A COMPREHENSIVE VIEW OF BACTERIAL GENETICS IN SIMPLE EASY TO UNDERSTAND ,SUMMARRIZED, ORGANISED POWERPOINTS

1. BACTERIAL GENETICS

2. Bacterial Genetics
1. Microbial Genome(CHROMOSOMAL DNA)
- Essential for the organism’s survival and contains genes responsible for
various cellular functions.
- Replication - Transcription – Translation
2. Plasmid(EXTRACHROMOSOMAL DNA)
-Responsible for genetic exchange & variation in bacteria; provide selective
advantages when in host, such as antibiotic resistance, toxin production, or
metabolic capabilities.
- Mutations - Transduction - Conjugation – Transformation - Transposition

3. Cell Division Of Bacteria “Binary Transverse Fission”
• Cell elongates as growth occurs longitudinal axis
• When certain length reached; septum produced in transverse axis
midway btn the cell ends NB; DNA replication preceeds septum
formation.

5. DNA Replication
• It is semiconservative type
• It begins at the origin of replication oriC; a 245 base pair sequence
1. The oriC opened by DnaA Protein
2. Parent strands unwind by DNA gyrase & @ acts as a template for synthesis
of complementary strand.
3. New strands synthesized by DNA polymerase III
4. Process goes bi-directional until replication forks meet; a point at which 2
double stranded daughter DNAs are formed
5. Ends of fully & newly formed strands are joined by DNA ligase to form circular
chromosomes

6. 1

7. Replication fork
1. Single-stranded DNA binding protein
- coats single strands preventing them from being denatured
2. DNA gyrase & helicase
-Unwind DNA duplex
3. DNA polymerase III
- Adds nucleotides to the 3’ end o the leading strand & Okazaki fragments to the RNA primers of the
lagging strand
4. DNA polymerase I & ligase
- Remove RNA primers replacing them with appropriate DNA segments & join the Okazaki fragments
4. RNA PRIMASE
-Adds RNA primers

10. Mut SHL Repair System
• During replication; base pair mismatch in growing chain are corrected by the 3’ to
5’ exonuclease activity of DNA polymerase III
1. Mut S binds to mismatched base pair
2. Mut H moves along duplex till it finds a methylated base in parent strand
3. Mut L binds to Mut S & Mut H; Mut H nicks the unmethylated strand
4. Helicase & single stranded exonuclease remove the DNA segment
5. Polymerase & ligase add the correct DNA segment in the gap
6. Daughter strand is then methylated

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4
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9

13. Transcription
• Copy of genes from DNA to RNA (tRNA & rRNA); where they are expressed as
proteins needed for sustaining life.
• Only short segments of DNA are copied
-Assembly of transcription complex
- Initiation
-Elongation
- Termination
-Regulation transcription

14. 1. Assembly of transcription complex
• Only 1 strand (antisense/template/non-coding) of DNA is copied
• RNA polymerase activity & sigma factor
• RNA polymerase & sigma binds to the promoter site (beginning of a gene)
forming a closed promoter complex.
• DNA unwinds to form open promoter complex

15. 2. Initiation & Elongation
• RNA P begins mRNA synthesis by adding a purine nucleotide
• After addition of 5 or 6 nucleotides, sigma is released & RNA P continues down
the template. "elongation”

16. 4.Termination
• Rho-independent termination;
The G-C rich region with in the transcript forms a hair pin loop
Weak pairing of A (DNA template) : U (RNA transcript)
Double helix zips up and RNA transcripts dissociates from DNA

17. • Rho-dependent;
Rho protein(a helicase) binds to C-rich region in transcript & advances in 5’ to
3’ direction till it meets transcription bubble.
Bring about unwinding of the transcript & template; and mRNA is released

