Multiple origins of replication, one centromere, and two telomeres - replicates in interphase. SEMI-CONSERVATIVE, 2 template strands. DNA always synthesized in 5'>3'
What other models are possible for DNA replication?
Conservative (make a brand new copy of DNA), dispersive (chunks of new and old combined)
How was semiconservative nature of DNA replication proven?
Meselson-Stahl experiements with heavy nitrogen.
When does DNA replication occur?
In S phase of interphase, only once per cell cycle.
Describe the replication process
removes primers from lagging strands
The protein encoded by this gene removes 5' overhanging flaps in DNA repair and processes the 5' ends of Okazaki fragments in lagging strand DNA synthesis
Enzyme that facilitates the joining of DNA strands together by catalyzing the formation of a phosphodiester bond.
Describe the editing capability of DNA polymerase
DNA pols have 3'-5' exonuclease activity, contributes significantly to fidelity of overall process. Chews from outside.
DNA mismatch repair
Complex of mismatch repair proteins scans newly replicated DNA for nucleotide mismatches, removes a section of strand containing wrong base, and resynthesizes the strand. (Mutations in these genes can cause colon cancer)
Describe error-prone DNA pols
Damaged DNA templates might require specialized low-fidelity pols to replicate. Lesions can be avoided by replication restart copying first off lagging strange then reestablishing replication fork. OR Translesion synthesis and reestablishing replication fork.
Describe the replication stress response
Replication forks are vulnerable to various "replication stresses." Cells have evolved mechanisms called checkpoints to deal with these events to preserve the replication fork. If these fail, the fork fails and could lead to bad shit with DNA
What protein is responsible for replication stress response?
ATR - makes sure the replication stops while the cell checks for damage
What are examples of diseases caused by replication defects?
Ataxia telangiectasia - lymphomas, dilation of blood vessels, immune dysfunction - ATM protein mutated, important for replication stress response
Bloom syndrome - carcinomas, leukemias, etc. caused by mutation to BLM, another protein involved in replication fork stress response
Describe some cancer drugs and their effect on replication
AZT - chain terminator, interupts the formation of HIV DNA
5-FU - affects pyrimidine synthesis
Camptothecin - targets topoisomerase
Hydroxyurea - depletes cancer cells of deoxyribonucleotides
How can DNA damage arise?
Spontaneous events, exposure to environmental mutagens, DNA rep errors or stress
Examples of DNA damage
Mismatched base pair, depurination, pyrimidine dimer (UV light is cause), bulky adduct, alkylation, deamination
How is DNA damage fixed?
Some enxymes exist to directly repair some mutations, while other times bases are excised (via single strand or double strand breaks) and repaired
If one intact DNA strand exists, how is DNA repaired?
1. Altered DNA is removed by enzymes called DNA repair nucleases
2. DNA pol fills in the gap
3. DNA ligase completes process
Base excision, nucleotide excision, and mismatch repair only differ by the enzymes used in step 1
If both strands of DNA are damaged, how is it repaired?
Non-homologous end joining - wrong bases are excised, DNA pol puts right ones in, DNA ligase patches everything up
What kinds of diseases happen when there are defects in DNA repair mechanisms?
Xeroderma pigmentosum - severe predisposition to skin cancers