2. (1) Why Replicate DNA?
• Necessary for cell division.
• During the S Phase of the Cell Cycle, the
cell needs to make a copy of its DNA for the
new cell.
• Errors:
– Can have ZERO impact on the cells.
– Could provide a positive advantage to cell.
– Could kill the cell.
3.
4. (2) Semi-Conservative Replication
• Messelson & Stahl discovered that DNA
Replication is a Semi-Conservative process:
– Semi = ½ or Partial
– Conserve = To Keep or To Save
• You start with 1 whole, old molecule of DNA…
– You end with 2 new molecules of DNA.
– Each new molecule has:
• 1- old strand
• 1- new strand
• (1/2 of the old molecule is conserved in each new one)
5.
6. (3) Replication Basics:
• Overall Goal: Make an Identical copy of
cell’s DNA molecule.
• Basic Steps:
1- Un-twist strands.
2- Separate strands.
3- Make a complimentary copy of each
strand.
• End Result:
2 DNA molecules, each with 1 old strand
and 1 new strand.
7.
8. (4) Untwisting & Separating
• DNA is untwisted and separated by the
enzyme Helicase.
• DNA is opened in small chunks with
replication forks in between…
– Several helicase enzymes open up DNA at once.
– Results in several open “bubbles” along helix.
– This increases replicating efficiency!
9.
10.
11. (5) Complimentary Copies
• The enzyme DNA POLYMERASE will lay down
the new DNA nucleotides.
– DNA Polymerase can only lay down new DNA
nucleotides starting with a 5’ End!
– This means the enzyme must move along the original
strand 3’5’ to be able to lay down new nucleotides.
• The Leading Strand:
– Original strand that is positioned 3’ 5’.
– DNA is first laid down along this strand.
• The Lagging Strand:
– Original strand that is positioned 5’ 3’.
– RNA nucleotides are first laid down along this strand.
– Then DNA Polymerase replaces them with DNA .