18. Regulation Transcription
• Bacteria can adapt to specific environmental conditions by altering levels of mRNA.
1. Negative regulation;
The environmental signals interfere with transcription initiation
Regulation Of Lac Operon
-Genes of lac operon can only be expressed when lactose is the only carbon
source; Lac Z, Lac Y, Lac A, (lacZ βgalactosidase, lacY lactose permease, and lacA transacetylase )these are
necessary for lactose catabolism
- In absence of lactose repressor monomers form tetramer that binds to
promoter & blocks transcription
- In presence of lactose, it binds & inactivates the repressor

20. 2. Positive regulation;
environmental signals facilitate transcription
Regulation Of Mal Operon
- Presence of maltose activates maltose genes
- Active Malt gene product binds to promoter of genes involved in the
transport & catabolism of maltose

21. TRANSLATION
• Conversion mRNA to protein
• Components; Ribosomes & tRNA
• RIBOSOMES;
- Consist of both protein and ribosomal RNA (rRNA).
- Site where Amino acids are linked together by peptide bonds to form protein
-Prokaryotic ribosomes is 70s(svedberg unit) having 30s subunit & 50s
subunit

23. 1. Initiation
30s subunit binds to Shine-Delgarno Sequence (GGAGGU) on mRNA through its
complementary sequence ACCUCC with the help of initiation factors (IF1 & IF3) on
the 3’ end of the 16s rRNA
GGAGGU is 4-6 nucleotides from the initiation codon (AUG)

24. IF2 brings an initiator tRNA charged with initiator amino acid N-formyl-
Methionine
Pairing occurs between anticodon UAC & complementary codon AUG
Larger subunit 50s then binds to the complex & IF1, IF2, IF3 & 16S rRNA are
released.
A site; entry site for new tRNA charged
with amino acrid
P site; occupied by tRNA with amino acid
of growing polypeptide chain
E site; exit for tRNA after delivering amino acid

26. 2. Elongation
A new tRNA carrying an amino acid enters A site then matching between anticodon of tRNA
& codon on mRNA occurs; those with incorrect anticodons are rejected & replaced by new till a
right amino-acyl tRNA enters A site
A peptide bond made between the 2 adjacent amino acids
tRNA in P site releases amino acid/peptide chain to tRNA in A site
The ribosome moves to one triplet forward on mRNA; the empty tRNA moves to the E-
site
A site is now empty & ready to accept new tRNA

27. 3. Termination
Occurs when a termination codon occupies the A-site and the codon is
recognized by a release factor (RF)
RF1 recognizes termination codons UAA and UAG, whereas RF2 recognizes UAA
or UGA
RF3-GTP,( GTPase protein enhances activity of RF1 & RF2) then binds to the ribosome
and catalyzes the cleavage of the peptide chain from the last tRNA
GTP provides energy for the dissociation of the ribosomes and the release of
the RFs

28. Basic characteristics of plasmids
Extrachromosomal DNA, replication occurs by bacterial cell machinery
Circular double stranded DNA
Variable size (100 to 1000)bp
Copy number: 1-30 copies per cell
Some are transferable > conjugation; Only to closely related species except
for promiscuous plasmids
Plasmid encoded genes not essential to the growth of organism but rather
survival.

29. • Plasmids encode genes for specialized metabolism
Biodegradation of complex organic molecules e.g. some Pseudomonas
This allows the bacterium to utilize unusual organic compounds as
carbon and energy source

30. ORGANISM FACTOR DISEASE
Escherichia Coli Enterotoxin Diarrhea
Clostridium Tetani Neurotoxin Tetanus
Staphylococcus Aureus Coagulase Enterotoxin Food Poisoning/Skin
Infections/Boils
Streptococcus Mutans Dextransucrase Tooth Decay
Agrobacterium Tumefaciens Tumor Crown Gall
Staphylococcus Spp Antibiotic Resistance Various
Examples Of Virulence Factors Encoded In Plasmids

31. Resistance (R) -plasmids
• Classified as R-plasmids because of the R-factors that encode antibiotic-
resistance determinants
• A plasmid can acquire additional R-factors
• Implications: R-factors affect therapeutic efficacy
Antibiotic sensitive bacterium may become
resistant following acquisition of R-plasmids.
Neisseria gonorrhoeae has plasmid encoding
β-lactamase hence resistance to ampicillin.

