THE CHEMICAL NATURE OF THE GENE

The molecules which are genes must have two properties:

• The must be able to hold information and decode it (translate it) into an organism as it grows and develops.

• It must be able to copy itself so that it can be passed on to future generations.

Whatever genes are made of it must be a big molecule to hold the large amount of information required to build an organism.

It must be a complex molecule to provide the necessary variation to code the instructions which control growth and development.

There are four classes of molecules which could form genes:

Griffiths’ Experiment (1928)

Conclusion

Transformation of R-type to S-type
Transformation was brought about by some heat stable compound present in the dead S-type cells called the TRANSFORMING PRINCIPLE.

Avery, MacCleod & McCarthy (1944)

Tried purifying the transforming principle to change R-type Pneumococcus to S-type.

The compound that had the most effect was:

• Colourless, viscous and heat stable.

• It contains phosphorus.

• It was not affected by trypsin (a protease) or amylase.

• It is inhibited by RNAase and DNAase.

Conclusion

The transforming principle is a nucleic acid

Experiment

Conclusion

DNA is the transforming principle and it it is hereditary material.

Criticism

The DNA was not totally pure, it was contaminated by a small amount of protein. This protein could be the real transforming principle.

BUT

When Avery and his team prepared purer extracts of DNA they became better at transforming the bacteria types

© Paul Billiet 2009