1. Ch. 16 Warm-Up Draw and label a nucleotide. Why is DNA a double helix?
What is the complementary DNA strand to:
DNA: A T C C G T A T G A A C

2. Ch. 16 Warm-Up
What was the contribution made to science by these people:
Hershey and Chase
Franklin
Watson and Crick
Chargaff’s Rules: If cytosine makes up 22% of the nucleotides, then adenine would make up ___ % ?
Explain the semiconservative model of DNA replication.

3. Ch. 16 Warm-Up What is the function of the following: Helicase
DNA Ligase
DNA Polymerase (I and III)
Primase
Nuclease
How does DNA solve the problem of slow replication on the lagging strand?
Code the complementary DNA strand: 3’ T A G C T A A G C T A C 5’
What is the function of telomeres?

4. THE MOLECULAR BASIS OF INHERITANCE
Chapter 16

5. What you must know The structure of DNA.
The major steps to replication.
The difference between replication, transcription, and translation.
The general differences between the bacterial chromosome and eukaryotic chromosomes.
How DNA is packaged into a chromosome.

6. Problem:
Is the genetic material of organisms made of DNA or proteins?

7. Frederick Griffith (1928) HOW?
S strain of bacteria = pathogenic (causes pneumonia in mice)
R strain of bacteria = non-pathogenic
HOW?

8. The “Transforming Principle”
mix heat-killed pathogenic & non-pathogenic
bacteria
live pathogenic
strain of bacteria
live non-pathogenic
strain of bacteria
heat-killed pathogenic bacteria A. B. C. D.
mice die
mice live
mice live
mice die
Transformation = change in phenotype
something in heat-killed bacteria could still transmit disease-causing properties

9. Frederick Griffith (1928)
Conclusion: living R bacteria transformed into deadly S bacteria by an unknown, heritable substance (R bacteria must have acquired something from the S bacteria)
- Still unclear WHAT the transforming, heritable substance was…until…
Oswald Avery, et al. (1944)
Discovered that the transforming agent was DNA… But still skepticism from scientific community

10. Hershey and Chase (1952)
To answer the question: Is DNA or PROTEIN the heritable substance that determines traits?
Bacteriophages: virus that infects bacteria; composed of DNA and protein
Protein = radiolabel S (radioactive sulfur label)
DNA = radiolabel P
(radioactive phosphorus label)

11. Hershey & Chase Which radioactive marker is found inside the cell?
Protein coat labeled
with 35S
DNA labeled with 32P
Hershey & Chase
T2 bacteriophages
are labeled with
radioactive isotopes
S vs. P
bacteriophages infect
bacterial cells
bacterial cells are agitated
to remove viral protein coats
Which radioactive marker is found inside the cell?
Which molecule carries viral genetic info?
35S radioactivity
found in the medium
32P radioactivity found in the bacterial cells

12. Hershey and Chase (1952)

13. Hershey and Chase (1952)
Conclusion: DNA entered infected bacteria  DNA must be the genetic material!

14. Hershey & Chase 1952 | 1969 Martha Chase Alfred Hershey Hershey
Martha Cowles Chase (1927 – August 8, 2003) was a young laboratory assistant in the early 1950s when she and Alfred Hershey conducted one of the most famous experiments in 20th century biology. Devised by American bacteriophage expert Alfred Hershey at Cold Spring Harbor Laboratory New York, the famous experiment demonstrated the genetic properties of DNA over proteins. By marking bacteriophages with radioactive isotopes, Hershey and Chase were able to trace protein and DNA to determine which is the molecule of heredity.
Hershey and Chase announced their results in a 1952 paper. The experiment inspired American researcher James D. Watson, who along with England's Francis Crick figured out the structure of DNA at the Cavendish Laboratory of the University of Cambridge the following year.
Hershey shared the 1969 Nobel Prize in Physiology or Medicine with Salvador Luria and Max Delbrück. Chase, however, did not reap such rewards for her role. A graduate of The College of Wooster in Ohio (she had grown up in Shaker Heights, Ohio), she continued working as a laboratory assistant, first at the Oak Ridge National Laboratory in Tennessee and then at the University of Rochester before moving to Los Angeles in the late 1950s. There she married biologist Richard Epstein and earned her Ph.D. in 1964 from the University of Southern California. A series of personal setbacks through the 1960s ended her career in science. She spent decades suffering from a form of dementia that robbed her of short-term memory. She died on August 8, 2003.
Martha Chase
Alfred Hershey

