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12.Bacterial genetics

DNA molecule structure, basearrangements: double helix strands;

composed of many neucleotides; A & Gare purine; C & T are pyrimidine

Plasmids

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1. Structure : small circular DNA moleculethat can exist independently of host

chromosome. They have their ownreplication origins, carry fewer genescompared to DNA.

2. types of plasmids and importance : 6

different types.

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Mutation

1. (a)meaning : an inherited change inthe base sequence of the genome of

an organism2. (b)replica plating : a technique used

to detect auxotrophic* mutants.

3. (c)types of mutation

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(i)molecular types of mutations- base pair(pointmutation): affects only one base pair in a givenlocation. Transition : replacement of a pyrimidine on

one strand by a different pyrimidine/ replacement ofa purine on one strand by a different purine.Transversion : purine replace pyrimidine vice versa.

silent mutation : do not alter phenotype of organism

and go undetected. More than one codon for a givenamino acid. wobble variability in the 3rd baseposition.

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Missense mutation : involves a single basesubstitution in the DNA that changes a codon forone a.a. into a codon for another. Expressed at

the level of protein structure however at the level of protein function, the effect may

range from complete loss of activity to no change at all.

Many proteins are still functional after substitution

of a single a.a. but depends on the type andlocation of the a.a.

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Eg. 1. replacement of a nonpolar withpolar a.a.- drastic change

Eg 2. replacement of a critical a.a. at theactive site will destroy its activity

Eg. 3 replacement of one polar withanother polar a.a., may have little/ no

effect.

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Missense mutation

provides evolution

often non-lethal/remain in the pool.

(ii)frameshifts mutation: causes a change in he 3 basesequences read as codons; a change in the phase oftranscription arising from the addition and deletion ofnucelotides makes reading frame to be shifted for allcodons downstream. Often very deleterious and yield

mutants phenotypes.(iii)conditional lethal mutations: expressed only under certainenvironment conditions. Eg: E. coli- cannot grow under onecondition but can under another. T sensitive, does not expressat low T, die at high T, grow normally at a permissible T.

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chemically caused mutations: inducedmutations: mutagen that directly damages

DNA/interferes with repair mechanism. Intercalating agents: acridines such as proflavin &

acridine orange. These mutagens become insertedbetween two DNA base pairs, thereby pushingthem apart. Thery induce frameshift mutations..These mutagens are planar and insert themselvesbetween the stacked bases of the helix.

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5-bromouracil : base analogs similar tonitrogenous bases and can be incorporated

into the growing polynucleotide chainduring replication. These analogs haveslightly altered copying error properties. 5

bromouracil is incorporated into DNA in

place of thymine but often pairs with G.

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nitrous acid :converts the base A to a formthat no longer pairs with T but instead

pairs with C. Eventually, AT base pair ofthe parent will change to GC base pair in agrand daughter cell. Therefore it is a pointmutation.

alkylating agent: methyl-nitrosoguanidine- adds methyl groups to G, causing it tomispair with T

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physical causes of mutations UV: formation of thymine dimers. Most mutagenic

component of UV; WL = 260 nm. Formation of covalentbonds between adjacent T in a DNA forming dimers.

DNA repair mechanism: excision repair: general repairsystem that corrects damage that causes distortion in thedouble helix. Enzyme that cuts out the damage DNA is calledUvr protein (endonuclease)

photoreactivation repair :repair of T dimers by splitting

them apart into separate T with the help of visible light in aphotochemical reaction catalyzed by the enzyme PRE(photoreactivation enzyme). Can function in the dark but

PRE must absorb a light photon.

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Transformation in bacteria:

Genes are transferred from one bacterium toanother as DNA

In nature, some bacteria after death and celllysis, release their DNA into the environmentand other bacteria depending on species,growth conditions take up fragments of DNA

integrate them into their own chromosomesby recombination that results in hybrid.

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Conjugation in bacteria

1. requires direct cell to cell contact

2. cells must be of opposite mating type,

donor cells carry plasmid, recipientcells do not

3. sex pili needed

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Transduction in bacteria:

bacterial DNA is transferred from adonor cell to a recipient cell inside a

virus that infects bacteria,bacteriophage

*