© 2014 Pearson Education, Inc. Chapter 11 At a Glance  11.2 What Is the Structure of DNA?  11.3 How Does DNA Encode Genetic Information?  11.4 How Does DNA Replication Ensure Genetic Constancy During Cell Division?  11.5 What Are Mutations, and How Do They Occur?

DNA: The Molecule of Heredity Ch. 11

© 2014 Pearson Education, Inc What Is the Structure of DNA?  Knowing that DNA is made up of genes does not provide an answer to the critical questions about inheritance  The secrets of DNA function and, therefore, of heredity itself are found in the three-dimensional structure of the DNA molecule

© 2014 Pearson Education, Inc What Is the Structure of DNA?  DNA is composed of four nucleotides –DNA is made of chains of small subunits called nucleotides –Each nucleotide has three components 1. A phosphate group 2. A deoxyribose sugar 3. One of four nitrogen-containing bases 1. Thymine (T) 2. Cytosine (C) 3. Adenine (A) 4. Guanine (G)

© 2014 Pearson Education, Inc. Figure 11-3 DNA nucleotides phosphate sugar base  adenine phosphate sugar base  thymine phosphate sugar base  cytosine phosphate sugar base  guanine

© 2014 Pearson Education, Inc What Is the Structure of DNA?  DNA is composed of four nucleotides (continued) –In the 1940s Erwin Chargaff, a biochemist at Columbia University, analyzed the amounts of the four bases in DNA from diverse organisms –He discovered a consistency in the equal amounts of adenine and thymine, and equal amounts of guanine and cytosine for a given species, although there was a difference in proportion of the bases –This finding was called “Chargaff’s rule”

© 2014 Pearson Education, Inc What Is the Structure of DNA?  DNA is a double helix of two nucleotide strands –In the 1940s, several other scientists investigated the structure of DNA –Rosalind Franklin and Maurice Wilkins studied the structure of DNA crystals using X-ray diffraction –They bombarded crystals of purified DNA with X-rays and recorded how the X-rays bounced off the DNA molecules –The resulting pattern does not provide a direct picture of the DNA structure, but the researchers were able to extract specific information

© 2014 Pearson Education, Inc What Is the Structure of DNA?  DNA is a double helix of two nucleotide strands Wilkins and Franklin deduced the following information about DNA from the patterns they found: 1. A molecule of DNA is long and thin, with a uniform diameter of 2 nanometers 2. DNA is a helical, twisted like a Corkscrew or a spiral staircase 3. DNA is a double helix 4.DNA has repeating subunits 5. Phosphates are probably on the outside of the helix

© 2014 Pearson Education, Inc What Is the Structure of DNA?  DNA is a double helix of two nucleotide strands (continued) –James Watson and Francis Crick combined the X-ray data with bonding theory to deduce the structure of DNA –They proposed that a single strand of DNA is a polymer consisting of many nucleotide subunits –Within each DNA strand, the phosphate group of one nucleotide bonds to the sugar of the next nucleotide in the same strand –The deoxyribose and phosphate portions make up the sugar-phosphate backbone

© 2014 Pearson Education, Inc What Is the Structure of DNA?  DNA is a double helix of two nucleotide strands (continued) –The nucleotide bases protrude from the sugar- phosphate backbone –All the nucleotides within a single DNA strand are oriented in the same direction, and thus have an unbonded sugar at one end and an unbonded phosphate at the other end

© 2014 Pearson Education, Inc What Is the Structure of DNA?  Hydrogen bonds between complementary bases hold two DNA strands together in a double helix –Watson and Crick’s findings provided the following insight about the DNA model: –The DNA model consists of two DNA strands, assembled like a twisted ladder –The bases protrude inward toward each other from the sugar-phosphate backbone like rungs on a ladder –Hydrogen bonds hold the base pairs together, composing the rung

© 2014 Pearson Education, Inc What Is the Structure of DNA?  Hydrogen bonds between complementary bases hold two DNA strands together in a double helix (continued) –The two strands in a DNA double helix are said to be antiparallel; that is, they are oriented in opposite directions –From one end of the DNA molecule, if one strand starts with the free sugar and ends with the free phosphate, the other strand starts with the free phosphate and ends with the free sugar

© 2014 Pearson Education, Inc What Is the Structure of DNA?  Hydrogen bonds between complementary bases hold two DNA strands together in a double helix (continued) –Because of their structures and the way they face each other, adenine (A) bonds only with thymine (T) and guanine (G) bonds only with cytosine (C) –Bases that bond with each other are called complementary base pairs –Thus, if one strand has the base sequence CGTTTAGCCC, the other strand must have the sequence GCAAATCGGG

© 2014 Pearson Education, Inc What Is the Structure of DNA?  Hydrogen bonds between complementary bases hold two DNA strands together in a double helix (continued) –Complementary base pairing explains Chargaff’s rule that for a given molecule of DNA, adenine equals thymine and guanine equals cytosine –Since every adenine, for example, is paired with a thymine, no matter how many adenines are in the DNA molecule, there will be an equal number of thymines

© 2014 Pearson Education, Inc What Is the Structure of DNA?  Hydrogen bonds between complementary bases hold two DNA strands together in a double helix (continued) –Adenine and guanine are large molecules; thymine and cytosine are relatively smaller –Because base pairing always places a large molecule with a small one, the diameter of the double helix remains constant –In 1953, James Watson and Francis Crick consolidated all the historical data about DNA into an accurate model of its structure

© 2014 Pearson Education, Inc. Figure 11-5 The Watson-Crick model of DNA structure Hydrogen bonds hold complementary base pairs together in DNA Two DNA strands form a double helix Four turns of a DNA double helix nucleotide free phosphate base (cytosine) sugar hydrogen bonds free sugar free phosphate free sugar

© 2014 Pearson Education, Inc. BUILD DNA