1. 3’
5’
This single DNA molecule is ready to be broken
by DNA Helicase at the bottom, at the AT bond.
T
A
C
G
A
T
T
A
C
G
KEY:
- Phosphate
T
A
- Sugar
C
G
C
G
A
- Adenine
T
- Thymine
C
3’
T
A
5’
- Cytosine
G
- Guanine

2. 5’
T
A
C
G
A
T
T
DNA Helicase begins splitting the DNA
at the origin of replication, which is at
an AT bond. AT bonds are only double
bonds, whereas CG bonds are triple
bonds. Helicase breaks a double bond
easier, so it looks for an AT bond.
A
C
A
3’
G
KEY:
- Phosphate
T
- Sugar
C
C
G
A
T
G
- Adenine
- Thymine
C
3’
5’
T
A
- Cytosine
G
- Guanine

3. 3’
5’
T
A
C
G
A
T
T
A
G
C
KEY:
- Phosphate
T
A
- Sugar
C
G
C
G
A
- Adenine
T
- Thymine
C
3’
T
A
5’
- Cytosine
G
- Guanine

4. 3’
5’
T
A
C
G
A
T
A
T
G
C
KEY:
- Phosphate
T
A
- Sugar
C
G
C
G
A
- Adenine
T
- Thymine
C
3’
T
A
5’
- Cytosine
G
- Guanine

5. 3’
5’
A
T
C
G
A
Helicase has finished unwinding the
DNA molecule, leaving to strands for
replication.
T
A
T
G
C
KEY:
- Phosphate
T
A
- Sugar
C
G
C
G
A
- Adenine
T
- Thymine
C
3’
T
A
5’
- Cytosine
G
- Guanine

6. 5’
T
A
DNA replication occurs in a 5’ to 3’ pattern,
so the left strand becomes the first to get
a new strand attached.
C
A
T
C
KEY:
- Phosphate
A
- Sugar
C
C
A
- Adenine
T
- Thymine
C
3’
T
- Cytosine
G
- Guanine

7. 5’
A
T
C
G
A
DNA replication occurs in a 5’ to 3’ pattern,
so the left strand becomes the first to get
a new strand attached.
T
T
C
KEY:
- Phosphate
A
- Sugar
C
C
A
- Adenine
T
- Thymine
C
3’
T
- Cytosine
G
- Guanine

8. 5’
A
T
C
G
A
DNA replication occurs in a 5’ to 3’ pattern,
so the left strand becomes the first to get
a new strand attached.
T
A
T
C
G
KEY:
- Phosphate
T
A
- Sugar
C
C
G
A
- Adenine
T
- Thymine
C
3’
T
- Cytosine
G
- Guanine

9. 5’
T
A
3’
The leading strand (left strand) finishes
replication and becomes a new DNA
molecule.
G
C
T
A
A
T
C
G
KEY:
- Phosphate
T
A
- Sugar
C
C
G
A
T
G
- Adenine
- Thymine
C
3’
T
A
A
5’
- Cytosine
G
- Guanine

10. 3’
A
T
C
G
A
T
Since DNA replication happens in a 3’
to 5’ pattern, the right strand, or the
lagging strand, replicates a new strand
after the leading strand. However, DNA
can’t replicate this way, so DNA Primase
is added to temporarily make it a 5’ to
3’ strand.
A
T
G
KEY:
- Phosphate
T
- Sugar
G
A
T
G
- Adenine
- Thymine
C
A
5’
- Cytosine
G
- Guanine

11. 3’
A
T
C
G
A
T
Since DNA replication happens in a 3’
to 5’ pattern, the right strand, or the
lagging strand, replicates a new strand
after the leading strand. However, DNA
can’t replicate this way, so DNA Primase
is added to temporarily make it a 5’ to
3’ strand.
A
T
C
A
G
KEY:
- Phosphate
T
- Sugar
G
A
T
G
- Adenine
- Thymine
C
A
5’
- Cytosine
G
- Guanine

12. 3’
A
T
C
G
A
T
Since DNA replication happens in a 3’
to 5’ pattern, the right strand, or the
lagging strand, replicates a new strand
after the leading strand. However, DNA
can’t replicate this way, so DNA Primase
is added to temporarily make it a 5’ to
3’ strand.
A
T
C
G
KEY:
- Phosphate
T
A
- Sugar
C
G
C
G
A
- Adenine
T
- Thymine
C
A
5’
- Cytosine
G
- Guanine