DNA Replication- the process of making a copy of a DNA molecule
(replicate means to make an exact copy).
- Occurs in the nucleus of the cell (eukaryotes).
- Happens during the S phase of interphase.
Steps of DNA Replication
Remember that DNA is in the shape of a double helix (spiral staircase). In order to
begin replication:
1. The DNA must uncoil (looks like a ladder).
2. Next, the Hydrogen bonds between the Nitrogen bases are broken (or
“unzipped”) by an enzyme (DNA polymerase).
a. The process of breaking the hydrogen
bonds is often compared to unzipping a
zipper.
b. The DNA molecule is now separated into two strands. These strands
will serve as a template for each new strand.
3. The DNA molecule then produces two new complementary strands of code,
following the rules of base pairing.
a. To do this, free DNA Nucleotides match up the complimentary
nitrogen bases:
- Adenine bonds with Thymine
- Guanine bonds with Cytosine
b. Example: original strand –T A C G T T
complementary – ATGCAA
Outcome: Copy of double stranded DNA; each copy contains one new
(complementary strand) and one old (original) strand of DNA
REINFORCEMENT ACTIVITY ON NEXT PAGE 
Color the picture of DNA Replication using the key provided below. Label the new
strands (complementary) and the old strands (originals). Circle a free nitrogenous base.
Color the thymines orange.
Color the adenines green.
Notice how the old strands are on the outside and the
new strands form on the inside of the DNA molecule –
resulting in 2 identical strands (each with one old and one
new)
Color the guanines purple.
Color the cytosines yellow.
Color the phosphates pink .
Color the deoxyribose blue.