Chapter 5 Microbial Genetics
1Inheritance (heredity) and Variability
·Inheritance is the exact transmission of genetic information from parents to their progeny.
·Variability is the differences between parents and their progeny.
2Genotype and phenotype
·Genotype refers to the genetic constitution of the cell.
·Phenotype is the expression of the genotype in observable characteristics of the cell.
·Gene mutation is a change in the nucleotide sequence of a gene. This gives rise to a changed genotype.
·Spontaneous mutation: mutation occurs under natural conditions. The mutation rate is low.
·Induced mutation: mutations obtained by use of a mutagen,
mutagen: Any agent that increases the mutation rate (UV)
mutant: A microorganism which shows the effects of a mutation is called a mutant.
3.2 Types of mutation
Point mutations occur as a result of the substitution of one nucleotide for another in the specific nucleotide sequence of a gene
These mutations result from an addition or loss of one or more nucleotides in a gene and are termed insertion or deletion, respectively.
3.3 Phenotypes of microbial mutants
·mutants that exhibit an increased tolerance to inhibitory agents, particularly antibiotics. ( antibiotic-or drug- resistant mutants)
·Mutants that are nutritionally deficient, that is, they require a more complex medium for growth than the original culture which they were derived. (auxotrophic mutants)
·mutants that show changes in the surface structure and composition of the microbial cell ( antigenic mutants)
·mutants that demonstrate an altered fermentation ability or increased or decreased capacity to produce some end products.
·mutants that exhibit changes in colonial form or ability to produce pigments.
·mutants that exhibit some changes in morphological changes, for example, the loss of ability to produce spores, capsule, or flagellum.
3.4 The practical implications of mutation
·It is important to isolate mutants capable of producing high yield of an end product. For example, the yield of penicillin in commercial production has been dramatically increased through selection of mutant.
·Some microorganisms are known to develop resistante to certain antibiotics because of mutation. This fact is of great importance in the treatment of disease, since antibiotics originally effective for the control of a bacterial infection become less effective or ineffective as antibiotic-resistant mutants appear.
·Microbial mutants have been extensively used in microbial research.
△ Many mutants are able to revert to the original Phenotype (wild-type) by reverse mutation.
4Bacterial gene transfer and Recombination
·Recombinant DNA: A DNA molecule containing DNA originationg from two or more sources.
·Recombination: Process by which genetic elements in two separate genomes are brought together in one unit.
·Recombinant: A microorganism resulting from recombination.
In bacterial recombination the cells do not fuse, and usually only a portion of the chromosome from the donor cell is transfered to the recipient cell. There are three types of gene transfer in bacteria.
Transfer of genes between bacterial cells that are in physical contact with one another. (Fig.A)
Transfer of genes from one bacterial cell to another by a bacteriaphage. (Fig.B)
“naked” donor DNA passes into recipient and is integrated into the chromosome of recipient cell. (Fig.C.D)
4.4 The Griffith experiment, in 1928, English health officer
·He found the “transforming principle”
·Avery, Macleod and McCarty in 1944 proved this“transforming principle” is DNA, and defined DNA as the chemical substance responsible for inheritance.
Genetic engineering refers to the development of microorganisms with genetic structure altered by biochemical manipulation. This kind of biochemical procedure is termed recombination DNA technology, and involves the use of plasmids and certain bacteriophages. This technology consists of isolating, purifying, and identifying genetic material from one source, tailoring it for insertion into a new host; and isolating a colony of cells with the desired new genes (Fig.F).
Fig. A Conjugolion