8.4 dna synthesis

8.4 DNA SynthesisReplication origin – a specific sequence of DNA (or

region on a chromosome) at which DNA synthesis, or replication begins

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)

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

Steps of DNA Synthesis1. Proteins & enzymes bind at replication origin.

Helicase, an enzyme, unwinds/unzips the DNA molecule.

Steps of DNA Synthesis2. Another enzyme, RNA primase, lays down an RNA

primer so that the next enzyme knows where to begin DNA synthesis.

Steps of DNA Synthesis3. The enzyme DNA polymerase adds nucleotides to the

pre-existing DNA strand by matching the correct base pairs.

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

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.

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.

Replication Animation

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

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

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

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

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

Specific Types of Mutations

Insertion – when a nucleotide is added into a strand of DNA

Specific Types of Mutations

Deletion – when a nucleotide is removed from a strand of DNA

Specific Types of Mutations

Substitution – when one nucleotide is substituted for another