15. Edwin Chargaff (1947) Chargaff’s Rules:
DNA composition varies between species
Ratios:
%A = %T and
%G = %C

16. Rosalind Franklin (1950’s)
Worked with Maurice Wilkins
X-ray crystallography = images of DNA
Provided measurements on chemistry of DNA

17. James Watson & Francis Crick (1953)
Discovered the double helix by building models to conform to Franklin’s X-ray data and Chargaff’s Rules.

18. 1953 article in Nature
Watson and Crick
Watson
Crick

19. Structure of DNA DNA = double helix “Backbone” = sugar + phosphate
“Rungs” = nitrogenous bases

20. Structure of DNA Nitrogenous Bases Pairing: Adenine (A) Guanine (G)
Thymine (T)
Cytosine (C)
Pairing:
purine + pyrimidine
A = T
G Ξ C
purine
pyrimidine

21. Structure of DNA
Hydrogen bonds between base pairs of the two strands hold the molecule together like a zipper.

22. Structure of DNA
Antiparallel: one strand (5’ 3’), other strand runs in opposite, upside-down direction (3’  5’)

25. DNA Comparison Double-stranded Circular One chromosome In cytoplasm
Prokaryotic DNA
Eukaryotic DNA
Double-stranded
Circular
One chromosome
In cytoplasm
No histones
Supercoiled DNA
Double-stranded
Linear
Usually 1+ chromosomes
In nucleus
DNA wrapped around histones (proteins)
Forms chromatin

26. Problem:
How does DNA replicate?

27. Replication: Making DNA from existing DNA
3 alternative models of DNA replication
Newly made DNA is light blue

28. Pulse-Chase Primer: The Meselson-Stahl Experiment
Read the Introduction

38. Meselson & Stahl

39. Meselson & Stahl

40. Replication is semiconservative

41. DNA Replication Video
Amoeba Sisters – with handout

43. Major Steps of Replication:
Helicase: unwinds DNA at origins of replication
Initiation proteins separate 2 strands  forms replication bubble
Primase: puts down RNA primer to start replication
DNA polymerase III: adds complimentary bases to leading strand (new DNA is made 5’  3’)
Lagging strand grows in 3’5’ direction by the addition of Okazaki fragments
DNA polymerase I: replaces RNA primers with DNA
DNA ligase: seals fragments together

44. 1. Helicase unwinds DNA at origins of replication and creates replication forks

45. 3. Primase adds RNA primer

46. 4. DNA polymerase III adds nucleotides in 5’3’ direction on leading strand

47. Replication on leading strand

48. Leading strand vs. Lagging strand

49. Okazaki Fragments: Short segments of DNA that grow 5’3’ that are added onto the Lagging Strand
DNA Ligase: seals together fragments

52. Proofreading and Repair
DNA polymerases proofread as bases added
Mismatch repair: special enzymes fix incorrect pairings
Nucleotide excision repair:
Nucleases cut damaged DNA
DNA poly and ligase fill in gaps

53. Nucleotide Excision Repair
Errors:
Pairing errors: 1 in 100,000 nucleotides
Complete DNA: 1 in 10 billion nucleotides

54. Problem at the 5’ End DNA poly only adds nucleotides to 3’ end
No way to complete 5’ ends of daughter strands
Over many replications, DNA strands will grow shorter and shorter

55. Telomeres: repeated units of short nucleotide sequences (TTAGGG) at ends of DNA
Telomeres “cap” ends of DNA to postpone erosion of genes at ends (TTAGGG)
Telomerase: enzyme that adds to telomeres
Eukaryotic germ cells, cancer cells
Telomeres stained orange at the ends of mouse chromosomes

56. Telomeres & Telomerase

57. DNA Replication Video
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58. BioFlix: DNA Replication
io/bioflix/bioflix.htm?8apdnarep