3. DNA Replication Flashcards
why does our body limit the amount of proteins we make?
limit amount of mutations
DNA replication error rate
1 in a million
Meselson-Stahl Experiment
proved DNA is semiconservative
DNA replication in heavy nitrogen, centrifuge, 1 band
DNA replication light nitrogen, centrifuge, 1 band (15N, 14N)
DNA replication light nitrogen, centrifuge, 2 bands (14N both and 14/15)
DNA replication begins at an _____ and occurs __________
origin; bidirectionally
direction of synthesis
DNA ALWAYS synthesized 5’ to 3’
DNA template is read 3’ to 5’
leading v. lagging strand
synthesis occurs continuously for leading strand
discontinuously for lagging strand (via Okazaki fragments), spliced together by DNA ligase
DNA pol
catalyzes nucleophilic attack (elongation of DNA strand)
purpose of Mg2+ ions?
stabilizes Asp residues on DNA pol
DNA pol requires?
template & primer
responsible for elongating DNA chain and fixing errors while replicating
RNA pol does not require a primer
2 ways to correct mistakes in DNA replication:
- during replication
2. after replication
3’ to 5’ exonuclease activity of DNA pol
slides back and excise the mismatch at the exonuclease site, then continues with DNA synthesis
DNA replication mistake (tautomer)
tautomer form cytosine (looks like thymine)
binds active site of adenine instead of thymine, then converted back to normal cytosine
all 3 DNA polymerases & _________ has ___________
have 3’ to 5’ exonuclease activity
DNA pol I has 5’ to 3’ exonuclease activity as well
DNA replication rate
can’t make DNA pol move faster, can try to initiate more DNA polymerases to act
3 steps of DNA replication
initiation, elongation, termination
initiation
BTHPT
- binds: DNAa binds R&I sequence
- torque: induces torque, springs A-T rich DUE
- helicase: binds, opens DNA sequence
- polymerase: binds DNA strands
- topoisomerase: relives torque from helicase activity
elongation
leading strand is read 5’ to 3’
synthesized continuously
lagging strand read 5’ to 3’
synthesized discontinuously via okazaki fragments
lagging strand elongation
primase adds RNA primer at replication fork, allows DNA pol to synthesize the strand by extending from the primer
single stranded binding proteins
protects the single-strands of DNA
elongation steps
- continuous synthesis of leading strand proceeds as DNA is unwound by the DnaB helicase
- clamp loader bound to ATP & B clamp
- B clamp is loaded around the new template primer
- ATP is hydrolyzed and B clamp closes around the DNA & dissociates from the clamp loader
- DNA pol adds nucleotides until it hits the pervious primer
- previous RNA primer is removed by 5’ to 3’ exonuclease activity of DNA pol I
- fragments joined by ligase
termination
Ter sequences bound by Tus enzymes signals DNA pol to terminate
topoisomerase breaks both DNA strands
minichromosome maintenance protein (MCM)
helicase that creates the origin recognition complex (ORC), once ORC is formed, cell cycle is approved to divide
replication requires what?
synthesis & activity of S-phase cyclin & CDK complexes
telomeres
created by telomerase
RNA serves as template
