1. Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Chapter 17 Nucleic Acids and Protein Synthesis 17.3 DNA Double Helix 1

2. Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Complementary Base Pairs 2 DNA contains complementary base pairs, equal amounts of A and T and equal amounts of G and C bases in which adenine is always linked by two hydrogen bonds with thymine (A−T) guanine is always linked by three hydrogen bonds with cytosine (G−C)

3. Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. DNA Double Helix A double helix has two strands of nucleotides that wind together is held in place by two hydrogen bonds that form between the base pairs A−T is held in place by three hydrogen bonds that form between the base pairs G−C 3

4. Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. DNA Double Helix In the double helix of DNA, the two chains are held together by hydrogen bonds that link bases A–T and G–C the bases along one strand complement the bases along the other 4

5. Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Learning Check Write the complementary base sequence for the matching strand in the following DNA section: —A—G—T—C—C—A—A—T—C— 5

6. Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Solution Write the complementary base sequence for the matching strand in the following DNA section: —A—G—T—C—C—A—A—T—C— 6 —T—C—A—G—G—T—T—A—G—

7. Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. DNA Replication In DNA replication, genetic information is maintained each time a cell divides the DNA strands unwind each parent strand bonds with new complementary bases two new DNA strands form that are exact copies of the parent DNA 7

8. Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. DNA Replication In DNA replication, the separate strands of the parent DNA are the templates for the synthesis of complementary strands, which produces two exact copies (daughter DNAs). 8

9. Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Learning Check How many hydrogen bonds link each of the following base pairs? A. A−T B. G−C 9

10. Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. Solution How many hydrogen bonds link each of the following base pairs? A. A−T bases are linked by two hydrogen bonds. B. G−C bases are linked by three hydrogen bonds. 10

11. Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. DNA Fingerprinting In a process called DNA fingerprinting, enzymes are used to cut DNA chains into smaller sections the resulting fragments are separated by size and treated with a radioactive isotope that adheres to specific base sequences the resulting pattern of bands is known as a DNA fingerprint One application of DNA fingerprinting is in forensic science, where DNA from samples such as blood, hair, or semen is used to connect a suspect with a crime. 11

12. Chemistry: An Introduction to General, Organic, and Biological Chemistry, Eleventh Edition Copyright © 2012 by Pearson Education, Inc. DNA Fingerprinting Dark and light bands on X-ray film show up as a result of the radioactive isotope that adheres to specific sequences. 12