8.4 DNA Synthesis

1. 8.4 DNA Synthesis • Replication origin – a specific sequence of DNA (or region on a chromosome) at which DNA synthesis, or replication begins

2. 8.4 DNA Synthesis Prokaryotes vs. Eukaryotes • Prokaryotes – only 1 replication origin • Eukaryotes – many replication origins (because they contain so much more DNA; would take too long to replicate)

3. 8.4 DNA Synthesis At the replication origin: • Helicase = enzyme that unwinds & unzips DNA • RNA primase = produces an RNA primer • DNA polymerase = enzyme that makes new DNA • Other enzymes/proteins • This whole combination of the enzymes, proteins, & DNA = replisome

4. Steps of DNA Synthesis 1. Proteins & enzymes bind at replication origin. Helicase, an enzyme, unwinds/unzips the DNA molecule.

5. Steps of DNA Synthesis 2. Another enzyme, RNA primase, lays down an RNA primer so that the next enzyme knows where to begin DNA synthesis.

6. Steps of DNA Synthesis 3. The enzyme DNA polymerase adds nucleotides to the pre-existing DNA strand by matching the correct base pairs.

7. 8.4 DNA Synthesis • Because DNA is antiparallel, we call one strand the leading strand (5’ → 3’) and the other the lagging strand (3’ → 5’). • Leading strand = continuous DNA synthesis • Lagging strand = discontinuous DNA synthesis

8. 8.4 DNA Synthesis WHY 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.

9. Steps of DNA Synthesis 4. DNA polymerase replaces the RNA primers with DNA and replication continues until the entire chromosome has been replicated, resulting in 2 identical DNA molecules.

10. Replication Animation

11. 8.4 DNA Synthesis • End 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

12. 8.5 DNA Repair • New DNA strands must be EXACT complements to the parental strand • Mutation - any change in the sequence of a cell’s DNA • Can be silent, harmful, or even lethal to cells • Ex – mutations play a major role in cancers

13. 8.5 DNA Repair • Mutagenic 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

14. 8.5 DNA Repair Processes 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 pairs • After adding the nucleotide it checks to see if the base pair is correct & if not, it removes the incorrect one & replaces it

15. 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

16. Specific Types of Mutations • Insertion – when a nucleotide is added into a strand of DNA

17. Specific Types of Mutations • Deletion – when a nucleotide is removed from a strand of DNA

18. Specific Types of Mutations • Substitution – when one nucleotide is substituted for another