Bacterial Genetics PNG 2011

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Key Words:

Key Words Genetics Bacterial genetics Mutation & its types Point mutation Frameshift mutation Lethal mutation Suppressor mutation Missense & nonsense mutation Bacteriophage Lysogenic cycle Mechanisms of gene transfer Transformation Transduction Lysogenic conversion Conjugation Transposition (Jumping Genes)

Bacterial Genetics:

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. Introns - non coding sequences on a gene. Exons - coding sequences on a gene translated into gene products. Bacterial genetics is used as a model to understand DNA replication, genetic characters, their changes & transfer to next generations.

Nucleic Acids :

Nucleic Acids DNA (deoxy ribonucleic acid) : Stores information for protein synthesis. RNA (ribonucleic acid) : Transcription & translation of information for protein synthesis. Central Dogma : DNA RNA Protein

Structure Of DNA :

Structure Of DNA Proposed by Watson & Crick . Double helix model. Composed of 2 chains of polypeptides, each chain has a backbone of deoxyribose sugar and phosphate residues arranged alternately. 4 nitrogenous bases: Adenine (A) Purine Guanine (G) Thymine(T) Pyrimidine Cytosine (C)

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Double helical structure of DNA by Watson & Crick

Structure Of RNA :

Structure Of RNA Structurally similar to DNA, except for 2 major differences: Ribose sugar Uracil in place of Thymine. 3 types of RNA m RNA (messenger RNA) t RNA ( transfer RNA ) r RNA ( ribosomal RNA )

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GENE – A segment of DNA that specifies for a particular polypeptide is called a “GENE”. CODON – Genetic information is stored in DNA as a code. Codon consist of sequence of three nucleotide bases i.e. the code is triplet. Each codon specifies for production of a single amino acid. But more than one codon may exist for the same amino acid. For e.g.- AGA codes for Arginine & CGU, CGC, CGA, & AGG can also code for the same amino acid, Arginine. IMPORTANT DEFINATIONS

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IMPORTANT DEFINATIONS NON SENSE CODON– Three codons (UAA, UGA, & UAG) do not code for any amino acid & act as a “ Stop codon” for terminating the message for the synthesis of a polypeptide. ANTICODONS- The word anticodon used in connection with tRNA. This is specific for nitrogen base triplet known as Recognition site placed on the exposed part of tRNA. It is complementary to codon on mRNA with which it associates during protein synthesis.


TRANSCRIPTION Transfer of genetic information from a DNA to mRNA. Part of DNA unzipped by breaking H 2 bond only one of the strand is used as “Template” for synthesis of mRNA. In synthesis of mRNA thymine is replaced by uracil. mRNA synthesis takes place in presence of enzyme “ RNA Polymerase” following pairing of A=U & C G.


Role of mRNA is acting as “Messenger”. It carries transcribed information from DNA to Ribosome, the actual site of protein synthesis. This process is called as “Transcription”. TRANSCRIPTION


TRANSLATION Actual process of protein synthesis which takes place in cytoplasm on ribosome. The mRNA passes into cytoplasm & then mRNA & tRNA come together on surface of ribosomes containing rRNA is known as “Codon”. The base sequence on mRNA are recognized by the anticodon (present on tRNA) sequences of tRNA. The tRNA contains the triplet (anticodon) at one end & amino acid at other end.


Ribosome moves along the mRNA untill the entire mRNA molecule has been translated into corresponding sequence of amino acids. The transferred amino acids are joined by peptide bonds in presence of enzymes “Peptide synthesis” present in ribosomes. This process is called as “Translation”. TRANSLATION

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The formation of polypeptide chain is terminated by one of the three “Terminating codons i.e. UAG, UGA, UAA called as “Stop Codon” OR “Non sense Codon” . Which do not code for any amino acid. The tRNA, mRNA, & ribosome are dislocated. tRNA goes in cytoplasm for reuse. mRNA being a labile structure get dissolved in cytoplasm, & peptide chain is released to form protein. TERMINATION


Thus in protein synthesis DNA acts as an “Architect”. mRNA as “Blue print”. tRNA as “Builders”. Which build up proteins by using amino acids as a raw material under the supervision of ribosomes. TERMINATION

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SYNTHESIS OF POLYPEPTIDE – RNA polymerase DNA mRNA mRNA with ribosome rRNA +protein Amino acid tRNA Anti codon mRNA Growing polypeptide

Genetic Information In Bacteria:

Genetic Information In Bacteria Chromosome Carries properties like virulence, pathogenicity & resistance. Plasmid Extrachromosomal genetic material in the cytoplasm. Replicate independently. Bacteriophage Virus infecting bacteria


EXTRACHROMOSOMAL GENETIC ELEMENTS -PLASMIDS Circular DNA molecules, Extrachromosomal. Important vectors in genetic engineering. Transmissible & non transmissible plasmids. EPISOME Plasmid DNA integrated with chromosomal DNA. TYPES OF PLASMIDS R plasmid (drug resistance)= (Resistant Transfer Factor) RTF + r determinant. F plasmid (maleness ) Carry properties like drug resistance, toxigenicity conjugation etc.

