Replication and Repair

Semiconservative Replication

• semiconservative → partly saving the old

• when DNA is replicated we untwist DNA strand and separate it into 2

• GOAL: creates 2 identical DNA molecule with one parent strand and one new strand

Replication

Step 1: Strand Separation

• takes place in replication origin → the specific location of where it’s happening

  • there’s several replication origins = replication bubbles

    • eventually the helicase meet and the DNA becomes 2 strands

• Helicase unzips the DNA by breaking up h-bonds

  

• Problem #1 → when untwisting DNA it will tangle at the lower on the strand

  • ex: untwisting hair

  • SOLUTION → topoisomerase enzyme will cut the DNA, then reattach it

• Problem #2 → the 2 strands of the DNA while unzipping want to reconnect because they’re attracted to each other

  • SOLUTION → Single-Strand Binding Proteins (SSBs) will hold the 2 strands inplace to prevent it from reattaching

Step #2: Building Complementary Strands

• DNA polymerase III uses parent strand to match the complementary nucleotide pair

  • ex: AT GC

• To build it uses nucleoside triphosphates

  • breaking the 2 extra phosphate groups off produces energy

• Problem #1 → polymerase III only works if it has something to attach to

  • can’t lay it’s first nucleoside

  • SOLUTION → RNA primer creates something for the DNA polymerase III to attach to

• Problem #2 → DNA polymerase only reads from 5’ to 3’, making the leading strand easy to build as long as the RNA primer is at the end because it follows helicase

  • The lagging strand needs to be done in chunks because it’s not going in the same way as helicase

• Problem #3 → RNA primer are not allowed in the Final DNA strand

  • SOLUTION → DNA polymerase I replace RNA nucleotides to DNA ones and DNA ligase binds together the last 2 nucleotide fragments when primer is removed

Step #3 →