Bacterial genetics

Assistant prof. Buroooj M.R. Al-aajemDepartment of Microbiology

College of Medicine

Bacterial genetics

Genetics is the science that defines & analyzesheredity & variation. It aims to understanding thestructure & functions of microbial genome, its geneproducts & their role in infection & disease. Theunit of heredity is gene.

The gene is a segment or portion of DNAthat carries in its nucleotide sequence informationfor a specific biochemical or physiological property,through coding for a single polypeptide sequence.

Phenotype refers to the observableproperties (or characters) of an organism which areproduced by interaction of genotype with theenvironment, i.e. the effect of both genes &environment. These include the structural &physiological properties of a cell or an organism(e.g. the eye color in human, resistance to anantibiotic in bacterium).

Genotype refers to the geneticconstitution of an organism.

Bacterial genetics

Genome is the sum of the genes of an organism, or the totality of genetic information in an organism.

Gene expression refers to a gene product that can be observed under appropriate condition at the level of phenotype.

Much of the genetic information inbacteria is contained on a single chromosomelocated in the cytoplasm. Bacterial genomes differin size, a feature which determines the bacterialcharacteristic traits or phenotype.

Properties of bacterial cell such asantimicrobial resistance & virulence aredetermined by bacterial genome. The genomicstructure consists of 3 types of genetic information:chromosome, plasmids and bacteriophages, thelater 2 structures provide additional geneticinformation & are transient in some instances.

Bacterial genetics

Most bacteria are defined as haploid with asingle circular chromosome consisting ofdouble-stranded DNA. e.g. in E.coli, thechromosome is circular ds-DNA molecule ofapproximately 4.6x106 base pairs.

Although the chromosome exists freein the cytoplasm, it is compacted throughsuper-coiling & looping of its structure. Thecentral function of genetics consists ofexpression of a gene from its locus on thechromosome or on a plasmid throughtranscription (production of messenger RNA)& finally translation: decoding of mRNA toproduce a polypeptide. The gene sequence &its subsequent expression through thesebiochemical pathways accounts for thephenotypic variation observed amongbacteria.

Bacterial genetics

Replication of bacterial DNA:As bacteria replicate by binary fission, the

daughter cells produced are usuallyindistinguishable genetically. Replication ofchromosome in bacteria begins at a specificlocation called origin of replication. It proceed at arate of 1000 uncleotide/second.

During replication, the purine [adenine (A)& guanine (G)] & pyrimidine [thymine (T) &cytocine (C) ] (Each of the 4 bases is bound tophospho-deoxyribose to form a nucleotide.Adenine always pairs with thymine and theguanine pairs with cytocine by hydrogen bondingin the center of the molecule) nucleotides in eachDNA strand are accurately copied into 2 new dsdaughter molecules. Each of these molecules iscomposed of a strand from the parent molecule & anewly synthesized complementary strand, a processtermed semi-conservative replication.

Bacterial genetics

As the 2 parent strands of the helical DNA unwindunder the influence of the enzyme DNA helicase,each acts as a template for the synthesis of acomplementary strand. In this manner, 2 identicalhelical DNA molecules are formed through theaction of replicating enzyme DNA polymerase. Theends of the new fully formed strands are joined byDNA ligase.

RNA most frequently occurs in singlestranded form. The base uracil (U) serves in RNAthe hybridization function that thymine (T) servesin DNA. So the complementary bases thatdetermine the structure of RNA are A-U and C-G.

Bacterial genetics

Mechanisms of genetic variation:The genotype of a cell determines its inheritablepotential, while the phenotype represents thoserecognizable characteristics expressed by the cellularnucleic acid. Thus both the genotype of a bacterium& its environment can influence phenotypicexpression.

Mutations are changes in the DNA sequence.Spontaneous mutations for a given gene generallyoccur with a frequency of 10-8 - 10-6 in a populationderived from a single bacterium. The mutationsinclude base substitution, deletion, insertion andrearrangement.

Base substitution can arise as aconsequence of mispairing between complimentarybases during replication. Many base substitutionsescape detection at the phenotypic level becausethey do not significantly disrupt the function of thegene product, for example substitution of one aminoacid for another.

Bacterial genetics

The consequences of deletion or insertion mutationsare severe because they can drastically alter theamino acid sequence of gene products..

Mutagenes:

The frequency of mutation is greatlyenhanced by exposure of cells to mutagens.Physical mutagen, e.g. ultraviolet light (UV) thatdamage DNA by linking neighboring thyaminebases to form diamer. Chemical mutagens may actby altering either the chemical or the physicalstructure of DNA. Biological mutagens refers tothose pathogens that are able to induce alterations inthe host DNA.

Recombination:Recombination occurs when sequences of DNAfrom 2 separate sources are integrated in bacteria,recombination includes an unexpected inheritablechange due to introduction of new genetic materialfrom a different cell. This genetic material can beintroduce by: Conjugation, Transduction orTransformation.

