GENETICS
OF
MICROORGANISMS
QUESTIONS OF LECTURE
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•
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1. Introduction
2. Genetics of microorganisms
3. Genotypic and phenotypic variations
4. Transmission of genetic material
GENETICS
• Genetics is the study of heredity and
variation, seeking to understand the
causes of the resemblances and
differences between parents and their
progeny
Genetics of microorganisms
• The «central dogma» of molecular
biology is that DNA carries genetic
information, which is transcribed on to
RNA and then translated as the particular
polypeptide
• (DNA - RNA - polypeptide)
• An exception exists in the case of some
viruses genetic material is RNA instead
of DNA
Genetics of microorganisms
• The DNA molecule is composed of two chains
of nucleotides bound together in the form of a
«double helix»
• Each chain has a backbone of desoxyribose and
phosphate residues arranged alternately
• Attached to each desoxyribose is one of four
nitrogenous bases, the purines: adenine (A) and
guanine (G), and the pyrimidines: thymine (T)
and cytosine (C)
• Adenine is always linked to thymine, and
guanine to cytosine: A=T, G=C
Genetics of microorganisms
• Genetic information is stored in the DNA as a
code, the unit of the code (codon) consisting of a
sequence of three bases (the code is triplet)
• Each codon specifies for a single aminoacid, but
the code is «degenerate» so that more than one
codon may exist for the same aminoacid
• A segment of DNA carrying codons specifying
for a particular polypeptide is called a «cistron»
or gene
• The bacterial chromosome carries about 10004000 cistrons.
Genetics of microorganisms
Structure of operon
•
Lac
Lac P
Lac O
• Regulator Promotor Operator
Lac Z - Y -A
Structural genes
• For transcription of RNA for the enzyme
synthesis, the RNA polymerase has to attach to
the promotor region and travel along the
structural genes in sequence.
• The transfer of genetic information from DNA
to RNA is called transcription and from RNA to
protein is called translation.
Genetics of microorganisms
• RNA is structurally similar to DNA except for three
major differences:
• It contains the sugar ribose (desoxyribose in DNA)
• One of pyrimidine base is uracile instead thymine
in DNA
• There are 3 distinct types of RNA on the basis of
structure and function:
• 1. Messenger RNA (m RNA)
• 2. Ribosomal RNA (r RNA)
• 3. Transfer RNA (t RNA)
• DNA acts as the template for the synthesis of
mRNA.
Genotypic and phenotypic variations
• The sum total of genes the make up the genetic
apparatus of a cell establishes its genotype.
• The phenotype («phaeno»: display) is the
physical expression of the genotype in a given
environment.
Genotypic and phenotypic variations
• Phenotypic variations are influenced by the
environment, limited in range by the genotype,
temporary and not heritable.
• Variations are genotypic when they are due to
alterations in the genome. Genotypic variations
are stable, heritable and not influenced by the
environment.
• They may occur by mutation, or by one of the
mechanisms of genetic transfer or exchange,
such as transformation, transduction, lysogenic
conversion and conjugation.
Mutation
• Mutation is a heritable variation caused by an
alteration in the nucleotide sequence at some
point of the DNA
• The sequence of nucleotides in DNA can
change in either of 2 ways:
• (a) by substitution of one base pair for another
as the result of a replication error – transition
or transvertion.
• by breakage of the sugar phosphate back bone
with subsequent deletion or insertion of a
DNA segment.
MUTATIONS IN BACTERIA
– Spontaneous (replication error )
– Induced (mutations are induced by a variety of
physical, chemical and biological agents)
Physical agents are radiations, heats etc.
Chemical agents are nitroso compounds,
alkylating agents, base analogs, and others.
TRANSFORMATION
•
Definition: It is the transfer of genetic
information through the agency of free DNA.
