GENETICS

GENETICS

•Genetics is the study of heredity and variation, seeking to understand the causes of the resemblances and differences between parents and their progeny

•Genetics of microorganisms

•The «central dogma» of molecular biology is that DNA carries genetic information, which is transcribed on to RNA and then translated as the particular polypeptide

•(DNA - RNA - polypeptide)

•An exception exists in the case of some viruses genetic material is RNA instead of DNA

•Genetics of microorganisms

•The DNA molecule is composed of 

two chains of nucleotides bound 

together in the form of a «double 

helix»

•Each chain has a backbone of 

desoxyribose and phosphate residues 

arranged alternately

•Attached to each desoxyribose is one 

of four nitrogenous bases, the purines: 

adenine (A) and guanine (G), and the 

pyrimidines: thymine (T) and 

cytosine (C)

•Adenine is always linked to thymine, 

and guanine to cytosine: A=T, G=C

•Genetics of microorganisms

•Genetic information is stored in the DNA as a code, the unit of the code (codon) consisting of a sequence of three bases (the code is triplet)

• Each codon specifies for a single aminoacid, but the code is «degenerate» so that more than one codon may exist for the same aminoacid

•A segment of DNA carrying codons specifying for a particular polypeptide is called a «cistron» or gene

•The bacterial chromosome carries about 1000-4000 cistrons.

•Genetics of microorganisms

Structure of operon

•   Lac         Lac P          Lac O            Lac Z - Y -A

•Regulator  Promotor   Operator        Structural genes

•For transcription of RNA for the enzyme synthesis, the RNA polymerase has to attach to the promotor region and travel along the structural genes in sequence.

•The transfer of genetic information from DNA to RNA is called transcription and from RNA to protein is called translation.

•Genetics of microorganisms

•RNA is structurally similar to DNA 

except for three major differences:

•It contains the sugar ribose 

(desoxyribose in DNA)

•One of pyrimidine base is uracile 

instead thymine in DNA

•There are 3 distinct types of RNA on the 

basis of structure and function:

•1.  Messenger RNA (m RNA)

•2.  Ribosomal RNA (r RNA)

•3.  Transfer RNA (t RNA)

•DNA acts as the template for the 

synthesis of mRNA.

•Genotypic and phenotypic 

variations

•The sum total of genes the make up the genetic apparatus of a cell establishes its genotype.

•The phenotype («phaeno»: display) is the physical expression of the genotype in a given environment.

•Genotypic and phenotypic 

variations

•Phenotypic variations

are influenced by the environment, 

limited in range by the genotype, 

temporary and not heritable.

•Variations are genotypic

when they are due to alterations in the 

genome. Genotypic variations are 

stable, heritable and not influenced 

by the environment.

•

They may occur by mutation, or by 

one of the mechanisms of genetic 

transfer or exchange, such as 

transformation, transduction, 

lysogenic conversion and 

conjugation.

•Mutation

•Mutation

is a heritable variation caused by an 

alteration in the nucleotide 

sequence at some point of the DNA

•The sequence of nucleotides in 

DNA can change in either of 2 

ways:

•(a) by substitution of one base pair 

for another as the result of a 

replication error – 

transition or transvertion.

•by breakage of the sugar phosphate 

back bone with subsequent 

deletion or insertion of a DNA 

segment.

•MUTATIONS IN BACTERIA

–Spontaneous (replication error )

–

Induced (mutations are induced by a 

variety of physical, chemical and 

biological agents)

Physical agents

are radiations, heats etc.

Chemical agents

are nitroso compounds, alkylating 

agents, base analogs, and others.

•TRANSFORMATION

•Definition: It is the transfer of genetic information through the agency of free DNA. Pieces of DNA involved in transformation may carry 10 to 50 genes. Factors affecting transformation

–DNA size and state

–Competence of the recipient (Bacillus, Haemophilus, Neisseria, Streptococcus)

•TRANSFORMATION

–Recombination

•TRANSDUCTION

•Definition: Gene transfer from a donor to a recipient by way of a bacteriophage

•

•Bacteriophage (phage): A virus that infects bacteria

•Phage Composition and Structure

•Composition

–Nucleic acid

•Genome size

•Modified bases

–Protein

•Protection

•Infection

•Infection of Host Cells by Phages

•Irreversible attachment

–Base plate

•TRANSDUCTION

•Types of transduction

–Generalized - Transduction in which potentially any dornor bacterial gene can be transferred.

•Generalized Transduction

•Release of phage

.

•Mechanism of F+ x F- Crosses

•DNA transfer

–Origin of transfer

–Rolling circle replication

•

•Transposable Genetic Elements

•Definition:

Segments of DNA that are able to 

move from one location to another

•Properties

–“Random” movement

–Not capable of self replication (not a 

replicon)

–Transposition mediated by site-

specific recombination

•Transposase

–Transposition may be accompanied 

by duplication

•Types of Transposable Genetic Elements

•Insertion sequences (IS)

–Definition: Elements that carry no other genes except those involved in transposition

–Nomenclature - IS1

–Structure

•Types of Transposable Genetic Elements

•Transposons (Tn)

–Definition: Elements that carry other genes except those involved in transposition

–Nomenclature - Tn10

–Structure

•Composite Tns

•

•PLASMIDS

•Definition: Extrachromosomal 

genetic elements that are capable of 

autonomous replication (replicon)

•Episome - a plasmid that can 

integrate into the chromosome

•Classification of Plasmids

•Transfer properties

–Conjugative are transferred from 

bacterium to bacterium (usually 

members of the same species or of very 

closely related species) through 

conjugation

–

Nonconjugative

•Phenotypic effects

–Fertility (F-plasmids)

–Bacteriocinogenic plasmid (Col-

plasmid)

–Resistance plasmid (R factors) (R-

plasmid)

–Enterotoxin plasmid (Ent-plasmid)

–Haemolysin plasmid (Hly-plasmid)

•Structure of R Factors

•RTF

–Conjugative plasmid

–Transfer genes

•Genetic Engineering

•It is now possible to isolate the genes coding for any desired protein from microorganisms and introduce them into suitable microorganisms, in which the genes would be functional directing the production of the specific protein. This is known as the Recombinant-DNA technology or Genetic engineering.

      

Plasmid DNA vaccination

•Multivalent DNA vaccine

   for malaria.

•Against tuberculosis

•Against hepatitis B virus.