Structure & functions of genetic material
Types of mutations
Genetics: The study of heredity and variation, of the
reasons for similarity and differences between
parents and their offspring.
Gene :The unit of hereditary is the gene, a segment of
DNA that carries in its nucleotide sequence
information for a specific biochemical or physiological
Genome: The sum total of all the genetic information
contained in a cell is its genome.
• DNA: or Deoxyribonucleic acid - the principal molecule
responsible for the storage of genetic information.
• Chromosome: The single largest DNA molecule in a cell
that contains the information necessary for the cell’s survival.
• Replicon: Any segment of DNA that is capable of encoding
its own replication in the cell. Thus, the chromosome is the single
largest replicon in the cell.
• Plasmid: Any extra-chromosomal DNA that is capable of
encoding its replication. In other words, non chromosomal
• PHENOTYPE:-Characteristics expressed by a cell
in a given environment. Adaptable in response to
• GENOTYPE:-Collection of genes encoding the
characteristics. Remains Unchanged.
• RECESSIVE :- A gene that does not achieve
phenotypic expression in the presence of its
homologue is recessive.
• DOMINANT:- A gene that over rides the effect Of
its homologue is dominant.
Structure and Function of Genetic
DNA & RNA
• DNA=deoxyribonucleic acid
• RNA=ribonucleic acid
• Basic building blocks:
• Phosphate group
• Pentose sugar
• Nitrogenous base
Structure of DNA
Double stranded (double helix)
Chains of nucleotides
5’ to 3’ (strands are anti-parallel)
Complimentary base pairing
• Bacteria have closed, circular DNA
• Genome: genetic material in an
• E. coli
• 4 million base pairs
• 1 mm long (over 1000 times larger that
actual bacterial cell)
• DNA takes up around 10% of cell volume
DNA Replication-occurs at the replication
• 5’ to 3’
• DNA helicase-unzips + parental DNA strand
that is used as a template
• Leading stand (5’ to 3’-continuous)
- DNA polymerase-joins growing DNA strand
after nucleotides are aligned (complimentary)
• Lagging strand (5’ to 3’-not continuous)
- RNA polymerase (makes short RNA primer)
- DNA polymerase (extends RNA primer then digests RNA
primer and replaces it with DNA)
- DNA ligase (seals Okazaki fragments-the newly formed
• DNA------- mRNA------ protein
of Molecular Genetics
• One strand of DNA used as a template to
make a complimentary strand of mRNA
• Promoter/RNA polymerase/termination site/5’
• Ways in which RNA & DNA differ:
• RNA is ss
• RNA sugar is ribose
• Base pairing-A-U
Types of RNA:
• Three types:
• mRNA: messenger RNA
• Contains 3 bases ( codon)
• rRNA: ribosomal RNA
• Comprises the 70 S ribosome
• tRNA: transfer RNA
• Transfers amino acids to ribosomes for protein
• Contains the anticodon (3 base sequence that
is complimentary to codon on mRNA)
• DNA: triplet code
• mRNA: codon (complimentary to
triplet code of DNA)
• tRNA: anticodon (complimentary
• Codons: code for the production of a
specific amino acid
• 20 amino acids
• 3 base code
• Degenerative: more than 1 codon
codes for an amino acid.
• Universal: in all living organisms.
-Act as punctuation mark.
-Termination the message for the sythesis of the
- UAA, UAG, UGA
• Three parts:
• Initiation-start codon (AUG)
• Elongation-ribosome moves along mRNA
• Termination: stop codon
reached/polypeptide released and new
• rRNA=subunits that form the 70 S
ribosomes (protein synthesis occurs
• tRNA=transfers amino acids to
ribosomes for protein synthesis.
• It is a heritable change in genotype, that can occur
spontaneously or induced by physical or chemical
• Organism selected as referance strain is called as
• Their progeny with mutation is called mutagen.
• Process of mutation is called as Mutagenesis.
• The agent inducing mutation is called as mutagen.
• Spontaneous mutation occurs naturally about one in
every million to one in every billion divisions & occurs
during the DNA replication.
• Mutation rates of individual genes in bacteria range
from 10‾2 to 10‾10 /bactrium/division.
Mechanism of division:
• Substitution of Nucleotides:
- Also called as point mutation.
- MC mechanism of the mutation.
- It invovles the changing of single base in the DNA
sequence, which is copied during replication, to
produce permanent changes.
- If one purine or pyrimidine is replaced by the other ,
the substitution is called as Transition.
- If a purine is replaced by purine or vice-versa, the
substitution is called as Transversion.
• Deletion or addition of nucleotide:
- Occurs during the DNA replication.
