Experimental work investigating the role of nucleic acids

• The experiment which convinced scientists that DNA did carry the genetic code was carried out using bacteriophage viruses.
• A bacteriophage is a virus that binds to the surface of bacteria and passes DNA into the cell. The protein remains outside the cell. The viral DNA then uses the amino acids, nucleotides and ribosomes in the cell to make new viruses, which break out of the cell and destroy it.
• Experiment one: The scientists made viruses that had DNA with a radioactive isotope of phosphorus in it.
• Experiment one: The scientists made proteins with radioactive isotopes of sulphur in them.
• In experiment one, the viruses were allowed to infect bacteria, then the bacteria were tested to see whether or not they had taken up the DNA or proteins.
• Experiment one found that radioactive sulphur was found only outside the cell, whereas the radioactive phosphorus was inside.
• Experiment one indicates that it is the viral DNA that carries the genetic code for new proteins and not the viral protein which remained outside the cell.
• For experiment two, the RNA code was deciphered by a team which made synthetic mRNA and then used this to make proteins. They made three types of RNA (a sequence entirely made of: adenine, uracil and cytosine).
• In experiment two, all 20 amino acids attached to tRNA were added to the the types of mRNA. One of the amino acids (phenylalanine) was labelled with a radioactive isotope of carbon. This process was repeated with different amino acids being labelled each time. The radioactivity of each polypeptide produced was measured.
• In experiment two, it was not possible to decipher all codons using the same method as three codons are stop codons which do not code for any amino acids.