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 helix In 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.