Bio 230 Topic 5: DNA Replication

descent with modification

Darwin observed descendants are similar but not identical to their parents

Semi-conservative nature of DNA Replication

each daughter molecule is 50% old and 50% new

helicase

seperates two DNA strands at the start of replication

How does polymerase get started?

primase starts replication because it puts OH group on 3’ end for dNTP binding, it creates a short RNA primer complementary to template DNA, DNA ligase then closes gap between synthesized DNA and DNA that replaced RNA primer

how are both strands simultaneously replicated in same direction?

lagging strand replicated in short fragments called Okazaki fragments that each have their own RNA primers, then RNA primers are removed and replaced by DNA synthesized by DNA polymerase and DNA ligase joins segments together

what keeps DNA from getting tangled?

Topoisomerase enzymes receive supercoiling in DNA by cutting the DNA

How are the ends of the DNA copied?

Telomerase extends the end of the chromosomes by more than 500 bases removing the gap from the RNA primer

Telomere

protective repetitive DNA caps at the ends of linear chromosomes

Telomerase

riboprotein with both a reverse transcriptase component (TERT) and an RNA molecule (TERC) that extends of upper strand leaving a 5’ to 3’ strand with an overhang, only found in germ cells, stem cells, and cancer cells

What happens to telomeres after each replication

the shorten until after ~70 divisions they reach a critical length which causes cell death

Tautomers

alternative structure of nucleotide bases

How’s is accuracy of replication ensured

through proofreading, where DNA polymerase can detect a misfired base, back up, remove and put down the right base (exonuclease activity)

Post-replication repair: Mismatch repair

mismatch repair proteins detect improperly paired bases and remove base and surrounding bases then DNA polymerase fills the gaps and ligase seals

Repair mechanism for methylation of bases

Direct repair by methytransferase (suicide enzyme)

Repair process for breakage of bond connecting the base to sugar phosphate backbone

base excision repair

repair process for changes to multiple adjacent bases or more extensive damage (caused by uv light forming dimers)

nucleotide excision repair (strands are locally separated and one strand excised to create gap of 10-20 nucleotides which is then filled by DNA polymerase

repair process if double-stranded DNA breaks

break repair

transition mutation

one pyrimidine replaces another pyrimidine or purine with purines

transversion mutation

one purine replaces a pyridine (or vice versa)

silent mutation

change in nucleotide that leads to no change in amino acid

nonsense mutation

causes a stop codon

missense mutation

change in amino acid

indel

insertion or deletion of bases through strand slippage