conexión suficiente
CAPÍTULO 4 LAS CLASES DE ACREEDORES EN LOS
4. LAS CLASES DE ACREEDORES
18
© BRIHASPATHI ACADEMY ׀ SUBSCRIBER’S COPY ׀ NOT FOR SALE
BACTERIAL GENETICS
BACTERIAL DNA
• A single circular double stranded DNA
• Length 1000 micrometer or 1µm
• When straightened (expressed as kilo base, around 4000 kb)
• Super coiled like a thread to accommodate within the bacterial cell
• Replication starts at a certain point (origin of replication)
• Replicates in a semi-conserved manner (one of the original strands is retained in the new double strands)
• Most bacterial genes code for proteins
• Introns are absent (non-coding interposed sequences)
• Operons are DNA sequences arranged sequentially at specific locations and code for functionally related proteins
CHARACTERISTICS- PROKARYOTIC AND EUKARYOTIC CHROMOSOMES
Prokaryotic (Bacterial)
Chromosome Eukaryotic Chromosome Replicates just prior to
binary fission
Replicates just prior to mitosis
Single molecule of DNA per genetic trait (haploid)
Two molecules of DNA per genetic trait (diploid) some haploid
Closed loop Linear
Histones absent Histones present No dominance or with a nuclear membrane No introns present Introns present
DNA also in plasmids
DNA also in
mitochondria and chloroplasts
Mutations occur in DNA Mutations occur in DNA Genetic recombination
Replication by semi- Replication by
semi-conservative method, rolling circle method occurs
conservative method. No evidence of rolling circle method
EXTRA CHROMOSOMAL GENETIC ELEMENTS Plasmid
• Autonomous DNA molecules present in the cytoplasm of bacteria
• Small & closed loops of DNA
• Number of copies varies from1-2 to100 per cell depending upon the size
• Can multiply independent of the chromosome
• Transferred easily between bacteria
• Many carry genes that code for certain characteristics
• Eg: R Plasmids code for drug resistance, F plasmids code for sex pili, Virulence plasmids code for toxin production like entero toxin
TRANSPOSONS
• Genetically discrete segments of DNA
• Can move in a cut and paste manner between chromosomal DNA & extra chromosomal DNA elements like plasmids within the cell
• Not self replicating & depend upon chromosomal or plasmid DNA for replication
GENOTYPE
• Sum total of the genes that make up the genetic apparatus of a cell (genome)
• It is the hereditary constitution of the cell
• It is transmitted to its progeny
• Includes complete genetic potential of the cell
• All the genetic potential may or may not be expressed in a given environment
PHENOTYPE
• It is the physical expression of the genotype in a given environment
• Exhibit different phenotypic appearances in different situations
• Eg. Synthesis of enzyme beta galactosidase by E.coli only in the presence of lactose
Genotypic Phenotypic Alteration in the genome
Stable
OCCURRENCE OF GENOTYPIC VARIATION
• By mutation
• By mechanisms of genetic transfer or exchange 1. Transformation sequence at some point of the DNA of the cell
• Occurrence
Spontaneous
Induced by external agents like chemicals, radiation
Point Mutation
Affects one point (base pair) in a gene
May change to or substitution of a different base pair Or Result in the deletion or addition of a base pair
Mutation Rate
Frequency of mutations expressed as mutation rate
Mutation rate is the probability of mutation per gene per cell division
In bacteria the rate ranges from 104 to 109 per bacterium per division
Consequences of Mutations
• May be lethal or may affect some minor functions that may not be expressed
• It is of vital importance when it confers a survival advantage. Eg-if a streptomycin resistant mutant of the tubercle bacillus develops in a patient under treatment with the drug, it multiplies selectively and ultimately replaces the original drug sensitive bacteria
• Practical importance of mutation is the development of drug resistance
GENETIC TRANSFER MECHANISMS Transformation
• Process in which a live bacterium acquires DNA fragments from the environment
• Fragments of DNA (composed of 10-20 genes) obtained from naturally dying& lysing bacteria in the environment
Mechanism
Binding of DNA fragment to recipient bacteria
Passage of single stranded DNA into the recipient
Incorporation of DNA fragment into the recipient chromosome
Significance
Acquisition of virulence genes
Drug resistance genes
Eg. Streptococcus pneumoniae, Haemophilus influenza
Transduction
• A process where bacterial virus called bacteriophage carries the DNA fragment from donor to recipient cell
• Bacteriophages are viruses that parasitize bacteria and consist of a nucleic acid core & a protein coat
After replication inside bacteria, phage assembly occurs
• When this particle infects another bacterium it acquires the new donor DNA & new characteristics
Types
1. Generalized
Any segment of donor DNA is transduced at random
2. Restricted/Specialized
When a specific phage transduces only a particular genetic trait
Sometimes apart from bacterial DNA plasmid can also be transduced
• Eg. Plasmids determining penicillin resistance in Staphylococcus aureus is transferred from one bacterium to another by transduction
Lysogenic conversion
• A process where phage infecting the bacterium gets incorporated in the bacterial chromosome (such a phage is called prophage)
MICROBIOLOGY
Bacterial Genetics20
© BRIHASPATHI ACADEMY ׀ SUBSCRIBER’S COPY ׀ NOT FOR SALE
• Prophage multiplies synchronously with the host DNA and passed on to daughter cells
• The prophage confers genetic information to bacteria and is called lysogenic conversion
• Eg. Prophage in Corynebacterium diphtheriae confers the property of toxin production
Conjugation
• A process where a donor bacterium (F+) makes physical contact with recipient (F-) bacterium and transfers genetic elements into it
• Donor status is determined by the presence of a plasmid which Codes for the sex pilus forming the conjugation tube
• Plasmid DNA replicates and a copy of it passes into the recipient
• Along with the plasmid DNA sometimes host DNA is also transferred
• Recipient becomes F+ and can in turn conjugate with another F- bacterium
Importance
• Transfer of multi drug resistance through R factor
R factor-A plasmid with 2 components (rtf+ r determinants)
RTF is responsible for conjugational transfer
R determinant responsible for drug resistance
• Resistance to as many as 8 or more drugs is possible simultaneously
• Commonly occurs among gram negative bacteria
• This transfer of drug resistance is also called transferable or infectious drug resistance
• Wide spread resistance has considerably diminished the clinical efficacy of antibiotics
F-FACTOR
• Plasmid contains genetic information for synthesis of sex pilus & self transfer
• Does not carry drug resistance markers
• Some times when integrated with the host chromosome is called as Episome
• Such cells are able to transfer host chromosomal genes with high frequency Hence called as Hfr cells
COL FACTOR
• A plasmid carries genes for production of bacteriocins called colicins
• Colicins are antibiotic like substances produced by intestinal bacteria called coli forms which are selectively and specifically lethal to other bacteria
• Col factor is capable of self transfer to other bacteria
COMPARISON OF MUTATIONAL &
TRANSFERABLE DRUG RESISTANCE Mutational Drug
Resistance
Transferable Drug Resistance One drug resistance at a
time Multiple drug resistance
Low degree resistance High degree resistance Can overcome by high
drug dose High dose ineffective
Drug combinations can prevent
Combinations cannot prevent
Resistance does not spread
Spreads to same or different species
Mutants may be defective Not defective
Virulence may be low Virulence not decreased