Experimental Work With Nucleic Acids

Question 1

What were the 2 experiments involved in investigating the role of nucleic acids?

Answer 1

Experiment 1: to show that DNA rather than proteins carries the genetic code

Experiment 2: to work out the genetic code

Question 2

Describe how scientists discovered that DNA carried the genetic code and not proteins

Answer 2

1) Carried out using bacteriophage viruses.
2) scientists made viruses that had DNA with a radioactive isotope of phosphorus in it and proteins with radioactive isotopes of sulphur in them
3) 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.
4) found that radioactive sulphur was found only outside the cell whereas radioactive phosphorus was inside - new viruses were made as normal and released from the cell
5) this 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

Question 3

What is a bacteriophage virus?

Answer 3

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 amino acids, nucleotides and ribosomes in the cell to make new viruses. The new viruses break out the cell and destroy it.

Question 4

How did scientists work out the genetic code?

Answer 4

1) RNA code was deciphered
2) made synthetic RNA and used that to make proteins
3) made 3 types of mRNA (shown in table) and added all 20 amino acids attached to tRNA
4) one of the types of amino acids was labelled with a radioactive isotope of carbon but the others were not
5) process repeated using different radioactively labelled types of amino acid and different sequences of mRNA each time