1. DNA
Double Helix

2. What are the chemical components of a DNA nucleotide?
a phosphate (or 3 phosphates)
A sugar (deoxyribose)
A nitrogenous base
(Adenine, Thymine, Guanine, Cytosine)

3. Nucleotides
Pyrimidines
Purines

4. 2. The instructions for the traits of an organism are determined by The order of nucleotides in a DNA molecule

5. 3. The two strands of a DNA molecule are held together by hydrogen bonds between the Nitrogenous bases on each strand

6. In the diagram below, strands I and II represent the two complementary strands of a portion of a DNA double helix. The sequence of strand I is indicated below. What is the sequence of strand II?
Strand I ----C-A-T-A-C----
Strand II ----?--?--?--?--?----
GTATG

7. Patterns of Base Pairing
Bases in DNA strands can pair in only one way:
A always pairs with T; G always pairs with C.
The sequence of bases is the genetic code, and variation in base sequences gives life diversity.
Base-pairing in DNA follows Chargaff’s first rule. A always pairs with T. G always pairs with C. The sequence of bases (order of A, C, G, and T) is what determines the genetic code. All organisms use the same genetic code, but there is enough variation in the code to be responsible for all of life’s diversity.

8. 5. In the 1950’s when Watson and Crick were working on their model of DNA, many scientists did not think that DNA carried the genetic code.
What was the other type of molecule that some scientists thought might carry genetic information?
proteins
Why did this other type of molecule seem like a likely candidate?
proteins and DNA were found in the nucleus of the cell
proteins are much more complex and diverse than DNA molecules.

9. Frederick Griffith’s Experiments
Study: make a vaccine for pneumonia.
Method: two strains of bacterium Streptococcus pneumoniae on mice (Mus musculus).
Result: transformation of bacterium was permanent and heritable.
What knowledge eventually led to Watson and Crick’s discovery? The next few slides will describe some of these experiments and what was learned from them.
We start with Griffith’s experiment. He was not trying to discover the structure of DNA. He was attempting to make a vaccine for pneumonia and stumbled across some interesting information. Vaccines are often composed of weakened or dead forms of the particular disease-causing bacteria. Sometimes harmless bacteria are used instead. The goal is to stimulate the immune system to produce antibodies to protect the body when it is exposed to that particular pathogen again.
Griffith injected mice with a harmless strain of living bacteria cells and compared them to mice injected with a killer strain of bacteria. The mice injected with the harmless strain lived, but the mice injected with the killer strain died.
Then Griffith injected mice with dead bacteria cells of the killer strain. These mice lived. None of these results so far are unusual.
The next thing he did, however, was inject mice with a combination of harmless living bacteria and dead killer bacteria. In this part of the experiment, the mice died. Neither of these things alone killed the mice in the previous trials, but together, something different was occurring.
Griffith concluded that there was something in the dead bacteria that could transform the living bacteria cells. This factor was also shown to be heritable (meaning that new bacteria cells that divided from these harmless bacteria now possessed the same trait as the killer bacteria cells).

10. Griffith’s Experiment
6. The mice died of the same disease caused by the pathogenic bacteria. Based on this information, which statement would be a valid conclusion? When DNA from one organism is transferred to another organism, it can give new traits to the second organism.

11. Chargaff’s Results

12. Chargaff’s Results
Which of the following observations can be supported by the data in the table? X

13. Chargaff’s Results
B. In one or two sentences, explain how these observations helped Watson and Crick build their model of DNA.

14. Chargaff’s Results
C. In one or two sentences, explain why the proportions of nitrogenous bases in the DNA of different human tissues (thymus and sperm) are about the same.
Hydrogen Bonding between
A and T
C and G

15. Chargaff’s Rules
Rule 1 - The amounts of thymine and adenine in DNA are the same, and the amounts of cytosine and guanine are the same: A = T and G = C.
Rule 2 - The proportion of adenine and guanine differs among species.
This molecular diversity among species helped establish DNA as a more credible candidate for the genetic material than protein.
Hopefully, in the previous slide, you noticed that there were approximately equal proportions of Adenine to Thymine and Guanine to Cytosine. This observation has become what now call “Chargaff’s first rule.” In any organism studied, this rule applies – the amount of A equals the amount of T and the amount of C equals the amount of G.
Chargaff’s second rule states that the amount of A and G differes amoung species (which would mean the amount of T and C also differs).
You can observe rule 2 in the table on the right.

16. Rosalind Franklin

17. Rosalind Franklin Pyrimidines hydrogen bond with purines Purines

18. Rosalind Franklin
C. Was this information consistent with the data obtained by Chargaff (question 7)?