Pieces of DNA involved in transformation
may carry 10 to 50 genes. Factors affecting
transformation
– DNA size and state
– Competence of the recipient (Bacillus,
Haemophilus, Neisseria, Streptococcus)
TRANSFORMATION
• Steps
– Uptake of DNA
• Gram +
• Gram -
– Recombination
• Significance
Phase variation
– Recombinant DNA technology
–
TRANSDUCTION
• Definition: Gene
transfer from a donor to
a recipient by way of a
bacteriophage
• Bacteriophage (phage):
A virus that infects
bacteria
Phage Composition and Structure
• Composition
– Nucleic acid
Head/Capsid
• Genome size
• Modified bases
– Protein
• Protection
• Infection
• Structure (T4)
– Size (80 X 100 nm)
– Head or capsid
– Tail
Contractile
Sheath
Tail
Tail Fibers
Base Plate
Infection of Host Cells by Phages
• Adsorption
–Tail fibers
– Receptor is LPS for T4
• Irreversible attachment
– Base plate
• Sheath Contraction
• Nucleic acid injection
• DNA uptake
Microbe Library, American Society for Microbiology
www.microbelibrary.org
TRANSDUCTION
• Types of transduction
– Generalized
Transduction
in
which
potentially
any dornor bacterial
gene
can
be
transferred.
– Specialized (Restricted) - Transduction
in which only certain donor genes can
be transferred
Generalized Transduction
• Infection of Donor
• Phage replication and degradation of host DNA
•
•
•
•
Assembly of phages particles
Release of phage
Infection of recipient
Homologous recombination
Potentially any donor gene can be transferred
BACTERIOPHAGES EXHIBIT TWO
TYPES OF LIFE CYCLE:
In the virulent or lytic cycle, large numbers of
progeny phages are built up inside the host
bacterium, which ruptures to release them.
In the temperate or nonlytic cycle, the phage
DNA becomes integrated with the bacterial
chromosome as the prophage and is transferred
to the daughter cells. This process is called
lysogenic or phage convertion.
CONJUGATION
• Definition: Gene transfer from a
donor to a recipient by direct
physical contact between cells
• Mating types in bacteria
– Donor
• F factor (Fertility factor) is a
transfer factor that contains the
genetic information necessary for the
synthesis of the sex pilus and for
self-transfer. Cells carrying the F
factor are called F+ cells. F factor
may exist in the «integrated state» or
inserted into the host chromosome.
Such cells are known as Hfr cells.
– Recipient
• Lacks an F factor
Donor
Recipient
Mechanism of F+ x F- Crosses
• Pair formation
– Conjugation
bridge
• DNA transfer
– Origin of
transfer
– Rolling circle
replication
Microbe Library, American Society for Microbiology
www.microbelibrary.org
Transposable Genetic Elements
• Definition: Segments of DNA that are able to
move from one location to another
• Properties
– “Random” movement
– Not capable of self replication (not a replicon)
– Transposition mediated by site-specific
recombination
• Transposase
– Transposition may be accompanied by
duplication
Types of Transposable Genetic
Elements
• Insertion sequences (IS)
– Definition: Elements that carry no other genes
except those involved in transposition
– Nomenclature - IS1
– Structure
GFEDCBA
ABCDEFG
Transposase
– Importance
• Mutation
•Plasmid insertion
•Phase variation
Types of Transposable Genetic
Elements
• Transposons (Tn)
– Definition: Elements that carry other genes
except those involved in transposition
– Nomenclature - Tn10
– Structure
• Composite Tns
– Importance
• Antibiotic resistance
IS
Resistance Gene(s)
IS
IS
Resistance Gene(s)
IS
Most of the
transposons
in the genome
are stably
silenced and
many of them
have been
“tamed” and do
important
cellular jobs
PLASMIDS
• Definition:
Extrachromosomal
genetic elements
that are capable of
autonomous
replication
(replicon)
• Episome - a
plasmid that can
integrate into the
chromosome
Classification of Plasmids
• Transfer properties
– Conjugative are transferred from bacterium to
bacterium (usually members of the same
species or of very closely related species)
through conjugation
– Nonconjugative
• Phenotypic effects
–
–
–
–
–
Fertility (F-plasmids)
Bacteriocinogenic plasmid (Col-plasmid)
Resistance plasmid (R factors) (R-plasmid)
Enterotoxin plasmid (Ent-plasmid)
Haemolysin plasmid (Hly-plasmid)
Structure of R Factors
• RTF
RTF
– Conjugative
plasmid
– Transfer genes
• R determinant
– Resistance genes
– Transposons
R determinant
Genetic Engineering
• It is now possible to isolate the genes coding for any
desired protein from microorganisms and introduce them
into suitable microorganisms, in which the genes would be
functional directing the production of the specific protein.
This is known as the Recombinant-DNA technology or
Genetic engineering.
Plasmid DNA vaccination
• Multivalent DNA vaccine
for malaria.
• Against tuberculosis
• Against hepatitis B virus.