Bacterial genetics

1. Conjugation:

In conjugation only one strand of DNA istransferred. The recipient cell completes thestructure of ds-DNA by synthesizing thecomplementary stand. Plasmids are mostfrequently transferred by conjugation.

Genetic analysis of E. coli revealed the presence offertility factor carried on plasmid called (F+). Sexpilus; an extracellular protein extrusion thatattaches donor cell to recipient cell lacking thefertility factor. A bridge between the cells isformed & allows a strand of F+ factor plasmid topass into the recipient, where the complementarystrand is formed. The F+ factor can integrate in thechromosome of donor cell creating HFr donorsfrom which chromosomal DNA is transferred.

Bacterial genetics

Similarly another plasmids e.g. drugresistant plasmid (R factor) can promotechromosomal transfer from differentbacteria.

2. Trasduction:

In transduction, bacterial donor DNA iscarried in a phage coat & transferred torecipient by the mechanism used for phageinfection. So, transduction is phage-mediatedrecombination in bacteria. Temperate phageis preferred vehicle for gene transfer,because infection of recipient bacteriaminimize cell lysis & favor survival ofrecombinant strain. Furthermore, recipientbacteria carrying a prophage may form arepressor that renders the cell immune tolytic infection, such cell may continues takeup bacterial DNA from transducingplasmids.

Bacterial transduction

Bacterial genetics

Transformation:

This process involves the transferof free (nacked) DNA containing genes ona segment of chromosomal or plasmidDNA from a lysed donor bacterium to acompetent recipient.

Natural transformation is unusual amongbacteria. Many bacteria unable to undergonatural transformation, can be forced toincorporate plasmids by treatment withcalcium chloride & temperature shock.This process is the cornerstone of modernmolecular biology, because it enable DNAfrom diverse biological sources to beincorporated as part of well-characterizedbacterial replicon.

Bacterial genetics

Genetic engineering:Specified DNA fragments can be isolated

& amplified, & their genes can be expressed at highlevels. The nucleotide specificity required forcleavage by restriction enzymes allows fragmentscontaining genes or parts of genes to be covalentlybound to plasmids “ vector” that can be insertedinto bacterial host. Bacterial colonies or clonescarrying specified genes can be identified byhybridization of DNA or RNA with chemical orradiochemical probes.

The protein products encoded by thesegenes can be recognized either by enzyme activityor by immunological techniques.

The protein products of isolated genes offers greatpromise as vaccine because it can be preparedwithout genes that encode the replication of viralnucleic acid. Moreover, protein ,e.g. insulin can beprepared in large quantities from bacteria thatexpress cloned genes.

Bacterial genetics

Mobile genetic elements:

Plasmids:Although most bacteria carry all the genesnecessary for survival on their chromosome,many bacteria contain small additional geneticelements, termed plasmids, which also locatedin the cytoplasm. Plasmids are small geneticelements capable of independent replication inbacteria & yeast. plasmids carry genesassociated with specialized functions & genesthat mediate their transfer from one organismto another. plasmids are frequently involved inthe genetic engineering, because of their smallsize, that permits their genetic manipulation &after alteration can be easily introduced to thecell. In pathogenic bacteria, plasmids encodevirulence factors & antibiotic resistance.

Bacterial genetics

Mobile genetic elements:Transposons:

Are genetic elements containing several kbp ofDNA, including information necessary for theirmigration from one genetic locus to another.Almost all bacteria carry transposons, which differfrom one species to another. Plasmids also carrytransposones, which are important in theformation of high-frequency recombinant strains.Carrying on the plasmids, the transposons maintainits dissemination throughout a bacterialpopulation.

Complex transposons carry genes forspecialized functions, e.g. antibiotic resistance.Unlike plasmids, transposons do not containgenetic information necessary for their ownreplication. Therefore, they depend on theirphysical integration with a bacterial replicon.

Bacterial genetics

Mobile genetic elements:

Bacteriophages ( phages):Viruses that infect bacteria. Phages have narrow specifichost range. The phage genome may be ss- or ds- DNA orRNA. The NA of phage is surrounded by a protein coat.Many phages contain syringe-like structure that bind toreceptors on the cell surface & inject the phage nucleic acidinto the host cell. Replication of phage is similar to that ofanimal viruses.

Types of phages:

1. Lytic phage: produces many copies of themselves & killtheir host cells. e.g. T2, T4 phage of E. coli

2. Temperate phage: are able to enter a non-lyticprophage state in which replication of its NA is linkedto the replication of host cell DNA. Bacteria carryprophage are called lysogenic (e.g. E. coli phagelambda).

3. Filamentous phage: (e.g. E. coli phage M13) theirfilaments contain ss-RNA with protein that extrudedfrom their host cell, which are debilitated but not killedby the phage infection.