Bacterial variation Phenotype Nonhereditary Reversible Morphology Cultural Physiological
Bacterial variation Phenotype (Non-hereditary) Reversible Morphology Cultural Physiological Genotype (hereditary) Irreversible The genetic compliment of an organism Types of genotypic changes – – Mutation Conjugation Transduction Transformation
Mechanism of bacterial variation • Gene mutation • Gene transfer and recombination Transformation Conjugation Transduction • Lysogenic conversion • Protoplast fusion
Mutations: Changes in the Genetic Code • A change in phenotype due to a change in genotype (nitrogen base sequence of DNA) is called a mutation. • A natural, nonmutated characteristic is known as a wild type (wild strain). • An organism that has a mutation is a mutant strain, showing variance in morphology, nutritional characteristics, genetic control mechanisms, resistance to chemicals, etc.
Causes of Mutations • Spontaneous mutations– random change in the DNA due to errors in replication that occur without known cause. • Induced mutations – result from exposure to known mutagens, physical (primarily radiation) or chemical agents that interact with DNA in a disruptive manner.
Categories of Mutations • Point mutation – addition, deletion or substitution of a few bases. • Missense mutation – causes change in a single amino acid. • Nonsense mutation – changes a normal codon into a stop codon. • Silent mutation – alters a base but does not change the amino acid.
• Back-mutation – when a mutated gene reverses to its original base composition. • Frame- shift mutation – when the reading frame of the m. RNA is altered by the addition or deletion of nucleotides in a newly synthesized DNA.
Positive and Negative Effects Of Mutations • Mutations leading to nonfunctional proteins are harmful, possibly fatal. • Organisms with mutations that are beneficial in their environment can readily adapt, survive, and reproduce – these mutations are the basis of change in populations. • Any change that confers an advantage during selection pressure will be retained by the population.
Repair of Mutations • Since mutations can be potentially fatal, the cell has several enzymatic repair mechanisms in place to find and repair damaged DNA. – DNA polymerase – proofreads nucleotides during DNA replication – Mismatch repair – locates and repairs mismatched nitrogen bases that were not repaired by DNA polymerase – Light repair – for UV light damage – Excision repair – locates and repairs incorrect sequence by removing a segment of the DNA and then adding the correct nucleotides
- Slides: 9