A.  The Hereditary Material:  DNA

 

The Discovery of Genetic Material and Its Location

·       Fredrich Miescher first investigated a substance called nuclein – now referred to as DNA

·       Hammerling’s experiments of the 1930s with Acetabularia – a one-celled green algae – helped confirm that the genetic material that gave rise to a new individual was located in the nucleus of a cell – Figure 2, p. 207 illustrates his results

·       T.H. Morgan’s group showed that genes are located on structures called chromosomes – structures made of DNA and protein

·       the argument as to which substance, DNA or proteins, was considered as the “genetic material” that was passed onto offspring continued up until the 1940s

·       until then the case for proteins seemed stronger, especially since biochemists had identified them as a class of macromolecules with great heterogeneity and specificity of function, essential requirements for the heredity material

 

Evidence That DNA Codes for Changes in Phenotype – The Transforming Principle

·       in 1928, Frederick Griffith studied two strains of the bacterium Streptococcus pneumoniae in mammals – a disease-causing strain, and a harmless strain

·       he found that when he killed the pathogenic strain with heat and mixed the cellular remains with living harmless strains, some of the living cells were converted to the pathogenic form

·       the offspring of the transformed bacteria inherited the new trait of pathogenicity

·       the exact cause of this transformation was not known until Oswald Avery and his colleagues, in 1944, added purified chemicals, not cellular remains, from heat-killed pathogenic bacteria, to harmless strains

·       he found that it was only the DNA of the pathogenic strain that caused the harmless strain to develop disease-causing phenotypes

·       even though the results were significant and conclusive, they were not universally accepted by the scientific community

·       a lot of scientists still believed that proteins were the cause of such transformations, not DNA

·       many had a difficult time accepting the fact that complex organisms, like us, would possess the same genetic mechanisms as bacteria

·       additional evidence for DNA as the genetic material came from studies of a virus that infects bacteria

·       a virus is a simple organism that consists of DNA, surrounded by a protein coat

·       in order for a virus to reproduce, it must infect a cell and take over the cell’s metabolic machinery

·       viruses that infect bacteria are called bacteriophages or simply phages (see Figure 3, p. 208)

·       in 1952 Hershey and Chase discovered that DNA is the genetic material of a phage known as T2

·       a T2 phage can quickly turn the bacteria cell E. coli into a T2-producing factory that releases phages when the bacteria cell ruptures

·       Figure 4, p. 209 illustrates Hershey and Chase’s experiment

o        since sulphur is only found in proteins and phosphorous is only found in DNA, Hershey and Chase labeled the protein coat and DNA of T2 phages accordingly with S-35 and P-32

o        both labeled batches were allowed to infect the non-radioactive/unlabeled host bacterial cell

o        after centrifugation, Hershey and Chase found no evidence of protein-labeled substances in the bacteria, however they did find DNA-labeled substances in the E. coli

o        if the virus was aloud to run its course, eventually the E. coli would rupture and produce phages that contained radioactive phosphorous – not sulphur

o        DNA of the virus is injected into the host cell, while most of the proteins remain outside

o        injected DNA molecules cause the cells to produce new viral DNA and proteins

o        nucleic acids, rather than proteins, are the hereditary material

 

Additional Evidence That DNA Is the Genetic Material of Cells

·       in 1947, Chargaff reported that DNA composition varies from one species to another – the amounts of the nitrogenous bases are not all equal but are present in a characteristic ratio

·       this evidence of biodiversity between different species made DNA a more credible candidate for the genetic material

·       Chargaff found a regularity in the ratios of nucleotide bases – the number of adenine bases equaled the number of thymine bases, the number of guanines equaled the number of cytosines

·       these particular findings were unexplained until the discovery of the double helix model by Watson and Crick

 

Homework:              1-3, p. 209