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DESCRIPTION
DNATRANSCRIPT
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
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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