CALS Farm and Industry Short Course Program: Farm Microbiology: Special Pages

DNA, Transcription and Translation

Linked from Section 2 Lecture Notes.

Here is a simplified scheme of a typical bacterial cell showing DNA replication, transcription and translation – processes which occur simultaneously and continuously: The diagrams at right and below show transcription and translation in a little more detail. Transcription (synthesis of mRNA): As RNA polymerase moves along the DNA, a strand of mRNA is synthesized from nucleic acid bases and other compounds in the cytoplasm. The sequence of bases on the DNA serves as a template upon which mRNA is made. Translation (synthesis of protein): As the ribosome moves along the mRNA, a strand of peptide (protein) is synthesized from amino acids in the cytoplasm. the sequence of bases on the mRNA dictates the order in which the amino acids are added to form the particular protein coded for.

DNA may be changed by mutation or recombination. When either happens, the phenotype may be altered (i.e., a change in a visible characteristic may be seen), or the cell may die due to a lack of a needed function. The following discussion gives a brief overview ot the mutation concept.

A mutation is a spontaneous change in the DNA sequence which is passed on to the "offspring." This change, generally occuring during DNA replication, may be accomplished by substitution, deletion or insertion of one or more bases such that the base sequence is altered, leading to other than the intended protein being constructed. Most mutations are deleterious, leading to death of the cell. Those mutations which are not lethal may cause a visible alteration of the phenotype and may confer a special survival benefit to the organism, possibly allowing the organism to colonize a previously-hostile environment.

A mutation may lead to one or more of the following changes in the phenotype of the cell:

• Structural component mutation – altering a part of the cell – for example, the cell membrane, ribosome, flagella or capsule.
  
  
• Biochemical mutation – losing the ability to utilize certain nutrients or to synthesize certain needed organic compounds such as vitamins, fatty acids, nucleic acid bases, amino acids, and siderophores. The loss of the ability to synthesize a certain organic compound thus requires that the organism be provided with that organic compound which is now considered a growth factor.
  
  
• Resistance mutation – making the cell resistant to an agent which affects other cells in the population negatively. A resistance mutation can confer resistance to particular antibiotics, viruses (bacteriophages) or antibodies which would otherwise inhibit or kill the organism.
  
  
• Silent mutation – leading to no observable phenotypic change. An altered triplet code may still code for the same amino acid.
  
  
• Lethal mutation – leading to death of the cell.
  

The following over-simplified example shows selection of antibiotic-resistant cells from a culture of a species generally thought of as being sensitive to the antibiotic. A test tube culture containing sixteen cells is shown on the left in this diagram. All of the cells are sensitive to a particular antibiotic except for one cell which – because of a mutation – is resistant. If the culture is dumped onto a plate of an all-purpose medium, all of the cells will form colonies. One cannot distinguish colonies of antibiotic-resistant cells from colonies of antibiotic-sensitive cells. If the culture is dumped instead onto a plate of the same medium to which is added the antibiotic, the sensitive cells will be inhibited or killed by the antibiotic – the selective agent in the medium – while the resistant cell will form a colony.

A take-home lesson: Suppose that this test tube is your body and the bacterial cells are infecting it. If you are given this antibiotic to control the infection, will it be totally effective? Might the resistant mutant escape your body's own natural defenses and then multiply and continue the infection? This is an important way in which antibiotic-resistant strains of a pathogen can arise. In the antibiotic disc sensitivity test, we test various species of bacteria (potential pathogens) with a number of different antibiotics, and we can determine the best antibiotics to use against a given organism – where there is no indication of any growth of resistant cells.