Finding the genetic material

For a long time, many scientists thought proteins carried genetic information. Three famous experiments showed it is DNA.

Griffith (1928) used the bacterium Streptococcus pneumoniae and mice. The S strain (smooth, with a coat) causes pneumonia and kills mice; the R strain (rough) does not. Heat-killed S bacteria did not kill mice either. But heat-killed S mixed with live R did kill the mice, and live S bacteria were found in them. Something from the dead S cells had transformed the R cells. Griffith called it the transforming principle, but did not know what it was.

Avery, MacLeod and McCarty (1933-44) purified the parts of the S cells. Only DNA could transform R cells. Enzymes that destroy proteins (proteases) or RNA (RNases) did not stop the transformation, but DNase, which destroys DNA, did. So the transforming principle was DNA.

Hershey and Chase (1952) gave the final proof, with bacteriophages (viruses that infect bacteria). They grew some phages with radioactive phosphorus (³²P), which goes into DNA but not protein, and others with radioactive sulphur (³⁵S), which goes into protein but not DNA. After the phages infected bacteria, the viral coats were shaken off. The bacteria infected by ³²P phages were radioactive; those infected by ³⁵S phages were not. Only the DNA had entered the bacteria, so DNA carries the genetic information.

What makes a good genetic material? It must:

• copy itself (replicate) accurately;
• be stable, chemically and structurally;
• allow slow changes (mutations), needed for evolution;
• be able to express itself as Mendelian characters.

DNA is the main genetic material, but in some viruses, such as the tobacco mosaic virus, it is RNA. RNA has an extra –OH group at the 2' position of its sugar, which makes it more reactive and less stable, and it mutates faster. So DNA is better for storing information, and RNA for passing it on (as in making proteins). RNA was probably the first genetic material in early life; DNA evolved from it, being more stable.