1.6. Bacterial genetics Flashcards
list the sources of genetic information
- Bacterial DNA/Genome: Chromosome
- Extrachromosomal Genetic Material
- plasmids
- transposons
Bacterial DNA/Genome: Chromosome
- Single copy per cell
- Circular, double stranded DNA
- 1 origin of replication
- Few million base pairs
Plasmids
- Circular, double stranded DNA
- Not essential for life •Carry genes -code for toxins or resistance
- Number varies: 1 –20 copies per cell
- Can be passed on to other Bacteria
Transposons
- Jumping genes
- Pieces of DNA may ‘jump’ from one place in the chromosome/ plasmid to another –altering the cell’s genetic identity and even genome size = transposition
- Carry antibiotic resistance genes
name the two types of gene transfer
- vertical gene transfer
2. horizontal gene transfer
Vertical Genetic transfer
- Genetic information passed on from parent to offspring
* Mutations
Horizontal genetic transfer
•Genetic information passed on between bacterial cells
•Results in genetic variations
•These genetic variation are needed for evolution and sometimes for survival
•There are three ways in which bacteria transfer genomic information via horizontal gene transfer:
-Transformation
-Conjugation
-Transduction
Transformation
- Genes are transferred from one bacterium to the other as “naked” foreign DNA
•Recipient cells must be competent for the uptake of DNA.
•Some bacteria have evolved systems that transport free DNA from the outside of the bacterial cell into the cytoplasm.
•These bacteria are referred to as naturally competent for DNA transformation:
-Streptococcus pneumonaiae
-Haemophilus influenza
-Neisseria gonorrhoeae
•Other bacterial species such as Escherichia coli are not naturally competent for DNA transformation. However, scientists have devised ways to physically or chemically force non-competent bacteria to takeup DNA-artificial DNA transformation.
Conjugation
- transfer of DNA between bacterial cells –> requires cell-to-cell contact
•There are many different conjugal plasmids carried by members of most bacterial species.
•Conjugal plasmids that carry antibiotic resistance genes are called R factors. However, conjugative plasmids also express transfer functions that allow the movement of DNA from a donor to a recipient cell; this is the process of conjugation.
•The steps of bacterial conjugation are:
1. mating pair formation,
2. conjugal DNA synthesis
3. DNA transfer,
4. maturation.
•They have to establish a cytoplasm bridge (pilus) for this task, so the cells have to be in direct contact.
Transduction
- transfer of bacterial genes by bacteriophage from one cell to another
•Bacteriophage parasitize bacteria and use their machinery for their own replication.
•During the process of replication inside the host bacteria, the bacterial chromosome or plasmid
is mistakenly packaged into the bacteriophage capsid.
•Life cycle of bacteriophage may either be lytic or lysogenic. Lytic means the parasitized bacterial cell is killed with the release of mature phages while lysogenic means the phage DNA gets incorporated into the bacterial chromosome as prophage.
list the stages of transduction involving a lytic phage
- lytic bacteriophage attaches to a susceptible bacterium
- bacteriophage injects its DNA into the bacterial cell –> phage DNA directs the bacteria’s metabolic machinery to manufacture bacteriophage components and enzymes
- occasionally during maturation, a bacteriophage capsid incorporates a fragment of the bacteria’s chromosome or a plasmid instead of a phage genome by mistake
- the bacteriophages are released with the lysis of bacterium
- the bacteriophage carrying bacterial DNA attaches to another recipient bacterium
- the bacteriophage injects the bacteria’s DNA it is carrying into the new recipient bacterium
- the bacteria DNA from the phage is exchanged by recombination for some of the recipient’s DNA
Mutations
•Heritable variations •Random or induced •Caused by alterations in the nucleotide sequence due to: -Substitution -Addition/Insertion -Deletion
types of mutation
- missense mutations
- nonsense mutations
- silent mutations
- frame shift mutations
Missense Mutations
- Lead to changes in Amino Acid sequence -changes Protein products
- Point mutation
- Series of mutations
Nonsense Mutations
- Lead to formation of Stop codon
* Terminates protein synthesis = incomplete protein product
Silent Mutations
- Single substitution in DNA sequence
* Results in the same amino acid = same protein product
Frame Shift Mutations
- Addition or deletion of nucleotide
- Shifts the reading frame of the gene
- Wrong codon –Amino acid –protein
list the mutation outcomes
- deleterious
- beneficial
- random/ spontaneous
Deleterious
- Block or disrupts a gene
- Affect the vital function of the cell
- Lead to cell death
Beneficial
- Added advantage for the cell
* Antibiotic Resistance
Random/Spontaneous
- No obvious effect on the phenotype
* Silent mutations
genetic basis of antibiotic resistance
the ability of a microbe to resist the effects of an antibiotic that was once effective in killing that particular microbe
what is intrinsic resistance?
- Organism is naturally insensitive to a particular antimicrobial agent
•Usually influenced by absence or presence of a target site
•bacteria are intrinsically resistant to the Antifungal antimicrobial agent, Amphotericin B
-Acts by binding to the sterol component of the fungal cell wall
-Bacterial cell membranes do not contain sterols, therefore resistant to this agent
Acquired Resistance
•Changes in DNA content of the cell
-i.e. the cell acquires a phenotype that is not inherent in that particular species
-appearance of resistance, to particular antibiotic agents, in a normally sensitive bacterial population
•May occur during a course of antibiotic treatment
-Continued presence of the antibiotic exerts selective pressure in favor of resistant organisms!
what affects the frequency of acquired resistance?
- The amount of antibiotic being used
- The frequency with which the bacteria can undergo spontaneous mutations and become resistant
- The prevalence of plasmids able to transfer resistance from one bacterium to another
Causes of spread of genes conferring antibiotic resistance:
- Selection pressure by increased and inappropriate antibiotic use in humans and animals
- General Practitioners prescribing antibiotics for “common cold”
- Antibiotics used for growth promotion in farm animals
- →drug resistance in gut flora of animals → spread to human gut flora → genes transferred to human pathogens!
