1. The Molecular Basis of Inheritance
Chapter 16
AP Biology

2. Search For The Genetic Material
Once chromosomes were known to carry genes, the question became which of two organic compounds make up chromosomes.
Was DNA or protein the genetic material?
In 1952, Alfred Hershey and Martha Chase showed that DNA must be the genetic material.

3. Phage - virus that attacks bacteria and reprograms host to produce more viruses.

4. Hershey & Chase Experiment

5. Viruses Infecting Bacteria

6. Questions About DNA Structure
James Watson and Francis Crick were the first to solve the puzzle of DNA structure.
Maurice Wilkins and Rosalind Franklin contributed to their success.
X-ray crystallography produced a rough blueprint of the molecule.

7. Rosalind Franklin’s Photo of DNA
Found distance across the molecule with this x-ray diffraction data

8. Purines – Pure As Gold (2 gold wedding rings)

11. Watson and Crick’s Model
Determined three major features of DNA
DNA is a double helix (with Franklin’s image).
The nitrogenous bases are A (adenine), T (thymine), C (cytosine), and G (guanine). In DNA, A only pairs with T and C only pairs with G.
The strands are antiparallel. They run in opposite, upside-down directions.

12. How does DNA Replicate?

13. How does DNA Replicate? Meselson – Stahl, late 1950’s
Grew bacteria on two isotopes of N.
Started on 15N, switched to 14N.
Looked at weight of DNA after one, then 2 rounds of replication.

16. DNA Replication DNA replication is semiconservative.
This means at the end of replication, each of the daughter molecules has one old strand, derived from the parent strand of DNA, and one strand that is newly synthesized.

17. Replication - Preview
DNA splits by breaking the H-bonds between the backbones.
Then DNA builds the missing backbone using the bases on the old backbone as a template.

20. How does DNA replicate? Priming
DNA pol III cannot initiate DNA synthesis.
Nucleotides can be added only to an existing chain called a Primer.

21. Primer Make of RNA. 10 nucleotides long.
Added to DNA by an enzyme called Primase.
DNA is then added to the RNA primer.

22. Priming
A primer is needed for each DNA elongation site.

24. Anti-Parallel Alignment

25. Problem of Antiparallel DNA
The two DNA strands run antiparallel to each other.
DNA can only elongate in the 5’--> 3’ direction.

27. Leading Strand
Continuous replication toward the replication fork in the 5’--> 3’ direction.

28. Lagging Strand Discontinuous synthesis away from the replication fork.
Replicated in short segments as more template becomes opened up.

29. Okazaki Fragments
Short segments ( bases) that are made on the lagging strand.
All Okazaki fragments must be primed.
RNA primer is removed after DNA is added.

31. DNA Replication – Major Points
The replication of DNA begins at sites called the origins of replication.
Initiation proteins bind to the origin of replication and separate the two strands, forming a replication bubble. DNA replication proceeds in both directions along the DNA strand.
A group of enzymes called DNA polymerases catalyze the elongation of new DNA.
DNA polymerase adds nucleotides working in the 5’ to 3’ direction.

32. DNA Replication
The strand running in the 5’ to 3’ direction is called the leading strand. Replication occurs continuously along this strand.
The other strand running in the 3’ to 5’ direction is called the lagging strand. It is synthesized in separate fragments called Okazaki fragments, which are then sealed together by DNA ligase, forming a continuous DNA strand.

33. DNA replication animation

34. Summary of DNA Replication
Overview
Origin of replication
Leading strand
Lagging strand
Leading strand
Lagging strand
Single-strand binding protein
Overall directions of replication
Helicase
Leading strand 5
DNA pol III 3 3
Primer
Primase 5
Parental DNA 3
DNA pol III
Lagging strand 5
DNA pol I
DNA ligase 4 3 5 3 2 1 3 5

35. Enzymes DNA pol I - replaces RNA primers with DNA nucleotides.
DNA pol III – adds nucleotides in the 5’ -> 3’ direction
DNA Ligase - joins all DNA fragments together.

36. Other Proteins in Replication
Topoisomerase – relieves strain ahead of replication forks.
Helicase - unwinds the DNA double helix.
Single-Strand Binding Proteins - help hold the DNA strands apart.

37. DNA animation DNAi

38. Energy for Replication
From the triphosphate monomers.
Loses two phosphates as each monomer is added.

40. DNA Replication Error Rate
1 in 1 billion base pairs.
About 3 mistakes in our DNA each time it’s replicated.

41. Proofreading and Repairing DNA
During DNA replication, DNA polymerases proofread each nucleotide against its template as soon as it is added to the growing chain.
An incorrectly paired nucleotide is removed and the correct one inserted.
This action is similar to hitting the “delete” key and then entering the correct letter.

42. Mismatch Repair
Mismatched nucleotides sometimes evade proofreading by DNA polymerases.
Special repair enzymes fix incorrectly paired nucleotides.
If one of these enzymes was structured incorrectly (from a mutation), what would be the result?

43. Nucleotide Repair
Damage in skin cells due to ultraviolet radiation.
Repair prevents the onset of skin cancer.

44. Nucleotide Excision Repair
Incorrectly paired or altered nucleotides can also arise after replication.
DNA bases can undergo spontaneous chemical changes under normal conditions, but certain environmental conditions can also contribute to change.
Nucleases which are DNA cutting enzymes, cut out the damaged part and replace it with the correct nucleotides.

45. Telomeres
Eukaryotic chromosomes have special nucleotide sequences at their ends.
These are multiple repetitions ( ) of
one short nucleotide sequence (TTAGGG in humans).
These sequences are called telomeres.
They do not prevent shortening of the DNA molecule in replication, they just postpone the erosion of genes near the ends of the molecule.

46. Shortening of the Ends of Linear DNA Molecules
With each round of replication the DNA molecule becomes shorter and shorter.
This may limit the number of times a cell can divide.

47. Telomerase What prevents the loss of genes is gametes?
Germ cells (which give rise to sex cells) produce an enzyme called telomerase.
Telomerase catalyzes the lengthening of telomeres in the germ cells.
Zygote receives chromosomes with the maximum length of telomeres.
Telomerase is not active in body cells.

48. Telomerase in Cancer Cells
Cancer cells produce the enzyme telomerase.
This prevents the erosion of the ends of chromosomes and allows for unlimited cell division.

50. Nucleosomes

51. Chromatin Packing

52. Effects of DNA Packing
As DNA becomes more highly packed, it becomes less accessible for transcription.
This is one way gene expression is controlled in cells.
DNA methylation
DNA acetylation
Evidence of effect: Barr bodies (inactivated X chromosome in females) is highly packed.