The Structure of DNA
The building block of DNA (and RNA) is the
nucleotide.
Each nucleotide has 3 parts:
• A sugar (deoxyribose in DNA)
• A phosphate group
• A nitrogenous base
– There are 4 possible nitrogenous bases in DNA.
The 4 nitrogenous bases in DNA
nucleotides are:
•
•
•
•
Adenine
Thymine
Cytosine
Guanine
The nitrogenous bases
(A, G, C and T) are the
parts of the DNA
molecule that encode
genetic information.
In DNA, the nucleotides are arranged in
double helix: 2 strands connected together
and twisted like a spiral staircase.
Watson and Crick (with data from Rosalind
Franklin) determined this structure of DNA in
the 1950’s.
This knowledge helped scientists understand
the role of DNA in heredity.
In the double helix, the sugar
and phosphate group of the
nucleotide form the backbone.
Pairs of nitrogenous bases (A, C,
T or G) form the “rungs” of the
double helix.
Adenine always pairs
with Thymine
Cytosine always pairs
with Guanine
DNA Structure Video
Click once on image to start
https://www.youtube.com/watch?v=qa5gfOMVpr4
Build a DNA Molecule
Click here to build a DNA double helix
Applying the Concepts
If one strand of a DNA molecule has a base
sequence of AACGCTAT what with the other
strand be?
A A C G C T A T
T T G C G A T A
Applying the Concepts:
Chargaff’s Rule
In a sample of DNA, 20% of the bases are
adenine.
What percentage are T, C, and G?
Thymine =
Cytosine =
Guanine =
20%
30%
30%
Adenine and Thymine always pair,
so equal amounts.
If 40% is A and T, remaining 60%
is C and G, divided in equal
amounts since they pair.