DNA is a molecule that contains genetic information. It is composed of nucleotides that form two strands bonded together by hydrogen bonds between complementary nitrogenous bases. The sequence of these nucleotides determines an organism's traits and distinguishes one organism from another. DNA replicates through a process where it unzips its two strands and uses each strand as a template to create a new complementary strand, duplicating the genetic information.

1. DNA: the molecule of heredity
What is DNA?
Although the environment influences how an
organism develops, the genetic information that is held
in the molecules of DNA ultimately determines an
organism’s traits.
Living things contain proteins. Your skin contains protein,
you muscles contain protein, and your bones contain
protein mixed with mineral.
All actions, such as eating running and even thinking,
depend on proteins called enzymes. Enzymes are critical
for an organism’s function because they control the
chemical reactions needed for life.

2. The structure of nucleotides
• DNA is a polymer made of subunits called
nucleotides.
• Nucleotides have 3 parts: a simple sugar, a
phosphate group and nitrogenous base

3. The structure of nucleotides
• In DNA, there are four possible nitrogen
bases: adenine (A), guanine (G), cytosine
(C), and thymine (T).
Nucleotides join together. The phosphate group
of one nucleotide is bonded with the
deoxyribose of another nucleotide.

4. Nitrogenous bases

5. The structure of DNA
• DNA is composed of 2 strands that are held
together by a hydrogen bond between
nitrogenous bases. Think of teeth of a zipper
holding the two sides of the zipper together.
• Hydrogen bonds can only form between
specific paired nitrogenous bases called
complementary base pairs.
1. (A)---------(T) 2. (C)---------(G)
Hydrogen
bond
Hydrogen
bond

6. Nitrogenous base pairing

7. The structure of DNA
• The two strands of DNA twist together, and its
shape is called a double helix.

8. The importance of nucleotide
sequences
• In all organisms the DNA is made up of the
four nucleotides. (A) (T) (C) (G)
• The sequence of nucleotides forms the unique
genetic information of an organism.
• For ex. A nucleotide sequence of A-T-T-G-A-C
carries different information from a sequence
of T-C-C-A-A-A.

9. The importance of nucleotide
sequence
• The closer the relationship is between two
organisms, the more similar their DNA
nucleotide sequences will be.
• The DNA sequences of a chimpanzee are
similar to those of a gorilla, but different from
those of a rosebush.

10. How DNA replicates
• The two DNA strands serve as a templates.
• Replication begins as an enzyme breaks the
hydrogen bonds between bases that hold the
two strands together, thus unzipping the DNA.
• As the DNA continues to unzip, nucleotides
that are floating free in the surrounding
medium are attached to their base pair by
hydrogen bonding.

11. How DNA replicates
• The process continues until the entire
molecule has been unzipped and replicated.