Transcript bacterial genetics

BACTERIAL GENETICS
Dr. Waleed Khalid
Lec. : 3
Bacterial Genetics

Genetics is the study of heredity and variation.
The unit of heredity is gene, which is a segment of
DNA specifying for a particular polypeptide .
Most bacterial genes code for proteins (Exons) but
some genes are noncoding interposed sequences
(introns), like those seen in eukaryotes.
Exons - coding sequences on a gene translated into gene
products.
Introns - non coding sequences on a gene.

Bacterial genetics is used as a model to understand DNA
replication, genetic characters, their changes & transfer to
next generations

Bacteria possess two genetic structures: the chromosome
and the plasmid.
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Both of these structures consist of a circular double
stranded DNA molecule twisted about its helical axis.

Replication of this DNA molecule always starts at a
certain point (the origin of replication) and it is “semiconservative” meaning that one strand in each of the
two resulting double strands is conserved .
Nucleic Acids

DNA ( Deoxy Ribonucleic Acid ) : Stores information for
protein synthesis.
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RNA ( ribonucleic acid ) : Transcription & translation of
information for protein synthesis.
Structure Of DNA

The bacterial genetic information is stored in its
chromosome and plasmids.

DNA is composed of 2 chains of polypeptides,
each chain has a backbone of deoxyribose sugar
and phosphate residues with 4 nitrogenous bases:
Adenine (A)
Guanine (G)
Thymine(T)
Cytosine (C)
Double helical structure of
DNA by Watson & Crick
Chromosome:

The chromosome corresponds to the nucleoid .
The E. coli chromosome is composed of 4.63 * 106
base pairs (bp) and codes for 4288 proteins.

The gene sequence is colinear with the expressed
genetic products.

The chromosomes of E. coli and numerous other
pathogenic bacteria have now been completely
sequenced
Plasmids:
The plasmids are autonomous DNA molecules of
varying size (3 *103 to 4.5 * 105 bp) localized in the
cytoplasm.

Large plasmids are usually present in one to two
copies per cell, whereas small ones may be present
in 10, 40, or 100 copies.
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Plasmids are not essential to a cell’s survival and
many of them carry genes that code for certain
phenotypic characteristics of the host cell.

The following plasmid types are medically important:

Virulence plasmids:
Carry determinants of bacterial virulence, e.g.,
enterotoxin genes or hemolysin genes.

Resistance plasmids:
Carry genetic information causing resistance to
anti-infective agents.
 Some plasmids carry both virulence
and resistance genes.
Genetic Information In Bacteria
Chromosome
Corresponds to the nucleoid
Plasmid
Extrachromosomal genetic
material in the cytoplasm
Replicate independently
Bacteriophage
Virus infecting bacteria
Structure Of RNA

Structurally similar to DNA, except for 2
major differences:
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

ribose sugar
uracil in place of thymine.
3 types of RNA
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m RNA (messenger RNA)
t RNA ( transfer RNA )
r RNA ( ribosomal RNA )
DNA Replication:
-The identical duplication process of DNA is
termed semi-conservative because the double
strand of DNA is opened up during replication and
each strand serves as the matrix for synthesis of
a complementary strand. Thus each of the two
new double strands “conserves” one old strand.
-The doubling of each DNA molecule begins at a
given starting point called origin of replication.
This process continues throughout the entire
cycle.
Genotypic & Phenotypic Variations

Genotype – genetic constitution of a cell that is
transmitted to its progeny

Phenotype – physical expression of the genotype
in a given environment

Variations
Phenotypic variations –
1.


influenced by the environment
temporary & not heritable
Genotypic variations –
2.
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Not influenced by the environment
Stable & heritable
Mechanisms Of Genetic Variations
 1-
Parasexuality
 2- Mutation
1- Parasexuality

Although bacteria have no sexual heredity , they have
mechanisms that allow for intercellular DNA transfer
involving a unilateral transfer of genetic information from a
donor cell to a receptor cell and it is called parasexuality.
Mechanisms of parasexuality are :
1.
Transformation
2.
Conjugation
3.
Transduction
1.
Transformation
It is the transfer of “naked” DNA after cell lysis and this
transformation process has been observed mainly in
the genera Streptococcus, Neisseria, Helicobacter and
Haemophilus.
2.
Conjugation
It is the transfer of DNA from a donor to a receptor in a
conjugation process involving cell-to-cell contact.
Conjugation is made possible by two genetic elements:
the conjugative plasmids and the conjugative pilli .
Conjugation is seen frequently in Gram-negative rods
(Enterobacteriaceae), in which the phenomenon has been
most thoroughly researched, and enterococci
3.
Transduction
It is the transfer of DNA from a donor to a receptor with
the help of transport bacteriophages.
Bacteriophages
Infection of another bacterium
Transfer of host bacterial DNA to the new bacterium
Acquisition of new characteristics coded by the donor DNA.
Bacteriophages
Definition:-
Bacteriophages are viruses that infect bacteria . They are
therefore obligate cell parasites. They possess only one type of
nucleic acid, either DNA or RNA, have no enzymatic systems
for energy supply and are unable to synthesize proteins on
their own.
Morphology:Similar to the viruses that infect animals and
vary widely in appearance.
Composition:
Phages are made up of protein and nucleic acid. The
proteins form the head, tail, and other morphological
elements, the function of which is to protect the phage
genome.
The nucleic acid in most phages is DNA, which occurs
as a double stranded DNA .
-Following injection of the phage genome, it is
integrated into the chromosome by means of regionspecific recombination employing an integrase and
this process is called Lysogeny .
2-Mutation
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Random, undirected heritable variation

Caused by a change in the nucleotide base
sequence of the DNA
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Types of mutation:
1.
2.
3.

Point mutation
Frame shift mutation
Lethal mutation
Mutagens - Agents which can induce mutation e.g.
UV rays, 5 bromouracil, alkylating agents, etc.
1. Point Mutation

Cause - due to addition, deletion or
substitution of one or more bases.
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Types 
Transition : It is the most common type in
which a purine base is replaced by a purine
base or a pyrimidine base is replaced by
another pyrimidine base.
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Transversion : substitution of a purine base
by a pyrimidine base & vice versa
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Results of mutation 
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Missense mutation – triplet code is
altered so that a different amino
acid is present at a particular
position in the protein.
Nonsense mutation – converts a
codon that specifies an amino acid
into a termination codon.
2. Frame Shift Mutation

Cause - Deletion or
insertion of a base changing all of the
codons downstream
from the change
3. Lethal Mutation

Mutation which affects vital functions resulting
in the death of the organism – (i.e. nonviable
mutation) .

A conditional lethal mutant may be able to live under
certain conditions – permissive conditions.
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Commonest type of conditional mutant is the
temperature sensitive (ts) mutant which is able to live
at the permissive temperature of 35C but not at the
restrictive temp (39C).