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BACTERIAL GENETICS

Dr. Waleed Khalid Lec. : 3

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 . Most bacterial genes code for proteins (Exons) but some genes are noncoding interposed sequences (introns), like those seen in eukaryotes. Exons - coding sequences on a gene translated into gene products. Introns - non coding sequences on a gene.

Bacterial genetics is used as a model to understand DNA replication, genetic characters, their changes & transfer to next generations Bacteria possess two genetic structures: the chromosome and the plasmid. Both of these structures consist of a circular double stranded DNA molecule twisted about its helical axis.

Replication of this DNA molecule always starts at a certain point (the origin of replication) and it is “semi-conservative” meaning that one strand in each of the two resulting double strands is conserved .

Nucleic Acids

DNA ( Deoxy Ribonucleic Acid ) : Stores information for protein synthesis. RNA ( ribonucleic acid ) : Transcription & translation of information for protein synthesis.

Structure Of DNA

The bacterial genetic information is stored in its chromosome and plasmids. DNA is composed of 2 chains of polypeptides, each chain has a backbone of deoxyribose sugar and phosphate residues with 4 nitrogenous bases:

Adenine (A) Guanine (G) Thymine(T) Cytosine (C)

Double helical structure of DNA by Watson & Crick

Plasmids:-The plasmids are autonomous DNA molecules of varying size (3 *103 to 4.5 * 105 bp) localized in the cytoplasm. Large plasmids are usually present in one to two copies per cell, whereas small ones may be present in 10, 40, or 100 copies.Plasmids are not essential to a cell’s survival and many of them carry genes that code for certain phenotypic characteristics of the host cell.

Genetic Information In Bacteria

Chromosome
Corresponds to the nucleoid
Plasmid
Extrachromosomal genetic material in the cytoplasm
Replicate independently
Bacteriophage
Virus infecting bacteria

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 )


DNA Replication:-The identical duplication process of DNA is termed semi-conservative because the double strand of DNA is opened up during replication and each strand serves as the matrix for synthesis of a complementary strand. Thus each of the two new double strands “conserves” one old strand. -The doubling of each DNA molecule begins at a given starting point called origin of replication. This process continues throughout the entire cycle.

Genotypic & Phenotypic Variations

Genotype – genetic constitution of a cell that is transmitted to its progenyPhenotype – physical expression of the genotype in a given environmentVariationsPhenotypic variations – influenced by the environmenttemporary & not heritableGenotypic variations –Not influenced by the environmentStable & heritable

Mechanisms Of Genetic Variations

Mechanisms of parasexuality are : Transformation Conjugation Transduction

1. TransformationIt is the transfer of “naked” DNA after cell lysis and thistransformation process has been observed mainly inthe genera Streptococcus, Neisseria, Helicobacter andHaemophilus.

2. Conjugation It is the transfer of DNA from a donor to a receptor in a conjugation process involving cell-to-cell contact.

Conjugation is made possible by two genetic elements: the conjugative plasmids and the conjugative pilli .

Conjugation is seen frequently in Gram-negative rods (Enterobacteriaceae), in which the phenomenon has been most thoroughly researched, and enterococci

-Following injection of the phage genome, it is integrated into the chromosome by means of region-specific recombination employing an integrase and this process is called Lysogeny .

2-Mutation

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

1. Point Mutation

Cause - due to addition, deletion or substitution of one or more bases. Types - Transition : It is the most common type in which a purine base is replaced by a purine base or a pyrimidine base is replaced by another pyrimidine base. Transversion : substitution of a purine base by a pyrimidine base & vice versa

Results of mutation - Missense mutation – triplet code is altered so that a different amino acid is present at a particular position in the protein.Nonsense mutation – converts a codon that specifies an amino acid into a termination codon.

2. Frame Shift Mutation

Cause - Deletion or insertion of a base - changing all of the codons downstream from the change

3. Lethal Mutation

Mutation which affects vital functions resulting in the death of the organism – (i.e. 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 (ts) mutant which is able to live at the permissive temperature of 35C but not at the restrictive temp (39C).




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