DNA replication and repair

• What is the mechanism of DNA replication?

  It involves unwinding and synthesizing new strands
• What are the three models of DNA replication?

  Conservative, semi-conservative, dispersive
• Who proved the semi-conservative model of DNA replication?

  Meselson-Stahl in 1958
• What is the replication fork?

  It is where DNA strands are unwound
• What did Cairns confirm about E. Coli in 1963?

  1. Coli has a circular DNA structure
• How is DNA replication characterized in terms of directionality?

  It is bi-directional
• How do eukaryotes replicate DNA?

  From multiple origins of replication
• Who discovered DNA polymerase I?

  Kornberg in 1959
• What is required for DNA polymerization?

  dNTPs, enzyme, Mg2+, template, primer
• What provides energy during DNA polymerization?

  Release of pyrophosphate
• What are the functions of DNA polymerase I?

  Proofreading and removing RNA primers
• What is the main replicating enzyme in DNA replication?

  DNA polymerase III
• How are leading and lagging strands synthesized?

  Leading is continuous, lagging is discontinuous
• What are Okazaki fragments?

  Short DNA segments on the lagging strand
• What is the role of helicase in DNA replication?

  It unwinds dsDNA from the origin
• How does helicase function in relation to ATP?

  It is ATP dependent
• What structure does helicase form around DNA?

  Hexamer ring
• How does helicase act on the lagging strand?

  Acts 5’ to 3’ and dissociates
• What is the function of single-stranded binding proteins?

  They prevent secondary structure formation
• What is the role of topoisomerase in DNA replication?

  It unwinds DNA and relieves overwinding
• What type of breaks does topoisomerase I create?

  Single strand breaks
• What type of breaks does topoisomerase II create?

  Double strand breaks
• What initiates DNA synthesis?

  Primase joins two NTPs
• How is the RNA primer removed?

  By RNase H and DNA polymerase I
• What is the function of DNA ligase?

  It joins the DNA backbone
• What does the sliding DNA clamp do?

  Prevents release of DNA polymerase
• What is the origin of replication?

  Site where DNA unwinding occurs
• What is a replicator?

  Cis acting site sufficient for replication
• What is the role of the initiator protein DnaA?

  It binds to 9-mer repeats at the origin
• What happens after DnaA binds to the origin?

  Localised unwinding of AT rich region occurs
• What is the role of the helicase loader?

  It is replaced by DNA primase
• What happens to DNA strands after replication?

  They are methylated
• What do Dam genes encode?

  Methylase that acts on GATC sequence
• What is the state of DNA after replication?

  It is hemimethylated
• What is the binding site for the Origin Recognition Complex (ORC)?

  It binds to the origin of replication
• What does the ORC do?

  It recruits helicase binding proteins
• What forms a prereplicative complex in G1 phase?

  ORC binds to the origin
• What happens after entry into S phase?

  Helicase loading proteins dissociate
• What initiates unwinding of DNA?

  Activated helicase
• What is decatenation of chromosomes?

  Breaking and resealing DNA strands