CHAPTER 6

• Genetic material


  • It must contain the information necessary to make an entire organism
  • It must be passed from parent to offspring
  • It must be copied in order to be passed from parent to offspring
  • It must be capable of changes to account for the known phenotypic variation in each species
• The data of many geneticists, including Mendel, were consistent with these four properties
• The identification of DNA as the genetic material involved a series of outstanding experimental approaches

  
• Streptococcus pneumoniae


  A bacterium studied by Griffith
• Streptococcus pneumoniae strains


  • S (Smooth) - secrete a polysaccharide capsule, produce smooth colonies
  • R (Rough) - unable to secrete a capsule, produce rough colonies
• Griffith's experiments


  1. Inject mouse with live type IIIS bacteria

  2. Inject mouse with live type IIR bacteria

  3. Inject mouse with heat-killed type IIIS bacteria

  4. Inject mouse with live type IIR + heat-killed type IIIS cells
• Griffith concluded that something from the dead type IIIS bacteria was transforming type IIR bacteria into type IIIS
• Transformation


  The process Griffith observed where something from the dead type IIIS bacteria transformed type IIR bacteria into type IIIS
• Transformation principle


  The substance that allowed the transformation to happen
• Griffith did not know what the transformation principle was
• The formation of the capsule is guided by the bacteria's genetic material
• Transformed bacteria acquired information to make the capsule
• Variation exists in ability to make capsule
• The information required to create a capsule is replicated and transmitted from mother to daughter cells
• Major constituents of living cells

  • DNA, RNA, proteins, carbohydrates
• Avery, MacLeod and McCarty's experiments

  1. Prepared cell extracts from type IIIS cells and purified each type of macromolecule

  2. Only the extract that contained purified DNA was able to convert type IIR bacteria into type IIIS

  3. Treatment of the DNA extract with RNase or protease did not eliminate transformation

  4. Treatment with DNase did
• Avery et al. concluded that DNA, and not a contaminant (RNA or protein), is the genetic material

  
• Bacteriophage T2


  • Composed of only two macromolecules: DNA and protein
• Hershey and Chase experiment

  1. Used radioisotopes to distinguish DNA from proteins

  2. Radioactively-labeled phages were used to infect non-radioactive Escherichia coli cells

  3. Residual phage particles were sheared off the cells and the phage ghosts and E. coli cells were separated

  4. Radioactivity was monitored using a scintillation counter
• Most of the 35S was found in the supernatant, but only a small percentage of 32P
• These results suggest that DNA is injected into the bacterial cytoplasm during infection
• Purified RNA from the tobacco mosaic virus caused the same lesions as intact TMV viruses, therefore the viral genome is composed of RNA

  
• RNA viruses found since then

  • Many

  
• Data from A. D. Hershey and Martha Chase (1952) Independent Functions of Viral Protein and Nucleic Acid in Growth of Bacteriophage. Journal of General Physiology 36, 39–56.

  
• Extracellular 35S


  Most of the 35S was found in the supernatant
• Extracellular 32P


  Only a small percentage of 32P
• Data is not conclusive since less than 100% of the DNA or protein ended up in the cell or supernatant
• Data is consistent with the hypothesis that DNA is the genetic material

  
• A. Gierer and G. Schramm isolated RNA from the tobacco mosaic virus (TMV), a plant virus


  1956

  
• Purified RNA caused the same lesions as intact TMV viruses

  
• Therefore, the viral genome is composed of RNA

  
• Since that time, many RNA viruses have been found

  
• Pathogenesis and viral replication mechanism of SARS-CoV-2

  1. Attachment of SARS-CoV-2 at type II pneumocyte cells via ACE2 receptor

  2. Release of positive single stranded RNA (+ SSRNA) into intracellular environment of pneumocyte cells

  3. Viral RNA synthesis carried out by RNA dependent RNA polymerase (RnRP)

  4. Viral polypeptides formation carried out by ribosome

  5. Assembly of new SARS-CoV-2 virus before being transported out to extracellular environment

  
• DNA and RNA


  Large macromolecules with several levels of complexity
• Levels of complexity


  • Nucleotides form the repeating unit
  • Nucleotides are linked to form a linear strand
  • Two strands can interact to form a double helix
  • The 3-D structure of DNA results from folding and bending of the double helix
• Nucleotide


  The repeating structural unit of DNA and RNA, with three components: a phosphate group, a pentose sugar, and a nitrogenous base
• Nucleoside


  Base + sugar
• Nucleotide


  Base + sugar + phosphate(s)
• Nucleotides


  • Adenosine monophosphate (AMP)
  • Adenosine diphosphate (ADP)
  • Adenosine triphosphate (ATP)
• Phosphodiester bond


  Nucleotides are covalently linked together, with a phosphate connecting the 5' carbon of one nucleotide to the 3' carbon of another