L5 - Bacterial Genetics Lecture Notes (Gulf Medical University)

L 5 - Bacterial genetics Dr. Janita Pinto September 9, 2024 www.gmu.ac.ae COLLEGE OF PHARMACY  Learning objectives On completion of this unit, the student will be able to:...

L 5 - Bacterial genetics Dr. Janita Pinto September 9, 2024 www.gmu.ac.ae COLLEGE OF PHARMACY  Learning objectives On completion of this unit, the student will be able to: Describe the structure and function of DNA and RNA Describe the genetic transfer mechanisms used by bacteria to obtain new genes (transformation, conjugation, lysogenic conversion and transduction) Describe the various mechanisms of mutations as part of genetic variation Differentiate the genetic and non-genetic basis of drug resistance  Bacterial Genetics Bacterial genetics is used as a model to understand DNA replication, genetic characters, their changes & transfer to next generations.  The bacterial chromosome The bacterial chromosome is a single circular molecule of double-stranded DNA approximately 1000 times as long as the cell itself. It exists in a highly folded state and this supercoiling is brought about by topoisomerase enzymes. In prokaryotic cells the chromosome is not surrounded by a nuclear membrane but exists free in the cytoplasm condensed into areas called chromatin bodies. The chromosome may duplicate itself every 20 minutes during exponential growth. The bacterial chromosome Prokaryotic genomes are very small with very little space between the genes. E. coli has approximately 4000 genes and the length of the DNA molecule is about 1mm. On the other hand a yeast cell has approximately 6000 genes in a genome three times the size of E. coli.  Bacterial genetics The genetic material of a typical bacterium, Escherichia coli, consists of a single circular DNA molecule containing 4,000 genes; and a human cell has about 23,000 genes on 46 chromosomes. A few bacteria (Eg, Vibrio cholerae) have genomes consisting of two circular DNA molecules. Bacteria are haploid; in other words, they have a single chromosome and therefore a single copy of each gene. Central Dogma : DNA RNA Polypeptide Transcription Genetic information is stored in DNA in the form of a code (3 bases) called codon. RNA polymerase forms a single polyribonucleotide strand, called mRNA, using DNA as a template; this process is called transcription. Translation The ribosomes contain rRNA and proteins, transfer the message into the primary structure of proteins via – aminoacyl transfer (tRNA) Translation (polypeptide synthesis) Bacteria have no nucleus, transcription and translation is coupled, meaning that the nascent mRNA attaches to a ribosome and is translated at the same time it is transcribed. This coupled transcription and translation allows for the rapid response to changes in the environment.  Bacterial DNA Replication The replication of bacterial DNA begins at one point and moves in both directions (ie, bidirectional replication). Replication of the bacterial chromosome is tightly controlled, and the number of each chromosomes (when more than one is present) per growing cell falls between one and four. The two daughter chromosomes are separated, or resolved, before cell division, so that each progeny cell gets one of the daughter DNAs. This is accomplished with the aid of topoisomerases, enzymes that alter the supercoiling of dsDNA Some Enzymes Involved in DNA Replication and Their Functions 14 Bacterial chromosome replication  Antibiotics That Affect Transcription and Translation Drugs that inhibit protein synthesis may exert their influence on transcription or translation. For example, the rifamycins used in tuberculosis treatment bind to RNA polymerase, blocking the initiation step of transcription. Rifamycins are selectively more active against bacterial RNA polymerase than they are against eukaryotic RNA polymerase.  Antibiotics That Affect Transcription and Translation One group of antibiotics (including erythromycin and spectinomycin) prevents translation by interfering with the attachment of mRNA to ribosomes. Chloramphenicol, lincomycin, and tetracycline bind to the ribosome in a way that blocks the elongation of the polypeptide, and aminoglycosides (such as streptomycin) inhibit peptide initiation and elongation.  Genetic Information In Bacteria Chromosome Single, circular, supercoiled Haploid; carries ~ 2000 genes Properties: virulence, pathogenicity & resistance Plasmid Extrachromosomal; circular DNA Replicate independently Can integrate into bacterial chromosomal DNA (Episome) Transposons A transposon can jump from a plasmid to a bacterial chromosome or from one plasmid to another plasmid. In this manner, multiple drug-resistant plasmids are generated. Jumping genes, Cannot replicate independently They often contain several genes, including those necessary for their migration from one genetic locus to another. In doing so, they create insertion mutations.  