DNA - Transformation• 1928 – Fredrick Griffith discovers non-virulent bacteria

(Streptococcus pneumoniae) become virulent when in contact with dead pathogenic bacteria – calls the process transformation

• Bacteria are “transformed” by genetic material left by the virulent bacteria

• 1944 – Oswald Avery (&colleagues) announced that DNA, not proteins, transferred the genetic material that causes transformation – Used enzymes to isolate the

different macromolecules (Lipids, Carbohydrates, Nucleic acids, & Proteins)• Determined that ONLY

nucleic acids were involved with transformation

Bacteriophages• 1952 – Alfred Hershey & Martha

Chase formed experiments showing that viral DNA from the T2 bacteriophage (bacteria-eater) transforms E. coli and changes its function – Used radioactive markers (isotopes) to

determine if DNA or proteins were associated with cell infection by viruses

Double Helix

Early 1950s -many scientists, including Linus Pauling and Rosalind Franklin had discovered the arrangement of bonds of a single strand of DNA – Pauling’s helix consisted of 3

intertwined chains– Franklin’s improbable X-ray picture

of the top of a DNA molecule opened the door for the building of the double helix

DNA – a helixIn 1953, James Watson and Francis Crick discovered the structure of DNA . The discovery would not have been possible, though, without the work of Rosalind Franklin. Franklin's image of DNA using X-ray crystallography. From this picture Watson & Crick assembled their model of double helix model of DNA with the following properties:

– both strands are anti-parallel • 5' to 3' moving in opposite directions

– the sugar backbone (deoxyribose) resides on the outside

– nucleotides are on the inside and paired in the following function • purines (A & G) are attached to pyrimidines (C & T)

– A is attached to T with two hydrogen bonds – C is attached to G with three hydrogen bonds

• explained the basis for Chargaff's rule

DNA Packaging

• DNA in the form of chromatin is packaged around special proteins called Histones.– Makes structure more compact– Hides non-active regions of the

chromosome from transcription• Active regions are opened and

transcribed ONLY when needed.