DNA and the discovery of genetic material

• For a molecule to serve as the genetic material, it must be able to 

  • Replicate
  • Store info
  • Express info
  • Allow variation by mutation
• Central dogma of molecular genetics - DNA makes RNA (transcription), which makes proteins (translation)
• Until 1944, observations favored protein as the genetic material. Proteins and nucleic acids were thought to be genetic material
• Proteins were more diverse and abundant, and were most actively studied within genetics, having more knowledge than that of nucleic acids
• DNA was thought to be too simple, only have 4 types of nucleotides compared to 20 different amino acids for proteins
• Evidence favoring DNA as the genetic material was first obtained during the study of bacteria and bacteriophages
• Fredrick Griffith (1927) showed that virulent strains of diplococcus pneumoniae could be transformed to virulence.  He speculated that the transforming principle could be part of the polysaccharide capsule or a compound required to capsule synthesis
• Avery, MacLeod, and McCarty demonstrated that the transforming principles was DNA not protein
• Alfred Hershey and Martha Chase (1952), using E. coli and an infection virus (bacteriophage T2), demonstrated that DNa is the genetic material
• Using radioisotope 32P and 35S, Hershey and Chase demonstrated that DNA enters the bacterial cell during infection and directs viral reproduction
• Transfection - the process of infection by viral DNA into bacterial cells, proved conclusively that the viral DNA alone contains all the necessary info for production of mature viruses
• Indirect and direct evidence supports the concept that DNa is the genetic material in eukaryotes

  • Protein is abundant in the cytoplasm, but isn’t in the nucleus
  • Mitochondria and chloroplasts perform genetic function and both have DNA
  • DNA is found only where the primary genetic function occurs
  • This is indirect evidence for DNA
• More indirect evidence

  • UV light is capable of inducing mutations in the genetic material and is most mutagenic at as wavelength of 260 nm
  • DNA and RNA absorb UV light most strongly at 260 nk but protein absorbs most strongly at 280 nm, a wavelength at which no significant mutagenic effects are observed
• The strongest evidence for DNA as the genetic material comes from recombinant DNA technology; segments of eukaryotic DNA corresponding to specific genes are isolated and spliced in the bacterial DNA. This complex can be inserted into a bacterial cell and monitored.
• The presence of the eukaryotic gene product in bacteria containing the said gene provides evidence that this DNA is present and functional in the cell. Eg. insulin production by bacteria