DNA replication is necessary for cell division to create a copy of the DNA for each new cell. Messelson and Stahl discovered that DNA replication is a semi-conservative process where the original DNA molecule splits into two new molecules, each with one original strand and one newly synthesized complimentary strand. The basic steps of replication are to unwind the DNA strands, separate the strands, and then make complimentary copies of each single strand to result in two new DNA molecules with one old and one new strand each.

1. Unit 4
notes:
DNA Replication

2. (1) Why Replicate DNA?
• Necessary for cell division because 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) Complimentary Bases
• REMEMBER:
– A bonds with T
– G bonds with C
• EXAMPLE:
A
C
G
T
T