1. Bacterial Genetics

2. The rapid development of molecular genetics, in conjunction with the ability of scientists to manipulate and transfer genes, has elicited considerable interest among physicians, scientists, and the public in general.
The ability to manipulate the hereditary material of microorganisms and even our own species has far reaching consequences.

3. Gene Mutation Any change in the nucleotide sequence of a gene.
It may result from base substitutions, deletions, insertions and rearrangement.
Transition Vs transversion of nucleotides
Mismatch repair by proof reading enzymes (SOS system)
Mutations in bacterial populations can pose a problem, bacteria have mechanisms by which genes can be transferred to other bacteria. Thus,
a mutation arising in one cell can be passed on to other cells.

4. Types of Mutation 1) Silent mutation 2) Missense mutation
3) Nonsense mutation
4) Frame-shift mutation
5) Null mutation
6) Suppressor mutation (intragenic or
extragenic)

5. Spontaneous Mutation
10-4 (hot spots) to Generally 10-7 in progeny of a single cell.
Mechanism = electrochemical rearrangement, or moving around of electrons in the nucleotide molecule.
Shift from a keto state to the enol form
( tautomerization) promotes abnormal hydrogen bonding between nucleotides.
Thymine (keto) → adenine
Thymine (enol) → guanine

6. Induced Mutation Chemical Agents
Mutagens (Chemical and Physical agents)
Chemical Agents
5-Bromouracil: analogue of thymine binds to G.
2-Aminopurine: analogue of adenine binds to C.
Nitrous acid (HNO2): Nucleotide deamination
Alkylating agents (eg ethyl-methyl sulfonate)
Acridine dyes (Proflavin): insertion or deletion leading to frame-shift mutation.

7. Physical Agents Heat: deamination of nucleotides.
UV light: Pyrimidine dimmers (thymine), hydrated pyrimidines.
Gamma and x-rays: breaks in the sugar phosphate backbone.

8. Gene Transfer
Transfer of DNA from a donor cell to a recipient cell with the resultant recombination.
Gene transfer in bacteria is unidirectional:
Donor → recipient
It requires DNA homology
A small part of DNA is transferred
It is achieved by three mechanisms: Transformation, Transduction and Conjugation.

9. Transformation Uptake of naked fragmented single stranded DNA
Most primitive mechanism of gene transfer
It occurs in only a few genera
Bacillus subtilis
Streptococcus pneumoniae
Hemophilus influenzae
Neisseria gonorrhea
E. coli (in the presence of a high concentration of calcium salts and high temperature)

10. The probability that transformation has contributed much to genetic variety is very small.
Factors affecting transformation:
- DNA size and state
- Competence of the recipient
Steps:
- Uptake of DNA
- Legitimate/Homologous/General
recombination

11. Transduction Transfer of DNA via a bacteriophage
Lytic Vs lysogenic (temperate) phage life cycle.
Restricted Vs generalized transduction
Not affected by nucleases in the environment
Phages that mediate generalized transduction generally breakdown host DNA into smaller pieces and package their DNA by a "head-full" mechanism.
Examples: diphtheria toxin, erythrogenic toxin and resistance to penicillin in staphylococci.

12. Phage Composition and Structure

13. Plasmid- Mediated Conjugation
PIASMIDS
- Circular ds DNA molecules ranging in size
from 1/1000 to 1/10 of the chromosome.
- They are usually autonomous (replicons).
- As few as 1 to as many as 100/cell.
- May become integrated (episomes).
- Two classes: conjugative and nonconjugative

14. Conjugation Requires the sex (F) factor
Donor cell (F+) and recipient cell (F- )
F factor is a ds DNA that is 1/40 of the chromosome in size, not under the control of the bacterial chromosome, ant it can replicate autonomously.
It possesses several genes that code for the formation of sex pili.

15. Conjugation:
During the conjugation process, a copy of the sex factor is made and one of its (single) strands is transferred to the recipient cell.
The complementary strand of the donor single strand is synthesized in the recipient cell which is now converted to an F+ cell.

16. Mechanism of F+ x F- Crosses
Pair formation
Conjugation bridge F+ F-
DNA transfer
Origin of transfer
Rolling circle replication

17. Physiological States of F Factor
Integrated (Hfr)
Characteristics of Hfr x F- crosses
F- rarely becomes Hfr while Hfr remains Hfr
High transfer of certain donor chromosomal genes F+
Hfr

18. Functions of Plasmids
R Plasmids: Antimicrobial resistance, may be multiple but clustered.
Resistance to heavy metals (Ag, Co, Hg) in Staphylococci, coexist with R plasmids.
Virulence determinants: toxins
Bacteriocinogenic plasmids

19. Mechanisms of Action of Resistance plasmids
Modification of antibiotic
Alteration of target site
Alteration of uptake
Replacement of a sensitive pathway

20. Transposable genetic Elements
Move from one location to another (jumping genes)
1) Random movement (not totally=preferable sites)
2) Not capable of self replication
3) Transposition mediated by site-specific
recombination (mediated by transposases) no
homology is required
4) Transposition can be accompanied by duplication

21. Types 1) Insertion sequences (Is): Small stretches of
DNA that have at their ends repeated
sequences
2) Transposons: (Tn) Carry extra genes

22. Genetic Engineering Cloning (recombinant DNA)
- RE sticky ends (passenger DNA)
- RE sticky ends (vector DNA) of plasmids
- Annealing (ligase)
- Insertion to a cell (transformation)