DNA Repair

Question 1

Initiation

Answer 1

DNA “opened” at replication origins creating replication bubble - by group of enzymes

Question 1

Initiation: Enzymes

Answer 1

• Helicases unwind helix ahead of replication fork and break H-bonds
• Gyrase relieves tension and prevents rewinding
• DNA Polymerase III inserts nucleotides
• Single Stranded Binding Proteins (SSBs) stabilize single stranded DNA and prevent rejoining

Question 2

Elongation

Answer 2

RNA primer made by primase acts as starting point of new nucleotides - DNA polymerase III adds nucleotides to 3’ end

Question 3

Elongation: Leading and Lagging Strand

Answer 3

Leading Strand (from 5’–>3’ into fork): replication is continuous (only one primer needed)
Lagging Strand (from 5’–>3’ away from fork): needs okazaki fragments (one primer for each fragment)

Question 4

Elongation: After Nucleotides Inserted

Answer 4

DNA Polymerase I removes primers and replaces them with nucleotides
DNA Ligase joins gaps left between fragments with phosphodiester bonds

Question 5

Termination

Answer 5

Daughter molecules rewind
Gap left at 5’ end (since nucleotides can only be added to 3’ end) filled by telomeres (repetitive DNA that doesn’t code for anything - only in eukaryotes) by telomerase –> loss of telomeres is linked to aging

Question 6

DNA Repair: Proofreading

Answer 6

Occurs during replication: polymerases I and III look for improperly paired nucleotides, then calls exonuclease which backtracks past mistake, excises section, and replaces with right nucleotides

Question 7

DNA Repair: Mismatch Repair

Answer 7

Occurs after replication: works in same way as proofreading except with group of enzymes