DNA Replication 2 - Mechanics Flashcards
How many DNA polymerases do eukaryotes have?
3 main types for genome replication
- DNA polymerase I - repair of damaged DN + replication
- DNA polymerase II - implicated in repair
- DNA replicated III - enzyme responsible for de novo synthesis of DNA
Role of DNA polymerase?
Synthesize new DNA strand using template sequence.
Which end of nucleotide DNA is new DNA added on?
5’ to 3’ direction in eukaryotic and bacterial polymerases. Added to 3’ end.
Can DNA polymerases initiate synthesis?
No, they require DNA primers.
What is a primer?
A primer is a short single-stranded nucleic acid utilized by all living organisms in the initiation of DNA synthesis.
Role of primers?
Provide 3’ end where extension is viable.
What is the bond between nucleotides called?
Nucleotides are linked together by the formation of a phosphodiester bond which is formed between the 3’ -OH group of one sugar molecule, and the 5’ phosphate group on the adjacent sugar molecule. This results in a loss of a molecule of water, making this a condensation reaction, also called a dehydration synthesis.
What catalyses the reaction between adjacent bases on DNA nucleotides?
DNA polymerases
DNA is supercoiled. What relaxes DNA for seperation of double helix to occur?
Topoisomerase
What is DNA helicase?
They separate double-stranded DNA into single strands allowing each strand to be copied.
What helps DNA helicase to bind to the double helix DNA?
Initiator proteins that are binding to a specific site of DNA. This is usually within origin of replication sequence.
Define what a primosome is?
Conjunction of primase and helicase enzymes.
How are RNA primers synthesized?
- Primase binds to helicase
- Short RNA primer is synthesized on ssDNA
- This allows initiation to occur
- DNA polymerase III can add first nucleotide
What is the leading strand?
Strand of DNA in replication fork that follows the 3’ end. Synthesis occurs first here using DNA polymerase.
What is lagging strand?
Strand of DNA that is unavailable for synthesis.
What is the role of Okazaki fragments?
- Synthesis multiple short fragments of DNA in the normal 5’-3’ direction and joining them together later.
- Added to lagging strand
Describe the process of adding Okazaki fragments.
- RNA primers synthesized on lagging strand
- DNA polymerase III extends primer away from replication fork
- Displaces SSB proteins
- Process repeats as replication fork moves away.
What are SSB proteins?
Proteins on the lagging strand that stabilise the DNA and prevent it from forming an abnormal structure during replication.
Which enzyme joins Okazaki fragments together?
DNA ligase. After removal of SSB proteins by DNA polymerase III, DNA polymerase I removes 5’ RNA to replace with DNA nucleotides.
What is the complex of all enzymes used during DNA replication called?
The replisome complex
What is the role of exonucleases?
Remove RNA primers from both new synthesized strand of DNA. DNA polymerase adds in the rest of the DNA nucleotides and they are joined together with DNA ligase.
