Chemical nature of genes PDF

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first year pharmacy...

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Describe some of the early experiments (in the 1940s and 1950s) which helped to define the chemical nature of the gene Prior to the 1940s, it was widely accepted that genes were made of protein. It was thought that DNA merely provides a structural framework in chromosomes. However, conclusive experimental evidence that genes are made of DNA came from studies of (a) bacterial transformation and (b) viral replication in the 1940s and 1950s. Back in 1928, Frederick Griffith studied two strains of the bacterium Streptococcus pneumoniae. Bacteria of the S (smooth) strain are pathogenic (because they have a capsule that protects them from an animal’s defence system) and can cause pneumonia in mice while the rough (R strain) is non-pathogenic. However, when a heat killed S strain and living R strain was injected in the mice it still died. It has now been concluded that: 1. The killed Type IIIS did not come back to life! 2. Genetic material contained in the Type IIIS cells had leaked out and entered the living Type IIR cells 3. The genetic material had converted (or transformed) cells of the Type IIR strain into Type III S cells 4. The genetic material from the Type III S contains the information for making the capsule required for virulence (death causing effect) But Griffith didn’t realise this! Griffith’s discovery of bacterial transformation provided a way of identifying what chemical substance in the bacterium contained the genetic information i.e. the genes themselves. His work set the stage for a 14 year search by Oswald Avery for the identity of the transforming substance- what caused the change in virulence? DNA? RNA? or Protein? In 1944 Avery, Maclyn McCarthy and Colin MacLeod proved the transforming substance was in fact DNA. Avery broke open the heat-killed pathogenic S strain bacteria and extracted the cell contents. He then treated the extract and treated it with different enzymes: one that kills protein-protease, one that kills DNA-DNAase, one that kills RNA-RNAase. He then tested each treated sample for its ability to transform live non-pathogenic bacteria into pathogenic bacteria. Only when DNA was allowed to remain active did the experiment work-so it was concluded that the transforming substance was DNA. In 1953 the Hershey-Chase Experiment also confirmed that DNA was the genetic material. Alfred Hershey and Martha Chase used phages (viruses that infect bacteria) to investigate which substance contains the genetic information. Phages contain 50% protein and 50% DNA so they decided to label the phosphorus present in DNA with radioactive phosphorus, and the sulphur present in protein with radioactive sulphur which was incorporated into phage protein, and phages were grown with radioactive The results showed that in the case of the protein radioactivity did not enter the bacterial cell but with the radioactive DNA it did

Experiment 1

Experiment 2 Further evidence that DNA is the genetic material came from the work of Erwin Chargaff. In 1950 he reported the base composition of DNA varies from one species to another, and that the number of adenines equalled the number of tyrosines and the number of guanines equals the number of cysteines....