Chapter 12-1 DNA(deoxyribonucleic acid)

DNA function

• Nucleic acid• Store and transmit

hereditary information

Experimental Proof of DNA as Genetic Material...

1) Griffith’s Transformation Experiments

1) Griffith’s Transformation Experiments

• What two strains of bacteria did Griffith use?– S (smooth) strain –cause pneumonia– R (rough) strain –harmless

• What happened in the fourth experiment?– The mice died. There were live S strains found in the mice

body. • What can be concluded from the experiment?– Cell components in the dead S strains transformed the live R

strains to live S strains

2) Oswald Avery experiment

• The transforming factor is DNA

3) The Hershey-Chase’s bacteriophages experiment

• Bacteriophages – viruses that infect bacteria– consist of protein coat and a

DNA or RNA core– inject their hereditary

material into bacteria

3) The Hershey-Chase’s bacteriophages experiment

virus particle labeled with 35S

virus particle labeled with 32P

bacterial cell (cutaway view)

label outside cell

label inside cell

• Radioactive Markers– Radioactive

isotope – 32P could only

mark DNA – 35S could only

mark protein coat

• Made by nucleotide– 5-carbon sugar

(deoxyribose)– phosphate group– nitrogenous bases• purine (2 rings): adenine

, guanine;• pyrimidine (1 ring):

cytosine , thymine

DNA

14

23

5

14

23

5

Purines Pyrimidines

Adenine Guanine Cytosine Thymine

Phosphate group Deoxyribose

12

345

12

345

12

345

12

345

DNA Structure

• Chargaff’s Rule :– Amount of adenine = amount of thymine, amount of

guanine = amount of cytosine

– A=T and G=C

DNA Structure

• Rosalind Franklin– Used x-ray diffraction to

examine DNA fibers

– Concluded that DNA was helix

Rosalind Franklin's X-ray diffraction photograph of DNA, 1953Photo: courtesy HarperCollins

DNA Structure

• Watson and Crick’s model– DNA have two strands run in

opposite directions

– Strands are held together by

hydrogen bonds

– A binds with T and C with G

– Molecule is a double helixHydrogen bond

DNA Structure

Hydrogen bonds

Nucleotide

Sugar-phosphate backbone

Key

Adenine (A)

Thymine (T)

Cytosine (C)

Guanine (G)

Francis H. C. CrickJames D. Watson Wilkins

Rosalind Franklin

Questions:

• 20% A in DNA strands, how many C ?

• A=T=20%,• A+T+G+C=100% • G=C, • C=30%

I II IIIA. 3′ end purine hydrogen bondB. 5′ end pyrimidine covalent bondC. 3′ end pyrimidine hydrogen bondD. 5′ end purine covalent bond

The drawing below shows a short section of a DNA molecule. What is labelled by I, II and III?

I

II

III