1. Microbial Genetics

2. Bacterial Chromosome (DNA)
Single
Circular
Attached one or many sites to plasma membrane

3. Bacteria chromosome Escherichia coli 4.6 million base pairs 4300 genes
1mm long
1,000 X length of cell
Supercoiled
Topoisomerase II
DNA gyrase

4. Bacterial chromosome Genetic map Mapped in minutes
Based on time for chromosome exchanged between two cells

5. Review

6. Eukaryotic differences
Transcription takes place in nucleus
mRNA completed prior to entry in cytoplasm
Exons – Expressed DNA, code for protein
Introns – intervening DNA, do not code for protein
Removed by ribozymes

7. Regulation of Bacterial Gene Expression
All metabolic reactions are catalyzed by enzymes (proteins)
Feedback inhibition stops a cell from performing unneeded chemical reactions
Stops enzymes that are already synthesized
What prevents synthesis of enzymes that are not needed?

8. Regulation of Bacterial Gene Expression
Protein synthesis requires tremendous energy
Cell does not waste energy
Regulating protein synthesis economizes cells energy

9. Regulation of Bacterial Gene Expression
Genes
60-80% are constitutive
Not regulated
Products produced at fixed rate
Genes turned on all the time
Code for enzymes essential to major life processes
Enzymes needed for glycolysis

10. Regulation of Bacterial Gene Expression
Genes
Inducible genes
Production of enzymes is regulated
Inducible enzymes
Present only when needed

11. Regulation of Bacterial Gene Expression
Regulation of transcription
Repression
Decreases gene expression
Decrease enzyme synthesis
Response to overabundance of an end product
Regulatory proteins called repressors
Block RNA polymerase

12. Regulation of Bacterial Gene Expression
Regulation of transcription
Induction
Turns on genes
Substance that turns on gene
Inducer
Inducible enzymes

13. Regulation of Bacterial Gene Expression
Induction enzymes
β-galactosidase (E. coli)
Cleaves lactose
Medium without lactose = little to no β-galactosidase
Lactose added to medium large amounts of β-galactosidase produced
Lactose is converted to allolactose
Allolactose is the inducer
Enzyme reduction

14. Operon Model Three genes for lactose utilization
Located next to each other on bacterial chromosome
Regulated together
Called structural genes
lac structural enzymes are transcribed and translated
lac for lactose

15. Operon Model Operon model lac operon Promoter region Operator region
Region of DNA where RNA polymerase initiates transcription
Operator region
Go or stop signal for transcription of the structural genes
Structural genes
Genes for metabolism of lactose

16. Lactose regulation Lactose operon
Depends on level of glucose in medium
Enzymes for glucose metabolism are constitutive
When glucose is absent cAMP (cyclic AMP) accumulates in cell
cAMP binds to cAMP receptor protein (CRP)
This binds to lac promoter
Initiates transcription by allowing mRNA polymerase to bind to the promoter
Transcription of lac operon requires
Presence of lactose
Absence of glucose
cAMP is an alarmone
Chemical alarm signal the cell uses to respond to environmental or nutritional stress

17. Mutation Mutation Change in the base sequence of DNA
may cause change in the product coded by the gene
Beneficial
Lethal
Neutral
Occur commonly
Degeneracy

18. Mutations Types of mutations Base substitution (point mutation)
AT substituted for CG
mRNA carries incorrect base
Translation
Insertion of incorrect amino acid into protein
Missense mutation, nonsense mutation, frame shift mutation, and spontaneous mutations

19. Base substitution

20. Mutation frequency Mutation rate
Probability that a gene will mutate when a cell divides
Expressed in power of 10
10-4 mutation rate (1 in 10,000 chance of mutation)
10-6 ( 1 in 1,000,000)
Mutagens
Increase spontaneous mutation by 10 – 10,000 times
10-6 becomes 10-3 to 10-5

21. Genetic Transfer and Recombination
Genetic recombination
Exchange of genes between two DNA molecules to form new combinations of genes on a chromosome
Crossing over
Two chromosomes break and rejoin
Adds to genetic diversity

22. Genetic transfer and recombination
Eukaryotes
Meiosis
Prophase I
Prokaryotes
Numerous different ways

23. Genetic Transfer and Recombination
Vertical gene transfer
Genetic information passed from an organism to its offspring
Plants and animals
Horizontal gene transfer
Bacteria transfer genetic information form one organism to another in the same generation
Genetic information passed laterally

24. Horizontal Gene Transfer
Donor cell
Organism gives up its entire DNA
Part goes to recipient cell
Part is degraded by cellular enzymes
Recipient cell
Receives portion of donor cells DNA
Incorporates donor DNA into its own DNA
Recombinant DNA
Less than 1 % of population

25. Transformation
Genes transferred from one bacterium to another in solution
Naked DNA
Discovered by Griffith
Used Streptococcus pneumoniae
Two strains
Virulent (pathologic) strain
Had a polysaccharide capsule resists phagocytosis
Avirulent (non- pathogenic) strain
Lacked a capsule

26. Transformation
Bacteria after cell death and lysis could release DNA into environment
Recipient cell can take up DNA fragments and incorporate into their own DNA
Resulting in a hybrid (recombinant cell)
Recombinant cell must be competent
Able to alter cell wall to allow DNA (large molecule) to enter
Bacillus, Haemophilus, Neisseria, Acinetobacter, and some Staph and Strep

27. Genetic Transformation

28. Conjugation Conjugation Involves plasmid Requires cell to cell contact
Circular piece of DNA
Replicates independent of chromosome
Non essential for growth genes
Requires cell to cell contact
Opposite mating type
Donor cell carries plasmid
Recipient cell lacks plasmid

29. Conjugation Gram positive Gram negative
Sticky surfaces cause bacteria to come in contact with one another
Gram negative
Utilize sex pili

30. Transduction in Bacteria
Transfer of bacterial DNA transferred via bacteriophage
Bacteriophage
Virus that infects bacteria

31. Plasmids Plasmids Self replicating rings of DNA
1-5% size of chromosomal DNA
Non – essential genes
Conjugative plasmid
F factor
Dissimilation plasmids
Code for enzymes to breakdown unusual sugars and hydrocarbons
Help in survival of unusual environments

32. Plasmids Other plasmids Toxins (Anthrax, tetanus, Staph)
Bacterial attachment
Bacteriocins
Toxic proteins that kill other bacteria
Resistance factors (R factors)
Resistance to antibiotics, heavy metals, cellular toxins

33. Plasmids Resistance factors
Two groups
RTF – resistance transfer factor
Includes genes for plasmid replication and conjugation
r-determinant
Resistance genes
Codes for production of enzymes that inactivate drugs or toxic substances
Bacteria can conjugate and transfer plasmids between species
Neisseria
Penicillinase resists penicillin

34. R factor Plasmids