1. 9
DNA and Its Role in Heredity: identification, structure and replication

2. Chapter 9 DNA and Its Role in Heredity
Key Concepts
9.1 DNA Structure Reflects Its Role as the Genetic Material
9.2 DNA Replicates Semiconservatively
9.3 Mutations Are Heritable Changes in DNA

3. Concept 9.1 DNA Structure Reflects Its Role as the Genetic Material
Scientists had criteria for DNA to be accepted as the genetic material, including that it:
Be present in the cell nucleus and in chromosomes
Doubles in the cell cycle
Is twice as abundant in diploid cells
Has same the pattern of transmission as its genetic information

4. Concept 9.1 DNA Structure Reflects Its Role as the Genetic Material
DNA was found in the nucleus by Miescher (1868).
He isolated cell nuclei and treated them chemically.
A fibrous substance came out of the solution and he called it “nuclein”.
DNA was found in chromosomes using dyes that bind specifically to DNA.

5. In-Text Art, Ch. 9, p. 166

6. Concept 9.1 DNA Structure Reflects Its Role as the Genetic Material
Dividing cells were stained and passed through a flow cytometer, confirming two other predictions for DNA:
Nondividing cells have the same amount of nuclear DNA.
After meiosis, gametes have half the amount of DNA.

7. Figure 9.1 DNA in the Nucleus and in the Cell Cycle (Part 2)

8. Figure 16.1 Transformation of bacteria
Griffith, 1928

9. So what causes transformation?
Avery, McCarty, and MacLeod – studied this for 14 years
1944 – DNA is the transforming agent
Evidence?
Purification of cellular components from heat-killed bacteria, test for transforming power.

10. Concept 9.1 DNA Structure Reflects Its Role as the Genetic Material
In 1950 Erwin Chargaff found that in the DNA from many different species:
Amount of A = amount of T
Amount of C = amount of G
Or, the abundance of purines = the abundance of pyrimidines—Chargaff’s rule.

11. Concept 9.1 DNA Structure Reflects Its Role as the Genetic Material
Chromosomes contain DNA, but also contain proteins, so scientists had to determine whether proteins carried genetic information.
Viruses, such as bacteriophages, contain DNA and a little protein.
When a virus infects a bacterium, it injects only its DNA into it, and changes the genetic program of the bacterium.
This provides further evidence for DNA, and not protein, as the genetic material.
ANIMATED TUTORIAL 9.1 The Hershey-Chase Experiment

12. Figure 9.2 Viral DNA and Not Protein Enters Host Cells (Part 1)

13. Figure 16.2b The Hershey-Chase experiment - 1952

14. Figure 9.2 Viral DNA and Not Protein Enters Host Cells (Part 2)

15. Figure 9.4 X-Ray Crystallography Helped Reveal the Structure of DNA (Part 1)

16. Figure 9.4 X-Ray Crystallography Helped Reveal the Structure of DNA (Part 2)

17. Concept 9.1 DNA Structure Reflects Its Role as the Genetic Material
Chemical composition also provided clues:
DNA is a polymer of nucleotides: deoxyribose, a phosphate group, and a nitrogen-containing base.
The bases form the differences:
Purines: adenine (A), guanine (G)
Pyrimidines: cytosine (C), thymine (T)
See Figure 3.1

18. Composition of DNA

19. Figure 9.5 DNA Is a Double Helix (Part 1)

20. Concept 9.1 DNA Structure Reflects Its Role as the Genetic Material
Watson and Crick suggested that:
Nucleotide bases are on the interior of the two strands, with a sugar-phosphate backbone on the outside.
Per Chargaff’s rule, a purine on one strand is paired with a pyrimidine on the other.
These base pairs (A-T and G-C) have the same width down the helix.

21. Concept 9.1 DNA Structure Reflects Its Role as the Genetic Material
Four key features of DNA structure:
It is a double-stranded helix of uniform diameter.
It is right-handed.
It is antiparallel.
Outer edges of nitrogenous bases are exposed in the major and minor grooves.
See Figure 3.4

22. Figure 9.5 DNA Is a Double Helix (Part 2)

23. Concept 9.1 DNA Structure Reflects Its Role as the Genetic Material
Grooves exist because the backbones of the DNA strands are not evenly spaced relative to one another.
The exposed outer edges of the base pairs are accessible for hydrogen bonding.
Surfaces of A-T and G-C base pairs are chemically distinct.
Binding of proteins to specific base pair sequences is key to DNA–protein interactions, and necessary for replication and gene expression.
See Figure 9.5B

24. Figure 9.6 Base Pairs in DNA Can Interact with Other Molecules

25. Concept 9.1 DNA Structure Reflects Its Role as the Genetic Material
DNA has four important functions—
double-helical structure is essential:
Storage of genetic information—millions of nucleotides; base sequence encodes huge amounts of information
Precise replication during cell division by complementary base pairing

26. Concept 9.1 DNA Structure Reflects Its Role as the Genetic Material
Susceptibility to mutations—a change in information—possibly a simple alteration to a sequence
Expression of the coded information as the phenotype—nucleotide sequence is transcribed into RNA and determines sequence of amino acids in proteins
LINK Transcription and translation are described in more detail in Concepts 10.2 and 10.4
See Chapter 3
See Figure 3.5

27. Mechanism called semiconservative.
How does DNA replicate?
Watson and Crick suggested that each strand could act as a template for forming a new complementary chain.
Mechanism called semiconservative.
Other possibilities, conservative or dispersive exist.