32. DRUG MECHANISM OF RESISTANCE
Beta Lactams Synthesis Of Beta Lactamase
Chloramphenicol Synthesis Of Enzyme That Acylates The Drug
Aminoglycoside Synthesis Of Enzyme That Inactivate Drug By
Acetylation, Phosphorylation, Or Adenylation
Tetracycline Synthesis Of Membrane Protein Capable Of Pumping
Out Drug Before It Acts On Ribosomes
Erythromycin Synthesis Of Enzyme That Methylates 23s Ribosomal
RNA
TRIMETHOPRIM SYNTHESIS OF MUTANT TRIMETHOPRIM

33. GENETIC VARIATION IN BACTERIA
• Mechanisms of genetic variation in bacteria:
i) Transformation
ii) Conjugation
iii) Transduction (Generalized & Specialized)
iv) Transposition
v) Mutation

34. DNA Recombination
• Natural way by which genetic materials are linked

35. Transformation
• Bacterium takes in DNA from its environment. This DNA can come
from other bacteria that have shed it.
• Conversion of avirulent rough (R) type of S. pneumoniae to virulent smooth
(S) type by transformation of R type cells with DNA obtained from S type
cells

36. • Virtually all bacteria have an ability to take up DNA from the environment,
provided they are competent
• Transformation competence is a state of a bacterial cell during which the usually
rigid cell wall can transport a relatively large DNA macromolecule
• Some bacteria, such as Hemophilus, Streptococcus, or Neisseria (during a certain
stage of cell division) can take up DNA => natural competence.
• Treatment of such cells that are usually unable to take up DNA under natural
conditions with CaCl2 or RbCl alters their envelope, and they become competent
=> artificial competence

37. Fate of DNA after entering into a competent cell
• Transformation of bacteria with a linear DNA fragments is more complex because
the new DNA is a natural target for hydrolysis by intracellular enzymes that
degrade noncircular DNA.
• One mechanism whereby foreign DNA can escape degradation is incorporation
into the chromosome of the recipient cell via recombination

38. Conjugation
• Mechanism of DNA exchange mediated by plasmids
• Performed by transmissible plasmids
• Donor cells have transmissible plasmids; those cells that receive plasmids
recipients
• Fertility (F) plasmid of E. coli or the large R plasmids of a variety of other
bacteria (self-transmissible plasmids), encode genes responsible for their own
replication, maintenance within the bacterial cell and the promotion of DNA
transfer via conjugation

39. • Transfer functions involve pilus

40. Transduction
• Is the process of transferring genes between bacteria, which is mediated by
bacteriophages (viruses that infect bacteria)
• General transduction: random transfer of all bacterial genes at low but identical
frequencies
• Specialized transduction: certain genes are transferred at very high frequencies,
whereas others are transferred at low rates or not at all
• Specialized transduction requires incorporation of the viral DNA into the bacterial
chromosome, and it is therefore carried out only by lysogenic bacteriophages

41. • Specialized transduction results in phage mediated transfer of genes that are near
the attachment site of the lysogenic prophage on the chromosome.

44. Transposition
• An Intracellular gene transfer where a gene moves from one place on the
chromosome to another.
OR:
• A gene moves between a chromosomal site and a plasmid site
• The site-specific recombination utilizes DNA sequences that define the site of
transposition, and specialized enzymatic machinery that catalyzes the
transpositional event
• The simplest form of such a mobile gene is an insertion sequence (IS)

46. • A common feature of IS: they contain short (16 to 41 bp) inverted repeat
sequences at their ends.
• The IS also encodes at least one enzyme, called transposase, that specifically
mediates the site-specific recombination event during transposition