Grade 12 University

STEPS

 

1. Parent strand unwinds and separates

• 3 enzymes are used

  • HELICASE breaks the hydrogen bonds between the nitrogenous bases 

  • GYRASE relieve tension from unwinding

  • SINGLE STRANDED BINDING PROTEINS (SSBs) keep the DNA strands apart

 

2. Making DNA

• in the 5'-3' direction

  • DNA POLYMERASE adds nucleoside triphosphate

    • Note: the phosphate group gives energy

• LEADING STRAND: 

  • ​Daughter strand goes TO the replication fork

    • ​RNA primase binds to the RNA primers on the DNA​

    • When have 10-60 RNA bases, ready for DNA replication...

 

DNA REPLICATION

 

• DNA POLYMERASE III adds DNA nucleotides to RNA primer

  • New strand grows from 5'-3'

 

• LAGGING STRAND: 

  • AWAY from replication fork 

  • The 3'-5' parent strand is a problem b/c DNA poly works in the 5'-3' direction....

    • RNA PRIMASE adds a primer, then DNA POLY adds to it

      • This strand is RNA PRIMERS + SERIES OF DNA FRAGMENTS ---> aka OKAZAKI FRAGMENTS

  • DNA POLY III builds Okazaki fragments off RNA primers

  • DNA POLY I removes RNA primers when used and replaces with the right DNA sequence

  • DNA LIGASE join Okazaki fragments to 1 strand by PHOSPHODIESTER BONDS (sugar and phosphate)

• ALL RNA primers are removed and replaced by DNA

  • ​A-T has 2 H bonds

  • C-G has 3 H bonds

    • ​When H bonds forms--twist into double helix

 

3. Dealing with mistakes

• When base pairs mismatch....

  • ​DNA POLY III replaces the wrong bases

    • ​If still wrong: DNA Poly I proofreads and II removes the wrong base

• Quality control to avoid loss/ change in function/ cancer