28. Conservative
Semiconservative
Dispersive
How to test?

29. Meselson-Stahl – late 1950’s
Replication is Semiconservative!

30. Concept 9.2 DNA Replicates Semiconservatively
Two steps in DNA replication:
The double helix is unwound, making two template strands available for new base pairing.
New nucleotides form base pairs with template strands and linked together by phosphodiester bonds. Template DNA is read in the 3′-to-5′ direction.
VIDEO 9.2 Cell Visualization: From DNA to chromosomes

31. Concept 9.2 DNA Replicates Semiconservatively
During DNA synthesis, new nucleotides are added to the 3′ end of the new strand, which has a free hydroxyl group (—OH).
Deoxyribonucleoside triphosphates (dNTPs), or deoxyribonucleotides, are the building blocks—two of their phosphate groups are released and the third bonds to the 3′ end of the DNA chain.
ANIMATED TUTORIAL 9.3 DNA Replication, Part 1: Replication of a Chromosome and DNA Polymerization
See Concept 3.1
See Figure 3.2

32. Figure 9.7 Each New DNA Strand Grows by the Addition of Nucleotides to Its 3′ End

33. Figure 9.8 The Origin of DNA Replication (Part 1)

34. Figure 9.8 The Origin of DNA Replication (Part 2)

35. Concept 9.2 DNA Replicates Semiconservatively
DNA replication begins with a short primer—a starter strand.
The primer is complementary to the DNA template.
Primase—an enzyme—synthesizes DNA one nucleotide at a time.
DNA polymerase adds nucleotides to the 3′ end.

36. Figure 9.9 DNA Forms with a Primer

37. Concept 9.2 DNA Replicates Semiconservatively
DNA polymerases are larger than their substrates, the dNTPs, and the template DNA.
The enzyme is shaped like an open right hand—the “palm” brings the active site and the substrates into contact.
The “fingers” recognize the nucleotide bases.

38. Figure 9.10 DNA Polymerase Binds to the Template Strand (Part 1)

39. Figure 9.10 DNA Polymerase Binds to the Template Strand (Part 2)

40. Concept 9.2 DNA Replicates Semiconservatively
A single replication fork opens up in one direction.
The two DNA strands are antiparallel— the 3′ end of one strand is paired with the 5′ end of the other.
DNA replicates in a 5′-to-3′ direction.

41. Figure 9.11 The Two New Strands Form in Different Ways

42. Concept 9.2 DNA Replicates Semiconservatively
One new strand, the leading strand, is oriented to grow at its 3′ end as the fork opens.
The lagging strand is oriented so that its exposed 3′ end gets farther from the fork.
Synthesis of the lagging strand occurs in small, discontinuous stretches—Okazaki fragments.
ANIMATED TUTORIAL 9.4 DNA replication, Part 2: Coordination of Leading and Lagging Strand Synthesis
APPLY THE CONCEPT DNA replicates semiconservatively

43. Figure 9.11 The Two New Strands Form in Different Ways

44. Figure 9.12 The Lagging Strand Story (Part 1)

45. Figure 9.12 The Lagging Strand Story (Part 2)

46. Figure 9.12 The Lagging Strand Story (Part 3)

47. Concept 9.2 DNA Replicates Semiconservatively
DNA polymerase works very fast:
It is processive—it catalyzes many sequential polymerization reactions each time it binds to DNA
See Chapter 3

48. Concept 9.2 DNA Replicates Semiconservatively
Okazaki fragments are added to RNA primers to replicate the lagging strand.
When the last primer is removed no DNA synthesis occurs because there is no 3′ end to extend—a single-stranded bit of DNA is left at each end.
These are cut after replication and the chromosome is slightly shortened after each cell division.

49. Figure 9.13 Telomeres and Telomerase (Part 1)

50. Concept 9.2 DNA Replicates Semiconservatively
Telomeres are repetitive sequences at the ends of eukaryotic chromosomes.
These repeats prevent the chromosome ends from being joined together by the DNA repair system.
Telomerase contains an RNA sequence—it acts as a template for telomeric DNA sequences.
Telomeric DNA is lost over time in most cells, but not in continuously dividing cells like bone marrow and gametes.

51. Figure 9.13 Telomeres and Telomerase (Part 2)

52. Concept 9.2 DNA Replicates Semiconservatively
DNA polymerases can make mistakes in replication, but most errors are repaired.
Cells have two major repair mechanisms:
Proofreading—as DNA polymerase adds nucleotides, it has a proofreading function and if bases are paired incorrectly, the nucleotide is removed.
Mismatch repair—after replication other proteins scan for mismatched bases missed in proofreading, and replace them with correct ones.

53. Figure 9.14 DNA Repair Mechanisms (Part 1)

54. Figure 9.14 DNA Repair Mechanisms (Part 2)