Genotypic & Phenotypic Variations:

Genotypic & Phenotypic Variations Phenotype – Physical expression of the genotype in a given environment. Genotype – Genetic constitution of a cell that is transmitted to its progeny. Variations Phenotypic variations – influenced by the environment temporary & not heritable Genotypic variations – Not influenced by the environment Stable & heritable

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PHENOTYPIC VARIATIONS – Bacteria exhibit different phenotypic appearance. Reversible E.g. Synthesis of flagella of S typhi.

Mechanisms Of Genetic Variations :

Mechanisms Of Genetic Variations Mutation Transfer or exchange of genetic material Transformation Transduction Conjugation Lysogenic conversion Transposition

Mutation :

Mutation Random, undirected heritable variation. Caused by a change in the nucleotide base sequence of the DNA. Types of mutation: Point mutation (Base pair substitution, Frame shift) Multisite mutations Mutagens - Agents which can induce mutation. e.g. UV rays, 5 bromouracil, alkylating agents, etc.

1. Point Mutation:

1. Point Mutation Cause - due to addition, deletion or substitution of one or more bases. Types – Base pair substitution Transition : A purine base is replaced by a purine base or a pyrimidine base is replaced by another pyrimidine base. Most common type. i.e. AT replaced by GC or TA by CG. Transversion : substitution of a purine base by a pyrimidine base & vice versa GC changes to CG.

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FRAME SHIFT MUTATION - ► During replication , one or a few adjacent base pair (nucleotides) have been inserted to or deleted from the DNA. This shifts the normal transitional “ reading frame” of coded message from the point on forming an entirely new set of triplet codon. ► So , during translation, the coded message is read correctly up to the point of deletion or addition, but the subsequent codons will specify the wrong amino acids.

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OTHER TYPES OF MUTATIONS Nonsense mutation – Converts a codon that specifies an amino acid into a termination codon.

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Mutation which results in involve vital functions in the death of the organism – nonviable mutation. A conditional lethal mutant may be able to live under certain conditions – permissive conditions. Commonest type of conditional mutant is the temperature sensitive (t s ) mutant which is able to live at the permissive temperature of 35  C but not at the restrictive temp (39  C). Lethal mutations –

Suppressor Mutation :

Suppressor Mutation Reversal of a mutant phenotype by another mutation at a position on the DNA, distinct from that of the original mutation.

Transformation (Griffith, 1928) :

Transformation (Griffith, 1928) Transfer of genetic information by free DNA. i.e. by direct uptake of donor DNA by the recipient DNA. Live noncapsulated (R) pneumococci + heat killed capsulated (S) pneumococci Injected into mice Death of mice Live capsulated pneumococcus isolated from the blood of mice.

Transduction :

Transduction Transfer of a portion of the DNA from one bacterium to another by a bacteriophage. Packaging error within the infected bacteria during the assembly of progeny phages – presence of a segment of host DNA along with the phage nucleic acid in the core of phage Infection of another bacterium Transfer of host bacterial DNA to the new bacterium Acquisition of new characteristics coded by the donor DNA.

Transduction :

Transduction Types of Transduction – Generalized - When it involves any segment of the DNA. Restricted - When a specific bacteriophage transduces only a particular genetic trait. Episomes & plasmids can also be transduced. Most widely used mechanism of gene transfer among prokaryotes. Any group of bacteria for which bacteriophages exist, can be subjected to transduction.

Lysogenic Conversion :

Lysogenic Conversion Phage DNA itself is the new genetic element. Bacteriophages – 2 Types of life cycles. Lytic or virulent cycle – Progeny viruses build up inside host bacterium, which rupture to release them. Temperate OR Nonlytic OR Lysogenic cycle – Host bacterium is unharmed.

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This process is called Lysogeny & the bacteria harboring these prophages are called Lysogenic bacteria. Prophage behaves as an additional segment of bacterial chromosome, coding for new characteristics. This process by which prophage confers genetic information to a bacterium is called Lysogenic conversion . By lysogenic conversion Diphtheria bacilli acquire toxigenicity & therefore virulence. Elimination of phage renders a toxigenic strain non- toxigenic.

Conjugation :

Conjugation First described by Lederburg & Tatum in 1946 in a strain of E.coli called K12. A donor or male bacterium passes DNA directly to a recipient or female bacterium by a conjugation tube (sex pili). The female bacterium attains donor status & in turn can conjugate with other female cells. Maleness is determined by the presence of a plasmid which codes for sex pili. The plasmid is called the sex factor or fertility factor (F factor). R (resistance) factor can also be transferred by conjugation.

Process of Conjugation:

Process of Conjugation

Transposon (Jumping Genes):

Transposon (Jumping Genes) DNA segment that can move between chromosome & plasmids Transposons are not self replicative, they depend on chromosomal or plasmid DNA for replication. Insertion of transposon into a functional gene would destroy the function of the gene (internal mutagenic agents) Plasmid Chromosome Transposon

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