1. Unit 6 Notes: DNA Replication

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.

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)

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.

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! 

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 .

13. (6) Complimentary Bases
REMEMBER:
A bonds with T
G bonds with C
EXAMPLE: A T G C T