Griffith – Experimented on mice and
observed some harmless strains of
bacteria could change into harmful strains.
He called this transformation.
Avery – Discovered that DNA is the
nucleic acid that stores and transmits the
genetic information from one generation to
More DNA History
Concluded that the
genetic material in
bacteria was DNA not
Watson & Crick –
created the double
helix model for DNA.
Watson & Crick
The Importance of DNA
Structure of DNA
DNA is a long molecule made up of units called
Each nucleotide is made up of three parts: a 5-
carbon sugar called dioxyribose, a phosphate
group, and a nitorgenous base (Nitrogen
The backbone of DNA is formed by sugar and
phosphate groups of the nucleotide.
The nitrogenous base stick out from the sides
and can be joined together in any order,
meaning that any sequence of bases is possible.
There are four kinds of nitrogenous bases.
They are divided into two classes: purines
Purines – Adenine and Guanine
Pyrmidines – Cytosine and Thymine
how the nitrogenous
bases bond together.
He discovered that
bonds with Thymine
and that Cytosine
always bonds with
The Genetic Code
Prokaryotes & DNA
In prokaryotes, DNA
molecules are located
in the cytoplasm of
Most prokaryotic DNA
is a single circular
contains nearly all the
Eukaryotes & DNA
have 1000 times as
much DNA as
DNA is located in the
nucleus in the form of
DNA wound tightly
around proteins called
E. Coli have about 4,639,221 base pairs.
It is about 1.6mm in length. This sounds
small until you realize the bacteria is only
1.6µm in diameter.
Thus DNA must be wrapped tightly to fit
into cells. Imagine fitting 900 yards
(300m) of rope into a backpack.
During DNA replication, the DNA molecule
separates into two strands, then produces
two new complimentary strands following
the rules of base pairing (Chargaff Rules).
Each strand of double helix of DNA serves
as a template, or model, for the new
How It Occurs
DNA replication is carried out by a series
The enzymes unzip the DNA molecule
creating two strands that serve as
Complimentary bases are added to the
strands, for example a strand of DNA with
the bases ATTCGAG would have a
complimentary strand of TAAGCTC.
Each new DNA molecule has one new
stand and one strand from the original
The enzyme DNA polymerase, the
principal enzyme, “proofreads” the new
DNA strands, helping to maximize the
odds that each molecule is a perfect copy
of the original.
Replication of DNA