1. DNA Replication
Hydrogen bonds
Nucleotide
Sugar-phosphate backbone
Key
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Opposite sides of the DNA double helix are called complementary strands.

2. Why replicate DNA? Copying of the DNA occurs before a cell divides
Parent cell pinches off into two daughter cells
Each daughter has the exact same DNA (the full amount, not half!)

3. DNA Replication – step 1 Hydrogen bonds are weak between bases
Easily broken by an enzyme – like unzipping a zipper

4. DNA Replication – Step 2
Free floating nucleotides in nucleus form hydrogen bonds with the complementary bases on the parent strands

5. DNA Replication – Step 3 DNA winds back up
End result is 2 identical DNA molecules – each with one old strand and one new strand (SEMICONSERVATIVE)

6. DNA Replication Original strand New strand DNA polymerase Growth
Replication fork
Replication fork
New strand
Original strand

7. DNA Replication
Step 1: Helicase binds to the DNA and unwinds the 2 strands (exposing nitrogenous bases)
What kinds of bonds does helicase break?
Helicase

8. DNA Replication
Step 2: DNA Polymerase adds complementary nucleotides to the exposed bases on the original strand
This forms a NEW strand attached to the OLD one
What kinds of bonds are being formed?
Helicase
DNA
Polymerase
DNA
Polymerase

9. DNA Replication
Step 3: New DNA molecules wind back up to form a double helix
Why do we say DNA replication is “Semi-conservative?”
There are now two DNA molecules, each containing one OLD strand and one NEW strand
Helicase
DNA
Polymerase
DNA
Polymerase

10. DNA Replication - Photo

11. Self-Quiz!
The sequence of bases on one strand of DNA is: ACTGTACCGA
TGACATGGCT
What is the sequence of bases on the complementary strand?

12. DNA Replication - Summary
When?
Where?
How?
Before cell division
In the nucleus of the cell
DNA unzips, complementary free nucleotides latch on, (A=T, G=C), two complete strands are produced.