1. DNA and Its Role in Heredity9
DNA and Its Role in Heredity

2. Figure 9.4 X-Ray Crystallography Helped Reveal the Structure of DNA
Rosalind Franklin
X-ray crystallography: positions of atoms can be inferred from the diffraction pattern of X rays passing through the substance
Figure 9.4 X-Ray Crystallography Helped Reveal the Structure of DNA (A) The positions of atoms in a crystallized chemical substance can be inferred by the pattern of diffraction of X rays passed through it. The pattern of DNA is both highly regular and repetitive. (B) Rosalind Franklin’s crystallographs helped other scientists visualize the helical structure of the DNA molecule.

3. Concept 9.1 DNA Structure Reflects Its Role as the Genetic Material
Chemical composition:
Biochemists knew that nucleotides consisted of the sugar deoxyribose, a phosphate group, and nitrogen-containing bases:
Purines: adenine (A) and guanine (G)
Pyrimidines: cytosine (C) and thymine (T)

4. Concept 9.1 DNA Structure Reflects Its Role as the Genetic Material
In 1950, Erwin Chargaff found the amount of A always equaled the amount of T, and amount of G always equaled the amount of C.

5. Concept 9.1 DNA Structure Reflects Its Role as the Genetic Material
3-D model building:
Francis Crick and James Watson combined all the knowledge of DNA to determine its structure.
Franklin’s X-ray crystallography convinced them the molecule was helical.
Density measurements suggested there are two polynucleotide chains in the molecule.
Modeling showed that DNA strands must be antiparallel.

6. Figure 9.5 DNA Is a Double Helix (Part 1)
Figure 9.5 DNA Is a Double Helix (A) Francis Crick (right) and James Watson (left) proposed that the DNA molecule has a double-helical structure. (B) Biochemists can now pinpoint the position of every atom in a DNA molecule and have verified the essential features of the Watson–Crick model. Two sugar–phosphate chains provide the outer backbones of the double helix, with hydrogen-bonded nitrogenous bases forming “rungs” within the structure.

7. Figure 9.5 DNA Is a Double Helix (Part 2)
Figure 9.5 DNA Is a Double Helix (A) Francis Crick (right) and James Watson (left) proposed that the DNA molecule has a double-helical structure. (B) Biochemists can now pinpoint the position of every atom in a DNA molecule and have verified the essential features of the Watson–Crick model. Two sugar–phosphate chains provide the outer backbones of the double helix, with hydrogen-bonded nitrogenous bases forming “rungs” within the structure.

8. Concept 9.1 DNA Structure Reflects Its Role as the Genetic Material
Watson and Crick suggested that:
The bases are on the interior of the two strands, with a sugar-phosphate backbone on the outside.

9. Concept 9.1 DNA Structure Reflects Its Role as the Genetic Material
The two strands are antiparallel.
In the sugar–phosphate backbone, the phosphate groups are bonded to the 5ʹ carbon of one sugar and the 3ʹ carbon of the next.

10. Figure 3.4 DNA
Figure 3.4 DNA (A) DNA usually consists of two strands running in opposite directions that are held together by base pairing between purines and pyrimidines opposite one another on the two strands. (B) The two antiparallel strands in a DNA molecule are twisted into a double helix.

11. Concept 9.1 DNA Structure Reflects Its Role as the Genetic Material
Per Chargaff’s rule, a purine on one strand is paired with a pyrimidine on the other, making the base pairs (A–T and G–C) the same width down the helix.

12. Concept 9.1 DNA Structure Reflects Its Role as the Genetic Material
Key features of DNA structure:
Double-stranded helix of uniform diameter
The chains are held together by hydrogen bonds between the base pairs and by van der Waals forces between adjacent bases on the same strand.

13. Covalent phosphodiester bonds
Figure 3.4 DNA
H-bonds
Covalent phosphodiester bonds
Figure 3.4 DNA (A) DNA usually consists of two strands running in opposite directions that are held together by base pairing between purines and pyrimidines opposite one another on the two strands. (B) The two antiparallel strands in a DNA molecule are twisted into a double helix.

14. Purines Pyrimidines Pairing adenine (A) guanine (G) thymine (T)
Base pairing in DNA
Purines
adenine (A)
guanine (G)
Pyrimidines
thymine (T)
cytosine (C)
Pairing
A : T
2 bonds
C : G
3 bonds

15. Concept 9.1 DNA Structure Reflects Its Role as the Genetic Material
The outer edges of the nitrogenous bases are exposed in major and minor grooves.
The grooves exist because the helices are not evenly spaced.
There are 4 possible configurations of the flat, hydrogen-bonded base pairs within the major and minor grooves.

16. Figure 9.5 DNA Is a Double Helix (Part 2)
Figure 9.5 DNA Is a Double Helix (A) Francis Crick (right) and James Watson (left) proposed that the DNA molecule has a double-helical structure. (B) Biochemists can now pinpoint the position of every atom in a DNA molecule and have verified the essential features of the Watson–Crick model. Two sugar–phosphate chains provide the outer backbones of the double helix, with hydrogen-bonded nitrogenous bases forming “rungs” within the structure.

17. Figure 9.6 Base Pairs in DNA Can Interact with Other Molecules (Part 1)
Figure 9.6 Base Pairs in DNA Can Interact with Other Molecules These diagrams show the four possible configurations of base pairs within the double helix. Atoms shaded in purple are available for hydrogen bonding with other molecules. These differences in configuration allow proteins to recognize specific DNA sequences.

18. Figure 9.6 Base Pairs in DNA Can Interact with Other Molecules (Part 2)
Figure 9.6 Base Pairs in DNA Can Interact with Other Molecules These diagrams show the four possible configurations of base pairs within the double helix. Atoms shaded in purple are available for hydrogen bonding with other molecules. These differences in configuration allow proteins to recognize specific DNA sequences.