- When a transposon (jumping gene) inserts itself in
to a gene, it leads to distruption of gene , called as
Types of the mutation:
• Missense mutation:
- Caused by DNA mutatins which leads to changes
in to amino acid sequences of the protein products
-Caused by single point mutation or series of
• Nonsense mutation:
- formation of stop codon, which causes termination of protein
synthesis leads to incomplete protein products.
- Single substitution mutation change in the DNA sequence
result in a new codon , still coding for the same amino acid &
there is a no change in the product.
• Frameshift mutation:
The addition or deletion of base pair causing a shift in the
“reading frame” of the gene.
• Theses causes a reading frame shift & all of the codon & all
amino acids after that mutation are usually wrong.
• Lethal mutation:
Some mutation affect the vital function of the cell, & bacterial
cell become nonviable, & which can kill the cell.
It is a reversal of the mutant phenotype by another mutation at
a position on the DNA, differ from that of original mutation.
• True reversion or back mutation results in reversion of mutant
to original form, which occurs as a result of mutation occurring
at the same spot once again.
• Conditional lethal mutation:
Mutation may occur in such a way that mutation may
survive in certain enviromental condition.
Segment of the DNA is removed & reinserted in the reverse
Genetic Transfer in Bacteria:
• Genetic transfer-results in genetic variation
• Genetic variation-needed for evolution
• Three ways:
• Transformation: genes transferred from one
bacterium to another as “naked” DNA.
• Conjugation: plasmids transferred one bacteria to
another via a pilus.
• Transduction: DNA transferred from one bacteria
to another by a virus.
• Recipient cell must be competent for uptake
• Natural competence versus artificial
• Only certain bacteria are naturally
transformable - Streptococcus pneumoniae,
Haemophilus influenzae, Neisseria gonorrhoeae,
Transduction by a Bacteriophage:
• Bacteriophage (phage) are viruses of bacteria - can
be either lytic or temperate
• 1. lytic - always lyse (kill) host bacterial cell
temperate - can stably infect and coexist within
bacterial cell (lysogeny) until a lytic phase is induced
• 2. lysogeny
• The phage genome during lysogeny is called the
prophage, and the bacterial cell is called a lysogen
• If the phage genome encodes an observable function,
the lysogen will be altered in its phenotype - lysogenic
conversion (e.g., diphtheria toxin in Corynebacterium
• 3. Specialized transduction:
• Some prophages integrate into the bacterial genome
at a specific location.
• When a prophage is induced to lytic phase, it may
drag along a piece of the bacterial genome next to
the integration site and move that bacterial sequence
into the new recipient host cell, changing the
not very important medically since only selected
genes can be transferred
• 4. Generalized transduction:
• when a phage lyses the host bacterial cell, it normally
packages phage genome into the capsid.
• sometimes the capsid is accidentally filled with
random pieces of bacterial genome, possibly
• when the capsid injects the host genes into a new
recipient, the new gene can recombine into the
recipient genome and cause a change.
• virulence and antibiotic resistance genes can be
moved by generalized transduction.
• Difference between lysogeny and generalized
1. lysogeny - when a phage lysogenizes a host
bacterial cell and brings in a new gene that changes
the characteristics of the cell - it is a phage gene that
makes the change in lysogenic conversion and a host
gene in specialized transduction.
2.Generalized transduction involves the phage particle
acting as a vehicle to move bacterial genes from one
cell to another, the phage genome is not moved .
Possible states for conjugation
i. Conjugative - the plasmid encodes all of the
functions for conjugation and can move itself from the
donor cell to the recipient cell.
ii. Mobilizable - the plasmid cannot move itself, but
can be moved with help from a conjugative plasmid.
i. synthesis of sex pilus.
ii. cell to cell contact via pilus.
iii. copying plasmid DNA and transfer of copy into
• Bacteria containing a conjugative plasmid are called
donor, male, (F+)
Bacteria receiving the plasmid are called recipient,
The F plasmid is a specific E. coli plasmid that has
nothing to do with medicine other than its historical
and laboratory use (and standardized exams)
• Hfr - a plasmid integrates into the chromosome,
conjugation will move part of the chromosome into the
recipient (not of medical relevance)
• F' - plasmid integrates into the chromosome, excision
drags along piece of the chromosome.
• F' can move host DNA between cells (not of medical
relevance) (similar to specialized transduction)
• Phenocopy - a cell with the F plasmid, but lacking pili
•Unlike transduction by phages, most plasmids
are more promiscuous in their host range
•Importance of conjugation - moving plasmids
encoding multiple antibiotic resistance genes (R
plasmids) among bacteria.
Insertion Sequences and Transposons:
1. These sequences are part of other genetic
elements - chromosome, plasmid.
-They move from one site in DNA to another WITHIN
the same cell (transposition)
2. Insertion sequence (IS)
-Gene encoding transposition enzyme
(transposase) flanked by inverted repeats of DNA
- can interrupt genes if they insert into them.