1. BACTERIAL GENETICS FE A. BARTOLOME M.D., DPASMAP
Department of Microbiology
Our Lad of Fatima University

2. GENETIC MATERIAL
Bacterial Genome
Total collection of genes carried by a bacterium both on its chromosome and on its extrachromosomal genetic element

3. GENETIC MATERIAL DNA Most are double stranded Template strand
Coding strand
Complementary bases paired by H-bonding (A-T; G-C)
Length – kbp
Distance between base pairs = 0.34 nm (3.4 x 10-7 mm)

4. GENETIC MATERIAL RNA Single-stranded Complementary bases – A-U; G-C
Types:
a. mRNA – for translation of DNA gene sequences
b. rRNA – protein synthesis
c. tRNA – protein synthesis

5. GENETIC MATERIAL RNA Single-stranded Complementary bases – A-U; G-C
Types:
a. mRNA – for translation of DNA gene sequences
b. rRNA – protein synthesis
c. tRNA – protein synthesis

6. PROKARYOTIC GENOME
A replicon – DNA circles (chromosome + plasmid) which contain genetic information necessary for their own replication
Single circular double stranded DNA molecule
Usually haploid
No histones – structure maintained by polyamines (spermine & spermidine)

7. BACTERIAL DNA REPLICATION
Initiated at OriC
Semi-conservative
New DNA synthesis occurs at growing forks  proceed bidirectionally
Leading strand – continuous (5’  3’)
Lagging strand – Okazaki’s fragments

8. BACTERIAL DNA REPLICATION
Enzymes:
a. Helicase – unwinds the DNA at the origin
b. Primase – synthesize primers to start the process
c. DNA polymerase – copy the DNA; with proofreading functions
d. Topoisomerase (gyrase) – relieves torsional strain
e. DNA ligase

9. REGULATION OF GENE EXPRESSION
Mechanisms to adapt to changes in concentration of nutrients in the environment
Organization of biochemical pathways into operons
Gene transcription regulated by repressor proteins  bind to operators
Rate of protein synthesis by the ribosome can regulate transcription in prokaryotes
Absence of nuclear membrane allow the ribosome to bind to the mRNA as it is being transcribed from the DNA

10. MUTATION Types of Mutation
Spontaneous – occurs without apparent cause (e.g. polymerase mistakes)
Induced – caused by mutagen; increased frequency of mutation
a. Physical agents
(1) heat  deamination of nucleotides
(2) UVL  pyrimidine dimer formation  form cyclobutane ring between 2 pyrimidines
(3) ionizing radiation  form free radicals  cause single strand breaks

11. MUTATION Types of Mutation
Induced – caused by mutagen; increased frequency of mutation
b. Chemicals
(1) nucleotide base analogues  mispairing (e.g. 5-bromouracil and thymine)
(2) polycyclic flat molecules (e.g. EtBr)  intercalates DNA  inc. spacing of successive base pairs
(3) DNA reactive chemicals (e.g. nitrous acid & alkylating agents)  modify normal base into a chemically different structure

12. MUTATION Types of Alterations in the DNA (as reflected in mRNA)
Addition or insertion
Deletion
Substitution
a. Transition – purine to purine; pyrimidine to pyrimidine
b. Transversion – purine to pyrimidine and vice versa

13. MUTATION Effects of Mutation Single codon change
a. Silent mutation – change at the DNA level that does not result in any change of a.a. in the encoded protein
Occurs because >1 codon may encode an a.a. (e.g. val  ala)
b. Missense mutation – result in substitution of 1 a.a. for another
c. Nonsense mutation – codon encoding an a.a. is changed to a stop codon (e.g. TAG)

14. MUTATION Effects of Mutation Numerous bases involved
a. Frameshift mutation  (+) change in reading frame  premature truncation of protein
b. Null mutation – with extensive insertion, deletion or gross rearrangement of chromosome structure  completely destroy gene function

15. MUTATION Repair Mechanisms Direct DNA repair
Enzymatic removal of damage, such as pyrimidine dimers and alkylated bases
Excision repair
Excision of a DNA segment containing the damage, followed by synthesis of a new DNA strand
Damage recognized by endonuclease  cut phosphodiester backbone on both sides of damage
Excision of DNA with damaged backbone carried out by exonuclease

16. MUTATION Repair Mechanisms Recombinational or post-replication repair
Retrieval of missing information by genetic recombination when both DNA strands are damaged
SOS response
Induction of many genes after DNA damage or interruption of DNA replication
Error-prone repair
Last resort of a bacterial cell before it dies
Fill in gaps when a DNA template is not available for directing an accurate repair

17. GENE TRANSFER TRANSFORMATION
First mechanism of genetic transfer to be discovered
Involves the release of DNA into the environment by the lysis of some cells, followed by the direct uptake of the DNA by the recipient
Occurs among related species
Requires competence of recipient cells
Gram (+) and gram (-) bacteria  H. influenzae, S. pneumoniae, Bacillus sp., Neisseria sp.

18. GENE TRANSFER TRANSFORMATION + Donor cell Competent recipient cell
Free DNA
ds DNA adsorbs to the surface
Entry of ss-DNA
Look for area of homology
The new gene is expressed

19. GENE TRANSFER CONJUGATION
DNA passed directly by cell-to-cell contact during the mating of the bacteria  sex-like exchange
Results in a one way transfer of DNA from a donor (male) cell to a recipient (female) cell via the sex pilus
Depends on the presence or absence of a conjugative plasmid (e.g. F plasmid of E. coli)
F plasmid transfers itself  convert recipient into F+ male
DNA transferred is single stranded

20. GENE TRANSFER CONJUGATION Three forms of F:
F+ - F plasmid transferred to recipient cell  recipient cell becomes F+ male
F’ – if fragment of chromosomal DNA is incorporated into the plasmid  F’ male
Hfr – if F plasmid sequence is integrated into the bacterial chromosome  cell called Hfr cell (high frequency of recombination cell)

21. GENE TRANSFER

22. GENE TRANSFER CONJUGATION F plasmid
E. coli, Bacteroides sp., Streptococci, Streptomyces, Clostridia
R plasmid
does not use pili; transfer antibiotic resistance
Streptococci, Streptomyces, Clostridia

23. GENE TRANSFER TRANSDUCTION
Mediated by bacteriophages  picks up fragments of DNA & package them into bacteriophage particles  DNA delivered to infected cells  incorporated into bacterial genome
Types:
Specialized – phage transfers particular genes, usually adjacent to their integration sites
Generalized – selection of sequences is random; contain primarily bacterial DNA & little or no phage DNA

24. GENE TRANSFER
TRANSDUCTION

25. THANK YOU