DNA as the genetic material

• 1. The genetic material is chemical
  • Frederick Griffiths
• 2. DNA is the genetic material
  • Avery, McCarty, MacCleod
  • Hershey and Chase
  • Chargaff
• 3. The discovery of the double helix
  • Linus Pauling
  • James Watson and Francis Crick
  • Maurice Wilkins and Rosalind Franklin
• 4. DNA Replication

• Streptococcus pneumoniae
• Smooth colonies (S)
• polysaccharide coat
• Rough colonies (R)
• no polysaccharide coat
• Only Smooth strain pathogenic to mice

• R cells acquired from the dead S cells the ability to make polysaccharide coats

• The newly acquired trait was heritable

• Now call this phenomenon transformation

• In 1940’s known that chromosomes carry heredity information
  • Chromosomes: Protein and DNA

• Which molecule carries the hereditary information?

• DNA
• or Proteins?

• Discovered the nature of Griffiths transforming agent

• Purified the various chemicals from heat-killed S cells

• Showed that only DNA had "transforming agent"

• Worked with T2 bacteriophage
  • a virus that infects Escherichia coli

• T2 consists of DNA enclosed by a protein coat

• T2 reprograms the E. coli to produce T2 phages

• Is it the DNA or the protein which reprograms the cells?

• Grow phage in media with either ³⁵S or ³²P

• S labeled proteins

• P labeled DNA

• Infect E. coli with either protein labeled or DNA labeled phages

• Agitate to remove phages outside the bacteria

• Centrifuge to separate phage and bacteria
• Measure radioactivity in pellet and supernatant

• E. coli infected with protein labeled phage  radioactivity with viruses (supernatant)

• E. coli infected with DNA labeled phage  radioactivity with bacteria (pellet)

The Hershey-Chase experiment showed that in viruses DNA is the genetic material ê what about eukaryotes?

• Experimental evidence that DNA is the genetic material in eukaryotes
• The amount and ratios of nitrogenous bases varies in different species

DNA can be a source of molecular diversity

• Also showed that

A=T

C=G

Chargaff’s rules

Deoxyribonucleic acid

• A polymer of nucleotides linked by condensation reactions
  • Deoxynucleotide = building block of a nucleic acid
  • Deoxyribose
  • Phosphate
  • Nitrogenous base

Review of DNA Structure (fig 5.27)

Nitrogenous bases

Base pairing in DNA

• Purines-2 rings-
• Pyrimidines-1 ring
• Adenine and Thymine: 2 hydrogen bonds
• Guanine and cytosine: 3 H bonds

DNA structure-the double helix fig 5.28

• Polymer made by covalent bonds called phosphodiester linkages
• Backbone sugar phosphate
• Sequence of bases unique for each gene
• Sequence of bases dictates protein structure
• Double stranded
• A pairs with T and G pairs with C
• Strands are antiparallel
• Helical

• A technique central to the discovery of the double helix

The workers

• Linus pauling-
  • California Institue of Technology
• Maurice Wilkins and Rosalind Franklin
  • King’s College London
• James D. Watson and Francis Crick
  • Cambridge University

Rosalind Franklin

• Produced the X-Ray Crystallographic image which was used to determine the structure of DNA

Watson and Crick deduced

• DNA is a helix with uniform width of 2nm
• 2 strands
• Purines and pyrimidines are stacked 0.34 nm apart
• Helix makes one full turn every 3.4 nm (10 bases per turn
• Bases on inside
• Purines must pair with pyrimidines

How did Watson and Crick deuce the base pairing rules?

• Purine : Purine
  • too wide

• Pyrimidine: Pyrimidine
  • too narrow

• Purine: Pyrimidine
  • gives constant 2nm width

Base Pairing rules

----- Crick and Watson, 1953

4. DNA Replication

To be continued!