Chapter 12 DNA & RNA

DNA’s StructurePurines: A & G - small name big molecule Pyrimadines: T & C - big name small molecule

Monomer = building blockOne Nucleotide is a monomer = nucleic acid + deoxyribose + phosphate

(Nitrogenous Base)

DNA’s Structure

3’ has OH - hydroxide5’ has Phosphate Molecule

•Directional Structure•Anti-parallel to each other•Always builds 5’ to 3’•DNA Polymerase can only add to the 3’ end

If DNA were synthesized in the 3' to 5' direction, the energy for the process would come from the 5' end of the growing strand rather than from free nucleotides. If the 5' nucleotide needed to be removed this triphosphate end would be lost, losing the energy source required to add a new nucleotide to the end.

DNA’s Structurenm = 1 Billionth of a meter

DNA Replication

Semiconservative

Chromosome Structure

Key Terms:

Histone = the protein

Nucleosome = A cluster of histones

Chromatin = DNA + Protein in an uncoiled form

Chromosome = condensed chromatin

Chromatid = each side of a chromosome

Centromere = point of attachment of sister chromatids

Evidence for DNA as Genetic Material• Griffith - 1928

– transformation

• Avery, MacLeod, and McCarty - 1944– Proves DNA is the transforming agent

using enzymes

• Hershey & Chase -1952– Reproves Avery’s evidence using

bacteriophage

Time Line

• 1866 Mendel publishes paper

• 1900 Mendel’s paper rediscovered

• 1902 chromosome theory of inheritance

• 1905 the term “genetics” coined

• 1950 base pairing discovered

• 1953 Watson & Crick deduce the structure of DNA

Griffith’s Transformation Experiments

• Diplococcus pneumoniae– two strains: IIR & IIIS– two differences in genotype

• R = rough, avirulent; S = smooth, virulent

Griffith’s Transformation Experiments

Live IIR

Live IIIS

Heat-killed IIIS

Heat-killed IIIS + live IIR

Heat-killed IIIS

Live IIR

Heat-killed IIIS

Live IIR

Explanations?

• IIR cells mutated to S?• Serotype was changing

• IIR were changing into IIIS• 2 mutations?

• Only happened when IIR and dead IIIS mixed

• Non-living component of IIIS cells changing genetics of IIR cells

Avery, MacLeod and McCarty

heat-killed IIIS + IIR

removed polysaccharide coat

removed protein

heat-killed IIIS + IIR

Avery, MacLeod and McCarty

heat-killed IIIS + IIR

removed RNA

removed DNA

heat-killed IIIS + IIR

Avery, MacLeod and McCarty

• DNA is the transforming principle

• DNA could change IIR bacteria into IIIS– Now know bacteria may pick up pieces of

exogenous DNA and incorporate them into their own chromosomes = transformation

Hershey and Chase

• Studied bacteriophage– virus which attacks E. coli

• Ran two side-by-side studies– labeled DNA of phage w/ 32P in one study– labeled protein of phage w/ 35S in second

• In which study does radioactivity end up in E. coli?

32P labeled DNA

35S labeled protein

Hershey and Chase

• Mix phage and bacteria

• Allow infection

• Shear off what’s on outside - blender

• Centrifuge

• Look for radioactivity in pellet (bacteria)

infection blender

centrifuge

E. coli w/ hot DNA

Supernatant w/ protein coats

infection blender

centrifuge

E. coli w/ DNA

Supernatant w/ hot proteincoats

Summary Hershey & Chase

Chargaff’s Rules 1947

Provided base pairing and hydrogen bonding

Rosalind Franklin 1920-1958 (1951)

Rosalind FranklinX-Ray Diffraction

Provided idea of double helix

Nobel Prize (1962)

Frederick Wilkins James Watson Francis Crick

Made Model 1953