Genetics Lecture 2 Flashcards
Name and describe the three models of DNA replication.
Semiconservative - each dsDNA splits, and each single strand receives a new strand.
Conservative - each dsDNA splits, replicates, splits, then recombines with the original.
Dispersive - DNA is chopped, alternating fragments separate into daughter cells, gaps between are filled in.
What 3 factors are needed for DNA polymerase to begin elongating?
Template
Primer
Energy - comes from the nucleotide triphosphate
Name the sites of replication in prokaryotes.
Only one - oriC
Is replication unidirectional or bidirectional?
Bidirectional.
Describe the sequence that makes up oriC.
Has 13bp sequences repeated 3x and 9b repeated 4x.
How is a replication bubble formed in oriC?
DnaA proteins bind to the 9bp repeats, causing the 13bp repeats to unwind.
DNA helicase binds to the unwound DNA at each end, and continues unwinding.
How does the leading strand differ to the lagging strand?
Leading strand is continuous.
Lagging strand is elongated in fragments, each with a primer and a nick at the end.
Which enzyme adds a primer required for okazaki fragment elongation?
RNA primase.
How long is each okazaki fragment?
100-200bp
Is replication collectively continuous or semidiscontinuous?
Semidiscontinuous.
What ligates okazaki fragments together?
DNA ligase.
Can DNA polymerase 3 elongate with no primer?
No, must have a primer.
Which enzyme replaces the RNA primer with DNA after it is extended?
DNA polymerase 1.
Why cant DNA polymerase 1 seal the nicks in okazaki fragments?
It uses energy from NTPs. No NTPs left when it reaches the nick.
What prevents single strands from recombining when unwound?
Single strand binding proteins binding to the lagging strand.
Which enzyme prevents DNA from tangling during uncoiling?
Gyrase.
What composes the replisome?
One helicase in the middle, a primase above it, and 2 DNA polymerase 3s above and below the helicase.
Describe the rolling stone replication process.
Occurs in phage λ.
Has no replication bubble, instead, only one strand of the double breaks when the genome is circular.
A continuous fragment is formed on the broken strand, with mutiple genomes worth in this fragment.
They are broken by S termination enzyme, which leaves sticky ends so it can form a circular genome.
How many origin sites do humans have?
Thousands.
What happens to the ends of DNA after replication is complete?
An RNA primer is left at the 5’ end. Enzymes cannot extend from 3’ to 5’. These regions are therefore lost after each replication, and will encroach on essential genes, mediating ageing. Telomeres slow this process down.
Describe the structure of a telomere, and how it works.
Is an RNA primer attached to a polymerase, and complimentary to telomeric DNA repeats allowing it to extend them.
What is the error rate in base pairing and error rate in the complete replication process? Why the mismatch?
Error in base pairing – 1/10,000
Errors in completed replication process – 1/1,000,000,000
DNA polymerase 3 has proofreading to reduce it significantly.
