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
Chromosome:- The chromosome corresponds to the nucleoid . The E. coli chromosome is composed of 4.63 * 106 base pairs (bp) and codes for 4288 proteins. The gene sequence is colinear with the expressed genetic products. The chromosomes of E. coli and numerous other pathogenic bacteria have now been completely sequenced
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.
The following plasmid types are medically important: Virulence plasmids: Carry determinants of bacterial virulence, e.g., enterotoxin genes or hemolysin genes. Resistance plasmids: Carry genetic information causing resistance to anti-infective agents. Some plasmids carry both virulence and resistance genes.
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 progeny Phenotype – physical expression of the genotype in a given environment Variations Phenotypic variations – influenced by the environment temporary & not heritable Genotypic variations – Not influenced by the environment Stable & heritable
Mechanisms Of Genetic Variations 1- Parasexuality 2- Mutation Although bacteria have no sexual heredity , they have mechanisms that allow for intercellular DNA transfer involving a unilateral transfer of genetic information from a donor cell to a receptor cell and it is called parasexuality.
Mechanisms of parasexuality are : Transformation Conjugation Transduction
It is the transfer of “naked” DNA after cell lysis and this 1. Transformation It is the transfer of “naked” DNA after cell lysis and this transformation process has been observed mainly in the genera Streptococcus, Neisseria, Helicobacter and Haemophilus.
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
3. Transduction It is the transfer of DNA from a donor to a receptor with the help of transport bacteriophages. Bacteriophages Infection of another bacterium Transfer of host bacterial DNA to the new bacterium Acquisition of new characteristics coded by the donor DNA.
Bacteriophages Definition:- Bacteriophages are viruses that infect bacteria . They are therefore obligate cell parasites. They possess only one type of nucleic acid, either DNA or RNA, have no enzymatic systems for energy supply and are unable to synthesize proteins on their own. Morphology:- Similar to the viruses that infect animals and vary widely in appearance.
Composition: Phages are made up of protein and nucleic acid. The proteins form the head, tail, and other morphological elements, the function of which is to protect the phage genome. The nucleic acid in most phages is DNA, which occurs as a double stranded DNA .
-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 35C but not at the restrictive temp (39C).