Transcript 3. DNA Double Helix

Chapter 21 Nucleic Acids and
Protein Synthesis
21.3 DNA Double Helix
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
1
Percentage of Bases in DNA
In Chargaff’s rules for DNA of various organisms,
 the percentage of adenine (A) is the same as the
percentage of thymine (T)
 the percentage of guanine (G) is the same as the
percentage of cytosine (C)
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
2
Complementary Base Pairs:
A and T
DNA contains complementary base pairs in which
adenine is always linked by two hydrogen bonds to
thymine (A−T).
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
Complementary Base Pairs: G and C
DNA contains complementary base pairs in which
 guanine is always linked by three hydrogen bonds
to cytosine (G−C)
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
4
Double Helix of DNA
The DNA structure is a
double helix that
 consists of two strands
of nucleotides that form
a double helix structure
like a spiral stair case
 has hydrogen bonds
between the bases A–T
and G–C
 has bases along one
strand that complement
the bases along the
other
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
5
Learning Check
Write the complementary base sequence for the
matching strand in the following DNA section:
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
6
Solution
Write the complementary base sequence for the
matching strand in the following DNA section:
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
7