EXTRACHROMOSOMAL GENETIC MATERIAL – PLASMIDS Extra-chromosomal genetic (Circular DNA) material. Confer Drug resistance and toxigenicity No role in the normal functioning of host bacterium. Important vectors in genetic engineering. EPISOME Covalently closed DNA circles (bacterial chromosomes and plasmids), which contain genetic information necessary for their own replication, are called replicons or episomes. Types of plasmids R plasmid (drug resistance): contain resistance transfer factor (RTF) and the R determinant F plasmid (maleness )  Horizontal Gene Transfer in Bacteria Any transfer of DNA that results in organisms acquiring new genes that did not come directly from parent organisms is called horizontal gene transfer. DNA transfer between bacterial cells typically involves small pieces of DNA in the form of plasmids or chromosomal fragments. Chromosomal fragments that have escaped from a lysed bacterial cell are also commonly involved in the transfer of genetic information between cells. An important difference between plasmids and fragments is that while a plasmid has its own origin of replication and is stably replicated and inherited, chromosomal fragments must integrate themselves into the bacterial chromosome in order to be replicated and eventually passed to progeny cells. Mechanisms of Gene Transfer 1. Transformation: direct transfer 2. Transduction: transfer by means of a bacteriophage 3. Conjugation: transfer through sex pili Conjugation Transfer of DNA from one bacterium to the other through conjugation tube (sex pilus) 1.The donor (F+) cell makes a copy of its F factor and transmits this to a recipient (F−) cell. The F− cell is thereby changed into an F+ cell capable of producing a pilus and conjugating with other cells. No additional donor genes are transferred at this time. 2. In a variation on that process, called high- frequency Recombination (Hfr), the plasmid becomes integrated into the donor chromosome before instigating transfer to the recipient cell. Biomedical importance of conjugation Special resistance (R) plasmids, or factors, that carry genes for resisting antibiotics and other drugs are commonly shared among bacteria through conjugation. Transfer of R factors can confer multiple resistance to antibiotics such as tetracycline, chloramphenicol, streptomycin, sulfonamides, and penicillin. Other types of R factors carry genes for resistance to heavy metals (nickel and mercury) or for synthesizing virulence factors (toxins, enzymes, and adhesion molecules) that increase the pathogenicity of the bacterial strain.  Transformation Direct uptake and absorption of bacterial DNA by a recipient cell Griffith’s experiment  Transduction Transfer of a portion of the DNA from one bacterium to another by means of a bacteriophage. Plasmids can also be transduced. Most widely used mechanism of gene transfer among prokaryotes  Types of transduction In generalized transduction, random fragments of disintegrating host DNA are taken up by the phage during assembly. Virtually any gene from the bacterium can be transmitted through this means. In specialized transduction, a highly specific part of the host genome is regularly incorporated into the virus. Several cases of specialized transduction have medical importance. The virulent strains of bacteria such as Corynebacterium diphtheriae, Clostridium spp., and Streptococcus pyogenes all produce toxins with profound physiological effects, whereas nonvirulent strains do not produce these toxins. It turns out that the toxins are produced by bacteriophage genes that have been introduced by transduction. Generalised Transduction Specialized transduction  Pathogenicity Islands Some of the horizontally transferred genes in bacteria have the ability to make their new hosts pathogenic, or able to cause disease. These are termed pathogenicity islands. These islands contain multiple genes that are coordinated to create a new trait in the bacterium, such as the ability to scavenge iron (important for the bacterium causing the plague, Yersinia pestis) or the ability to produce exotoxins (seen in Staphylococcus aureus). Mutations: Changes in the Genetic Code Changes in the genetic code can occur by two means: mutation and recombination. Mutation means a change in the nucleotide sequence of the organism’s genome. Mutations can be either spontaneous or induced by exposure to some external mutagenic agent. All cells have enzymes that repair damaged DNA. When the degree of damage exceeds the ability of the enzymes to make repairs, mutations occur. Mutation-induced changes in DNA nucleotide sequences range from a single nucleotide to addition or deletion of large sections of genetic material. Summary of Genetic Transfer Mechanisms Learning Resources Barer MR, Irving W, Swann A, Perera N. Medical Microbiology: A Guide to Microbial Infections. 19th Ed. Elsevier Ltd; 2018, ISBN: 978-0-7020-7200-0. https://www.clinicalkey.com/#!/content/book/3- s2.0-B9780702072000000023 Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. 8th ed. Elsevier Ltd; 2016. ISBN: 978-0-323-29956-5. https://www.clinicalkey.com/#!/content/book/3-s2.0- B978032329956500080X Additional Reading Levinson W. Review of Medical Microbiology and Immunology, 13th Ed. USA: McGraw-Hill Companies; 2014. ISBN-13: 978- 0071818117. 9/9/2024