L7: DNA Replication Flashcards
Learning Objectives: • Outline the mechanism of DNA replication in prokaryotes. • Outline the importance and processes of proofreading and error correction by DNA polymerases. Outline key differences between the mechanisms of DNA replication in prokaryotes and eukaryotes.
Why is DNA replication important?
New cells are created by mitosis, which required DNA replication
What happens in DNA replication?
Identical copy of the parent DNA is used as a template for a new strand
Define semi-conservative replication
2 copies of the original DNA is produced, consisting of only half of the material
When is the replication fork created?
DNA helicase unwinds the double helix structure
Initiation of DNA replication in prokaryotes
1) Initiation proteins bind to replication origin
2) Separate strands of DNA
3) Two ‘replication forks’ are formed
Which directions do replication forks move?
In opposite directions
Which bonds are nucleotides joined by?
Phosphodiester Bonds
Structure of deoxynucleoside triphosphates (dNTPs)
3 phosphate groups attached to the 5’ carbon of deoxyribose
Role of DNA polymerase III
Synthesis the bulk of DNA in E.coli
What is a holoenzyme?
Composed of multiple subunits with different functions
What does DNA pol III require for DNA synthesis?
RNA primer
How is RNA primer created ?
Using enzyme PRIMASE
Which direction does DNA pol III synthesis new DNA in?
5’ to 3’ direction
What is the leading strand?
Replicates continuously
What is the lagging strand?
Replicates to form short fragments
Polarity problem in DNA replication
DNA double helix have opposite polarity but all DNA polymerase replicate in the same direction
What are Okazaki fragments joined by?
DNA ligase
Role of ligase in DNA replication
Joins Okazaki fragments into continuous DNA strands
Function of enzyme topoisomerase
Preventing DNA from getting tangled
- Relieving pressure supercoiling ahead of replication fork
Role of Type I topoisomerases
Cut 1 DNA strand
Role of Type II topoisomerases
Cut both DNA strands, pass coil through gaps (ATP) & reseal the strands
Role of helicase in DNA replication
Separates DNA strands
Role of single-stranded binding protein in DNA replication
Prevents DNA strands from re-annealing
Role of primase in DNA replication
Synthesise RNA primers
Role of DNA polymerase III
Synthesise bulk of DNA
Role of sliding clamp in DNA replication
Keeps DNA pol III firmly attached to DNA
What enzymes act as proofreaders during DNA polymerisation in DNA replication?
Exonucleases
How does the exonuclease activity proofread?
Remove last added nucleotide via exonuclease activity, then insert the correct nucleotide
Which DNA polymerase has exonuclease activity?
DNA polymerase I and III
What is a mismatch repair?
Errors missed by proofreading activity, corrected by repair machinery
How does the mismatch repair work?
1) Identify incorrect base pairs
2) Remove DNA in new strand
3) Synthesise replacement DNA
What is DNA replication in eukaryotic cells?
Part of the ‘cell cycle’
Where is DNA replication in eukaryotic cells
Nucleus
Where is DNA replication in prokaryotic cells?
Cytoplasm
Role of aminoacyl -tRNA synthetase in protein synthesis
Joining the amino acids to tRNA
Role of peptidyl transferase in protein synthesis
Formation of the peptide bond