DNA Structure
• By the early 1900’s it was
known that the chromosomes
carry the genetic (hereditary)
information
• Chromosomes consist of DNA
(deoxyribonucleic acid)
I. Chemical Structure of DNA
A. Nucleotide:
1. Thousands of repeating units that
make up the DNA (DNA is a polymer)
a) a phosphate group
b) a deoxyribose molecule (5-carbon sugar)
c) a nitrogenous base
adenine, thymine, guanine, cytosine
(There are four different types of
nucleotides, depending on which of the
four bases is present- A, T, C, or G)
II. The Watson-Crick Model of the
Structure of DNA
DNA consists of two chains of nucleotides
in a ladder-like structure which is twisted
(Double Helix)
James Watson & Francis Crick:
• Used data of M.H.F. Wilkins and
Rosalind Franklin, early 50’s
• Wilkins and Franklin studied the structure
of DNA crystals using X-rays.
• The X pattern suggested the structure of
DNA was a helix.
Distance between the two “backbones” of DNA
is constant along the length of the molecule
• Used data of Erwin Chargaff, 1940’s
and early 50's
• Chargaff’s Rule: His data showed
that in each species, the percent of A
equals the percent of T, and the
percent of G equals the percent of C.
Watson and Crick (1953)
Watson and Crick Model:
A. The sides of the ladder are
made up of alternating
molecules of phosphate and
deoxyribose
B. The bases make up the rungs of
the ladder
C. The bases that make up the
rungs of the ladder are
attracted by a weak chemical
bonds called hydrogen bonds
Base Pairing Rule
Adenine (A) pairs with Thymine (T)
Guanine (G) pairs with Cytosine(C)
Structure of DNA Animations
• http://207.207.4.198/pub/flash/24/menu.swf
http://www.pbs.org/wgbh/nova/genome/dna.html#
Watson and Crick’s Double Helix
Model explained:
1. How replication of DNA during
mitosis produces exact copies for
the daughter cells
2. How DNA acts as a code,
specifying how proteins are made
by the cell
DNA Replication
1. The double-stranded DNA
molecule separates or
“unzips” along the weak
hydrogen bonds between the
base pairs
2. Free nucleotides that are
present in the nucleus attach
themselves by hydrogen
bonds to the exposed bases
(A-T, C-G)
3. Two identical strands form
from each strand of DNA
DNA Replication Animations
http://www.lewport.wnyric.org/JWANAMAKER/animations/DNA%20Rep
lication%20-%20long%20.html
http://www.johnkyrk.com/DNAreplication.html
• If the order of bases along one strand of
DNA is
AGGTTACTGCAC
what is the order of bases on the
complementary strand?
TCCAATGACGTG
How Does DNA Act As
A Code?
• The order of bases on the
DNA strand instructs the
ribosomes how to
synthesize proteins
• Gene: portion of DNA
that codes for the
production of a specific
polypeptide
•Polypeptide: building
block of a protein
In terms of decreasing size:
Nucleus → Chromosome → Gene → DNA