DNA is the genetic material found in chromosomes in the nucleus of cells. It has a double helix structure like a twisted ladder, with sugar and phosphate molecules forming the sides and nitrogen base pairs forming the rungs. There are four nitrogen bases - adenine, thymine, guanine and cytosine - that always pair up in the same way: A pairs with T and G pairs with C. DNA replication is the process where the DNA double helix unwinds and each half acts as a template for a new partner strand, resulting in two identical DNA molecules.

1. The Structure & Replication of
DNA

2. What is DNA?
• Found in the nucleus of a cell in two
different structures: chromatin and
chromosomes
• Genetic Material
• Determine inherited characteristics

3. What is the Structure of DNA?
• It looks like a twisted ladder.
• The 2 sides of the DNA ladder are made
up of molecules of sugar (deoxyribose)
and molecules called phosphates.
• The deoxyribose and phosphates
alternate, one followed by the other, all the
way up both sides of the ladder.

4. Deoxyribose

5. The Sides of DNA
SIDES
DEOXYRIBOSE
(SUGAR)
RUNGS
PHOSPHATES

6. What are Nitrogen Bases?
• Each rung of the DNA ladder is made up
of a pair nitrogen bases.
• Nitrogen bases are molecules that
contain the element nitrogen combined
with other elements.
• DNA has 4 kinds of nitrogen bases:
adenine (A), thymine (T), guanine (G), and
cytosine (C).

7. The Rungs of DNA
SIDES
DEOXYRIBOSE
(SUGAR)
RUNGS
PHOSPHATES
NITROGEN
BASES

8. How do the Nitrogen Bases Pair?
•
•
•
•
•
ADENINE pairs only with THYMINE.
GUANINE pairs only with CYTOSINE.
A–T
G–C
This pattern of pairing is the key to
understanding how replication works.

9. Nucleotides
• One sugar, One phosphate and One base
• Make up DNA molecules
PHOSPATE
BASE
SUGAR

10. DNA REPLICATION
• Occurs during the 2nd Phase of
INTERPHASE: Copying of DNA
• It’s so important that we need to talk about
it in detail.
• First let’s look at what DNA looks like…

11. Here is a DNA Molecule
Notice how it is a TWISTED
ladder.

12. What is the Replication Process?
1. The 2 sides of the Parent DNA Molecule
unwind like a zipper unzipping. The ladder
separates between the paired bases.
2. The nitrogen bases that are floating in the
nucleus pair up with the bases on each half of
the Parent DNA molecule to make 2 new
Daughter DNA molecules.
• Because the bases only pair with certain
bases, the order of the bases in the new
Daughter DNA molecules exactly match the
order in the Parent DNA molecule.

13. STEP 1: Unzipping the
Parent DNA
Nitrogen
Base
Nitrogen
Base

14. STEP 2: Nitrogen Bases Pair
Up with New Bases
Daughter DNA
Called
a strand
of DNA
Parent DNA
Daughter DNA

15. Here is a Parent DNA molecule
replicating to make 2 Daughter
DNA molecules

16. STEP 1: Unzipping the
Parent DNA
A
A
T
T
G
C
G
C
T
A
G
C

17. STEP 2: Nitrogen Bases Pair
Up with New Bases
G
C
A
A
G
T
G
C
T
T
C
C
G
A
G
C