Analogies - Metaphors - Similies

ANALOGIES - METAPHORS - SIMILIES

by Dr. B

What is the chemical bases for Mendel’s determinants?

We now all the chemical composition of Mendel’s determents to be DNA (deoxyribonucleic acid). But until the early fifties no really knew what the determinants were composed of, so lets go back and see how this came about. How merging different fields finally lead to the conclusion that genes are made of DNA. In 1869, Miescher found a substance in nuclei of cells (nuclein) that was rich in phosphorous but had no sulfur like proteins. Further work found the substance to be acidic (nucleic acid) and in two forms, deoxyribonucleic (DNA) and ribonucleic (RNA). They still didn’t know that determinants were made of DNA. Then in 1931, a very interesting experiment was carried out by Frederick Griffith using a bacterium called Streptococcus pneumonia, or pneumococcus). This organism was always found associated with the disease pneumonia and fulfilled all of Koch’s postulates (meaning pneumonia was cause by the bacterium Streptococcus pneumonia). Normally the pneumococcus forms smooth colonies with bacteria that have a capsule around them. But every so often one could find a colony that was rough and when they looked at these bacteria they lacked the normal capsule. Griffith also knew that the rough pneumococcus did not produce pneumonia when injected into a test animal whereas the smooth did. In one of his experiments he injected smooth and rough killed bacteria into separate animals (the bacteria was killed by heating in boiling water). He found that neither produced pneumonia so his conclusion was he had destroyed the determinant that causes pneumonia. He then took heat killed smooth bacteria and injected them along side living rough bacteria and to his surprise the animal died of pneumonia. The autopsy only produced smooth bacteria. Because the only bacteria that he injected that were live rough bacteria he concluded that the determinant normally present in the smooth bacteria was passed to the rough bacteria now making them smooth. He called this phenomena transformation because the rough bacteria were transformed into smooth bacteria. But it was several years later that Avery et. al., describes what was being passed. Avery et. al., knew they could transform rough into smooth while in a test tube so they devised an experiment to see if they could interfere with the transformation process. They did this thought the use of enzymes that specifically destroyed certain macromolecules such as lipids, proteins, polysaccharides, RNA, and DNA. If the treatment did not interfere with the transformation process then it meant that macromolecule was not the one responsible for the transformation. The only enzyme that interfered with the transformation process was the enzyme that destroyed DNA. From this they concluded that Griffiths transforming principle was DNA and for the first time we knew what Mendel’s determinants were made of, DNA.

Some interesting sites:

http://ntri.tamuk.edu/cell/griffith.html

http://mywebpages.comcast.net/biologycentury/pages/dna2.html

http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookDNAMOLGEN.html