1. Objectives 12.2 The Structure of DNA -Identify the chemical components of DNA. -Discuss the experiments leading to the identification of DNA as the molecule that carries the genetic code. -Describe the steps leading to the development of the double-helix model of DNA.

2. Lesson Overview Lesson Overview The Structure of DNA Nucleic Acids and Nucleotides Nucleic acids are Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA). Nucleic acids are made up of nucleotides. The nucleotides that make up DNA are shown.

3. Lesson Overview Lesson Overview The Structure of DNA What are Nucleotides made of??? DNA’s nucleotides are made up of three basic components: 1. a 5-carbon sugar called Deoxyribose; 2. a phosphate group; 3. a nitrogenous base.

4. Lesson Overview Lesson Overview The Structure of DNA The Nitrogen Bases… DNA has four kinds of nitrogenous bases: 1. adenine (A), 2. guanine (G), 3. cytosine (C), 4. thymine (T).

5. Lesson Overview Lesson Overview The Structure of DNA Chargaff’s Rules? Erwin Chargaff discovered that the percentages of adenine (A) and thymine (T) bases are always equal in any sample of DNA. The same thing is true for the other two nucleotides as well, guanine (G) and cytosine (C). The observation that A = T and G = C became known as one of “Chargaff’s rules.”

6. Lesson Overview Lesson Overview The Structure of DNA Franklin’s X-Rays In the 1950s, Rosalind Franklin used a technique called X-ray diffraction to figure out the structure of DNA.

7. Lesson Overview Lesson Overview The Structure of DNA Along came Watson and Crick At the same time, James Watson and Francis Crick were also trying to understand the structure of DNA. Early in 1953, Watson was shown a copy of Franklin’s X- ray pattern. The clues in this picture enabled them to build an accurate model of DNA.

8. Lesson Overview Lesson Overview The Structure of DNA The Double-Helix Model A double helix looks like a twisted ladder. The double helix accounted for Franklin’s X-ray pattern and explains Chargaff’s rule of base pairing and how the two strands of DNA are held together.

9. Lesson Overview Lesson Overview The Structure of DNA Antiparallel Strands In the double-helix model, the two strands of DNA are “antiparallel”—they run in opposite directions. This arrangement enables the nitrogenous bases on both strands to come into contact at the center of the molecule.

10. Lesson Overview Lesson Overview The Structure of DNA Hydrogen Bonding Hydrogen bonds are weak chemical bonds that allow the two strands of the DNA to separate.

11. True or False DNA is a nucleic acid made up of nucleosomes joined into long strands or chains by covalent bonds. _________________________ The nucleotides within a DNA strand may have different nitrogenous bases, but all have identical sugar molecules. ________________________________ Chargaff contributed to the understanding of DNA by figuring out that the percentages of adenine and thymine were almost equal in any sample of DNA. _________________________

12. Answers DNA is a nucleic acid made up of nucleosomes joined into long strands or chains by covalent bonds. False, nucletotides The nucleotides within a DNA strand may have different nitrogenous bases, but all have identical sugar molecules. True Chargaff contributed to the understanding of DNA by figuring out that the percentages of adenine and thymine were almost equal in any sample of DNA. True