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FE A. BARTOLOME M.D., DPASMAP
Department of Microbiology
Our Lad of Fatima University
GENETIC MATERIAL
Bacterial Genome
• Total collection of genes carried by a
bacterium both on its chromosome
and on its extrachromosomal genetic
element
GENETIC MATERIAL
DNA
1. Most are double stranded
• Template strand
• Coding strand
2. Complementary bases paired by H-bonding (A-T; G-C)
3. Length – kbp
4. Distance between base pairs = 0.34 nm (3.4 x 10-7
mm)
GENETIC MATERIAL
RNA
1. Single-stranded
2. Complementary bases – A-U; G-C
3. Types:
a. mRNA – for translation of DNA gene sequences
b. rRNA – protein synthesis
c. tRNA – protein synthesis
GENETIC MATERIAL
RNA
1. Single-stranded
2. Complementary bases – A-U; G-C
3. Types:
a. mRNA – for translation of DNA gene sequences
b. rRNA – protein synthesis
c. tRNA – protein synthesis
PROKARYOTIC GENOME
1. A replicon – DNA circles (chromosome +
plasmid) which contain genetic information
necessary for their own replication
2. Single circular double stranded DNA molecule
3. Usually haploid
4. No histones – structure maintained by
polyamines (spermine & spermidine)
BACTERIAL DNA REPLICATION
1. Initiated at OriC
2. Semi-conservative
3. New DNA synthesis occurs at growing forks 
proceed bidirectionally
• Leading strand – continuous (5’  3’)
• Lagging strand – Okazaki’s fragments
BACTERIAL DNA REPLICATION
4. 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
REGULATION OF GENE
EXPRESSION
Mechanisms to adapt to changes in
concentration of nutrients in the environment
1. Organization of biochemical pathways into operons
2. Gene transcription regulated by repressor proteins 
bind to operators
3. Rate of protein synthesis by the ribosome can regulate
transcription in prokaryotes
4. Absence of nuclear membrane allow the ribosome to
bind to the mRNA as it is being transcribed from the
DNA
MUTATION
Types of Mutation
1. Spontaneous – occurs without apparent cause (e.g.
polymerase mistakes)
2. 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
MUTATION
Types of Mutation
1. 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
MUTATION
Types of Alterations in the DNA (as reflected in
mRNA)
1. Addition or insertion
2. Deletion
3. Substitution
a. Transition – purine to purine; pyrimidine to
pyrimidine
b. Transversion – purine to pyrimidine and vice
versa
MUTATION
Effects of Mutation
1. 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)
MUTATION
Effects of Mutation
2. 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
MUTATION
Repair Mechanisms
1. Direct DNA repair
• Enzymatic removal of damage, such as pyrimidine
dimers and alkylated bases
2. 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
MUTATION
Repair Mechanisms
3. Recombinational or post-replication repair
• Retrieval of missing information by genetic
recombination when both DNA strands are damaged
4. SOS response
• Induction of many genes after DNA damage or
interruption of DNA replication
5. 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
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.
GENE TRANSFER
TRANSFORMATION
+
Donor
cell
Competent
recipient
cell
The new gene
is expressed
+
Free DNA
Look for area
of homology
ds DNA adsorbs
to the surface
Entry of ss-DNA
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
GENE TRANSFER
CONJUGATION
Three forms of F:
1. F+ - F plasmid transferred to recipient cell 
recipient cell becomes F+ male
2. F’ – if fragment of chromosomal DNA is
incorporated into the plasmid  F’ male
3. Hfr – if F plasmid sequence is integrated into the
bacterial chromosome  cell called Hfr cell (high
frequency of recombination cell)
GENE TRANSFER
GENE TRANSFER
CONJUGATION
F plasmid
• E. coli, Bacteroides sp., Streptococci,
Streptomyces, Clostridia
R plasmid
• does not use pili; transfer antibiotic
resistance
• Streptococci, Streptomyces, Clostridia
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:
1. Specialized – phage transfers particular genes,
usually adjacent to their integration sites
2. Generalized – selection of sequences is random;
contain primarily bacterial DNA & little or no
phage DNA
GENE TRANSFER
TRANSDUCTION