1. Microbiology: A Systems Approach
PowerPoint to accompany
Microbiology: A Systems Approach
Cowan/Talaro
Chapter 9
Microbial Genetics
Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Chapter 9 Topics - Genetics - Flow of Genetics - Regulation - Mutation
- Recombination

3. Viral Genetics
Viruses consist of one or more pieces of DNA or RNA enclosed in a protective coating
Can be dsDNA, ssDNA, dsRNA or ssRNA
Viruses require access to the host’s genetic and metabolic machinery to reproduce
Also have the potential for genetically changing the cells they infect

4. Regulation
Operon – a coordinated set of genes regulated as a single unit
Found only in bacteria and archaea
Inducible operons – Catabolic; code for enzymes that act in catabolism
turned on (induced) by the substrate(s) for which the structural genes encode
Ensures that enzymes needed to metabolize a nutrient are only present when that nutrient is present in the environment
Lactose operon – regulates enzymes needed to process lactose; Lactose is the inducer

5. Operons Repressible operons: Antimicrobials
Contain genes coding for anabolic enzymes
Several genes in a series are turned off (repressed) by the product synthesized by the enzyme
common gene products that can act as repressors are amino acids, + nucleotides
Antimicrobials

6. Lac operon An inducible operon
Ensures that the cell does not make unnecessary enzymes to process lactose if lactose is not present in the cell.
Lactose is the inducer

7. Lac Operon Regulatory gene On the DNA Promoter region Operator region
Where RNA pol binds
Operator region
Can bind lactose OR the repressor protein
Structural genes that code for the enzymes that process lactose
Regulatory gene
Will be located somewhere else on the DNA
Codes for a repressor protein
If the repressor protein binds to the operator region, then transcription is blocked. No enzymes made
If repressor protein is not bound to the operator region, RNA pol is not blocked. Transcription proceeds; enzymes made; lactose processed

8. Lac Operon Induced when lactose is present
Operon is “on”
Lactose is the inducer
Lactose binds to the repressor protein.
Repressor protein falls off the operator region. Transcription is not blocked
RNA pol binds to the promoter. Gene is transcribed. Enzymes made. Lactose processed.
Repressed when lactose is not present
Operon is “off”
Repressor protein is able to bind to the operator region
RNApol can’t get past the blocked operator region.
Genes are not transcribed.
No enzymes to process lactose are made (not needed since no lactose is present in the cell)

9. The regulation of sugar metabolism such as lactose involves repression in the absence of lactose, and induction when lactose is present.
Fig The lactose operon in bacteria

10. The regulation of amino acids such as arginine involves repression when arginine accumulates, and no repression when arginine is being used.
Fig Repressible operon

11. Antimicrobials
Ex. Antibiotics and drugs can inhibit the enzymes involved in transcription and translation

12. Mutations Changes made to the DNA Spontaneous – random change
Induced – chemical, radiation.
Point – change a single base
Nonsense – change a normal codon into a stop codon
Back-mutation – mutation is reversed
Frameshift – reading frame of the mRNA changes

13. Examples of chemical and radioactive mutagens, and their effects.
Table 9.3 Selected mutagenic agents and their effects

14. Repair of mutations involves enzymes recognizing, removing, and replacing the bases.
Fig Excision repair of mutation by enzymes

15. The Ames test is used to screen environmental and dietary chemicals for mutagenicity and carcinogenicity without using animal studies.
Fig The Ames test.

16. Effects of mutations Positive effects for the cell
Allow cells to adapt
Negative effects for the cell
Loss of function
Cells cannot survive

17. Recombination Sharing or recombining parts of their genome Conjugation
Transformation
Transduction

18. Conjugation
Transfer of plasmid DNA from a F+ (F factor) cell to a F- cell
An F+ bacterium possesses a pilus
Pilus attaches to the recipient cell and creates pore for the transfer DNA
High frequency recombination (Hfr) donors contain the F factor in the chromosome

19. Conjugation is the genetic transmission through direct contact between cells.
Fig Conjugation: genetic transmission through direct contact

20. Transformation
Nonspecific acceptance of free DNA by the cell (ex. DNA fragments, plasmids)
DNA can be inserted into the chromosome
Competent cells readily accept DNA

21. DNA released from a killed cell can be accepted by a live competent cell, expressing a new phenotype.
Fig Griffith’s classic experiment in transformation

22. Transduction Bacteriophage infect host cells
Serve as the carrier of DNA from a donor cell to a recipient cell
Generalized
Specialized

23. Genetic transfer based on generalized transduction.
Fig Generalized transduction

24. Genetic transfer based on specialized transduction.
Fig Specialized transduction

25. Transposon “Jumping genes” Exist in plasmids and chromosomes
Contains genes that encode for enzymes that remove and reintegrate the transposon
Small transposons are called insertion elements

26. Movement of transposons can occur in plasmids and chromosomes.
Fig Transposons: shifting segments of the genome