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DNA Structure
Powerpoint #1
Chapter 12

The building blocks of nucleic acids are
nucleotides.
One Nucleotide
Nitrogen Base
Nitrogen Base
•DNA is found in
the nucleus of
the cell
• DNA’s function is to store and
transmit genetic information.
How did scientists
determine the
structure of DNA?
A little background in what was
discovered so far…
1940’s
Oswald Avery and
others determined that
DNA stores and
transmits genetic
information
1950 - Erwin Chargaff
Noticed a pattern:
Comparing ANY sample of DNA he discovered that
the percentages of the nitrogen bases guanine(G)
and cytosine(C) are equal. The same is true for the
percentages of adenine(A) and thymine(T).
CHARGAFF’S RULE
#A=#T
#G=#C
1951 The Structure (shape) of DNA
It was a RACE - lots of competition
USA
Linus Pauling
(Nobel prize chemical
bonds)
London, England
Rosalind Franklin and
Maurice Wilkins were
photographing the
DNA molecule.
England
Cambridge
University, James
Watson and
Francis Crick build
models
1952 - Rosalind Franklin
X-ray crystallography
Shine X-rays on crystalized DNA, bounce off
see an image.
Clues
Order/pattern
Equal width
Born:1920 Died:1958 (Age 37)
X-Ray crystallography
1953 Watson and Crick
With Franklin's and many other scientists
data, Watson and Crick were soon able to
determine the structure of DNA.
On March 18, 1953, Watson and Crick
published the first accurate model of the DNA
molecule.
DNA
DOUBLE
HELIX
1960 Watson, Crick, and Wilkins received
the Nobel prize for their work.
Watson & Crick accepting Nobel Prize
Franklin had died of cancer in 1958, at age 37.
Would you award her a Nobel Prize?
The four different nitrogen bases in DNA
(for different types of DNA nucleotides)
Adenine (A), Guanine (G),
Cytosine (C), and Thymine (T)
are:
A always pairs with T
G always pairs with C
The bases that pair with each other are
called complimentary
 The
code is read 3 bases at a
time (4x4x4=64) and can be
thousands or millions of base
pairs long.
 (64
x 64 x 64 x 64…….)
A
single DNA molecule may
contain thousands of nucleotides
(bases).
The order of the
nucleotides (bases) in a
DNA is a code that
provides instructions for
making proteins.


A segment of DNA that codes for a specific protein
is called a gene.
Genes code for traits such as:
◦ Hair color
◦ Eye Color
◦ How many fingers you have
◦ Skin color
◦ Enzymes to break down the food that you eat
In your cells, DNA wraps up tightly around
proteins called histones to produce a
substance called chromatin which
condenses during cell division (mitosis) to
form structures called chromosomes



Bases are either:
Purines (A & G) – a
nine-member, 4
nitrogen molecule
Pyrimidines (T & C)
– a six-member, 2
nitrogen molecule


The bases hydrogen
bond to each other
at the locations to
the right.
Look for these
donors and
acceptors to
determine where
bonds are located

G & C bond in three locations

A & T bond in two locations
1 Nucleotide
Base Pairs: Adenine,
Guanine, Cytosine,
Thymine
Backbone made of
sugar and phosphate