8.4 DNA Synthesis
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Transcript of 8.4 DNA Synthesis
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8.4 DNA SynthesisReplication origin – a specific sequence of DNA (or
region on a chromosome) at which DNA synthesis, or replication begins
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8.4 DNA SynthesisProkaryotes vs. Eukaryotes
Prokaryotes – only 1 replication originEukaryotes – many replication origins (because
they contain so much more DNA; would take too long to replicate)
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8.4 DNA SynthesisAt the replication origin:
Helicase = enzyme that unwinds & unzips DNA RNA primase = produces an RNA primerDNA polymerase = enzyme that makes new DNA Other enzymes/proteinsThis whole combination of the enzymes, proteins,
& DNA = replisome
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Steps of DNA Synthesis1. Proteins & enzymes bind at replication origin.
Helicase, an enzyme, unwinds/unzips the DNA molecule.
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Steps of DNA Synthesis2. Another enzyme, RNA primase, lays down an RNA
primer so that the next enzyme knows where to begin DNA synthesis.
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Steps of DNA Synthesis3. The enzyme DNA polymerase adds nucleotides to the
pre-existing DNA strand by matching the correct base pairs.
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8.4 DNA SynthesisBecause DNA is antiparallel, we call one strand
the leading strand (5’ → 3’) and the other the lagging strand (3’ → 5’).
Leading strand = continuous DNA synthesisLagging strand = discontinuous DNA synthesis
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8.4 DNA SynthesisWHY is the lagging discontinuous???
DNA polymerase can only work in one direction (3’ → 5’), so in lagging strand – DNA synthesis occurs in short, unconnected segments (called Okazaki fragments) that get joined by another enzyme, called ligase.
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Steps of DNA Synthesis4. DNA polymerase replaces the RNA primers with
DNA and replication continues until the entire chromosome has been replicated, resulting in 2 identical DNA molecules.
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8.4 DNA SynthesisEnd result = 2 identical double helices, each
with one original strand & one newly synthesized strand
Called Semi-conservative DNA synthesis b/c each helix has an original & a new strand
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8.5 DNA RepairNew DNA strands must be EXACT complements
to the parental strandMutation - any change in the sequence of a
cell’s DNACan be silent, harmful, or even lethal to cellsEx – mutations play a major role in cancers
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8.5 DNA RepairMutagenic chemicals – environmental factors that
introduce or cause mutations; typically cause a mismatched pair to occur (A-C, which can’t form H bonds)
How are errors detected & fixed: DNA polymerase proofreads as it goes/ excision repair
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8.5 DNA RepairProcesses to detect & correct errors
DNA polymerase proofreads its own work~1 in 10,000 bases is incorrect, but ends with only
~ 1 mutation in 10,000,000 base pairsAfter adding the nucleotide it checks to see if the
base pair is correct & if not, it removes the incorrect one & replaces it
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Excision repair:
1.Enzyme recognizes mismatch, binds to DNA, breaks the sugar-phosphate bonds of mismatched section, & removes mutant DNA.
2. DNA polymerase then fills in deleted DNA sequence & another enzyme (ligase) repairs the broken bonds
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Specific Types of Mutations
Insertion – when a nucleotide is added into a strand of DNA
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Specific Types of Mutations
Deletion – when a nucleotide is removed from a strand of DNA
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Specific Types of Mutations
Substitution – when one nucleotide is substituted for another