AIM

What is the structure of DNA?

DNA

• Deoxyribonucleic Acid• The material that contains the information

that determines inherited characteristics

Gene

• A segment of DNA that is located on a chromosome and codes for a specific trait

• Instructions for inherited traits• Instructions for proteins

Searching for Genetic Material

• Fredrick Griffith: Discovery of Transformation– 1928– 2 strains of bacteria (S causes pneumonia, R

doesn’t)• Transformation: A change in the genotype

(genes) when cells take up foreign genetic material

• Outcome: Genetic Material can be transferred between cells

Searching for Genetic Material

• Oswald Avery: DNA is responsible for transformation– 1940’s– Wanted to see what caused transformation– 3 choices: DNA, RNA and Proteins– Used enzymes to destroy each part– Only when DNA was missing did transformation

not occur• Outcome: DNA is the transforming agent

Searching for Genetic Material

• Alfred Hershey and Martha Chase: DNA is hereditary material– 1952– Studied bacteriophages (viruses that invade

bacteria)• Outcome: DNA is the hereditary material

The Shape of DNA

• James Watson and Francis Crick (February 28, 1953)

• Double Helix shape-strands of repeating subunits joined at the center

Nucleotides

• Subunit of DNA that consists of 3 parts• Phosphate• Deoxyribose Sugar• Nitrogenous Base• The information in DNA is contain in the order

of the nitrogenous bases• Base-pairing structure allows DNA to be

copied quickly

Nitrogenous Bases

Purines• Two rings of Carbon per

base• Adenine• Guanine

Pyramidines• One ring of Carbon per base• Thymine• Cytosine

Base-Pairing

• A purine is always paired with a pyramidine based on their chemical structure

• They are held together with weak Hydrogen bonds

• A always pairs with T• C always pairs with G

Complementary Strands

• Now that we know the base-pairing rule we can construct complementary strands

• If given one strand (side) of the DNA, we can construct the other strand

• EX: TATGAGACT

• This ensures that each strand has the same information, with one side being the reverse of the other

Discovering the Structure of DNA

• Erwin Chargaff: 1949– In every DNA the number of Adenine was equal to

the number of Thymine and the number of Guanine was equal to the number of Cytosine

Discovering the Structure of DNA

• Rosalind Franklin: 1952– Used X-ray diffraction to take pictures of the two

forms of DNA– These photographs show that DNA as it is in

nature has a tightly coiled helix made of two chains of nucleotides

Discovering the Structure of DNA

• James Watson and Francis Crick: 1953– Built models to figure out the structure of DNA– Double Helix with phosphate and deoxyribose

sugars on the outside and nitrogenous bases on the inside

• Watson, Crick and Wilkens were awarded the Nobel Prize in 1962 for their disocovery

• Franklin who had died of Cancer was not named in the award