16A

Intro to DNA

Science as a process

• What was the genetic material? At the time, most scientists believed it was protein.

Griffith Mouse Experiment (1928)

• Living S• Living R• Heat Killed S• Heat Killed S/Living R

• Transformation: change caused by dead pathogenic cells.

• Avery (1944): Concluded that ______ was the transforming agent, still viewed with skepticism.

Hershey/Chase (1952)

• DNA is the genetic material of T2 bacteriophage.– Bacteriophage: _______ that infects __________.– T2: phage that infects E Coli, turns it into a T2 producing

factory. Question: was it DNA or protein of the T2 that was causing the reprogramming?

– Radioactively labeled:• ___________: found in proteins only. Mixed T2 with E Coli, blended

and centrifuged, found that the pellet of bacteria cells were not radioactively labeled but the supernatant viral parts were.

• ___________: found in DNA only. Mixed T2 with E Coli, blended and centrifuged, found that the pellet of bacteria cells was labeled and the supernatant was not.

• CONCLUSION:

Chargaff Base Pairing Rules

• Already known that DNA is a nucleotide containing sugar (deoxyribose), phosphate and a nitrogenous base (adenine, thymine, guanine and cytosine).

• Chargaff found a characteristic ratio of bases in all species

• Number of adenines = number of _________• Number of cytosines=number of _________• Chargaff’s base pairing rules:

The Current Model

• The race to the 3-D structure of DNA was won by Watson and Crick.– Used X-ray crystallography from Franklin and Wilkins to

determine the size and double helix shape– Used wire model, chemistry (hydrophobic bases on

interior) to create the double helix model.– Used size and Chargaff’s rules to determine A to T (2

Hydrogen bonds) and C to G (Three hydrogen bonds)– Correct width (see page 298)– Adenine and Guanine are purines: two rings– Thymine and Cytosine are pyrimidines: one ring

Structure