Chapter 12: Chromosome Structure And Dna Sequence Organization

Concepts

• Viral and bacterial chromosomes are relatively simple DNA molecules
• Mitochondrial and chloroplast DNA resembles prokaryotic DNA
• Supercoiling is common in the DNA of viral and bacterial chromosomes
• Specialized chromosomes reveal variations in structure
• DNA is organized into chromatin in eukaryotes
• Chromosome banding differentiates regions along the mitotic chromosome
• Eukaryotic genomes demonstrate complex sequence organization characterized by repetitive DNA
• The vast majority of a eukaryotic genome does not encode functional genes

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Hershey / Chase Experiment

Fredrick Griffith’s pioneering work on the transforming principle, in 1928, was extended by Oswald T. Avery, Colin M. MacLeod and Maclyn J. McCarty.  In 1944, these workers showed that Griffith’s transforming principle was DNA.  Although Avery's experiments were straightforward, they did not entirely resolve the question of whether DNA or proteins encoded genetic information.  To answer this question, Alfred Hershey and Martha Chase hit upon the idea of using the bacteriophage T2, which consists of a DNA core surrounded by a protein coat.  They used radioactive isotopes to determine whether the phage DNA or protein entered the bacterial cell and directed the synthesis of progeny phage.  Their conclusions seem obvious now, but consider these experiments without the benefit of that knowledge

The story of the chemical identity of genes begins in 1928.   At that time, in England, Frederick Griffith discovered the transforming principle.  The transforming principle had the capacity to transform avirulent bacteria to a virulent form, to alter the phenotype of the organism.  Transformants passed the trait to successive generations, suggesting that the phenotypic transformation involved a change in genotype, a change that was inheritable.  Griffith worked without knowledge of the nature of the composition of his "transforming principle".  In 1944 , working at the Rockerfeller Institute, Oswald T. Avery,  Colin M. MacLeod, and Maclyn J. McCarty purified Griffith’s transforming principle and showed that it had the chemical and physical properties of DNA.  Additional evidence was needed to prove to the scientific community that this was a genetic event and not just a physiological response.  It was not long before Hershey and Chase offered more proof that DNA is the genetic material.