Download DNA Replication and Cancer

10-3 DNA Replication
Watson & Crick’s model explained exactly how DNA replicates when a cell divides!
How it occurs
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DNA REPLICATION – process by which DNA is copied in cell before it divides (through mitosis, meiosis,
OR binary fission)
o 2 nucleotide strands of doublehelix separate along strands
o Since the strands are complementary – they’re both template for new complementary strand
o Then after it’s replicated, 2 identical double-stranded DNA separate and move to new cells
1.
Helicases(enzymes) come & separate the DNA strands.
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2.
They BREAK THE HYDROGEN BONDS between bases
Now the 2 DNA strand separate, forming a replication fork
DNA Polymerases(enzymes) – add complementary nucleotides(found floating in nucleus) to each
original strand (
- As new nucleotides are added, covalent bonds form between adjacent nucleotides-between the
deoxyribose sugarof one and phosphate group of the other**
- Hydrogen bonds form between Complementary bases on old & new strand
3. DNAPolymerases are finished replicating, and fall off
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Now you’re left with 2 identical DNA molecules ready to go to new cells
Semi-conservative replication  DNA double helix has one strand from original molecule, other strand is
new.
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Kept (conserved) one of the two (semi) original DNA strands
Action at Replication Fork
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DNA synthesis occurs in opposite directions on each strand
As replication fork moves along DNA, synthesis of one strand follows the movement of rep. fork…
and synthesis on the other strand goes the other direction, away from rep. fork…
i. Leaves gap in newly made strand.
ii. These gaps are later joined together by enzyme DNA ligase
Prokaryotic & Eukaryotic Replication
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Prokaryotes
i. Circle-chromosome, replication happens at ONE place along chromosome
ii. 2 resulting rep.forks are copied in opposite directions ( like 2 ziippers going opposite ways)
iii. Replication continues until entire molecule is copied.
Eukaryotic cells
i. Chromosomes are long…not circular
ii. Rate that DNA polymerase adds nucleotides (approx. 50 nucleotides/sec in euks)
1. It’d take 53 days to replicate largest human chromosome in one take
iii. So replication occurs at MANY POINT , or origins along DNA, so that replication is more rapid
1. Rep. forks still move in opposite directions
DNA ERRORS IN REPLICATION
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Usually very accurate! Rarely errors…
i. Because DNA polymerases have repair
enzymes that “proofread” DNA like friend
reads over paper
ii. Ex. if A pairs with C rather than T, repair enzymes in DNA will replace & make it beter
When mistakes DO occur…. Base sequence of new DNA is different from base sequence of original DNA
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Change in nucleotide sequence of DNA molecule  mutation
i. Mutations can seriously affect function of important gene & disrupt cell function
some errors escape repair!!!
- Chemicals & UV radiation – damages DNA
- Some mutations lead to cancer,
- So…. effective mechanism for repair of damaged DNA is super important
DNA Replication and Cancer
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Process by which genetic info is passed down for generations and generatins’
Someee changes help individuals survive and reproduce beter…..
Mutations that aren’t repaired could develop into cancer!!!
i. Mutations that affect genes controlling how cell divides.. leads to
tumor