ii. Bacterial Genetics Flashcards
What is bacterial genetics?
Bacterial genetics is the study of the mechanism of heritable information in bacteria.
What are the sources of genetic information?
- Bacterial DNA/Genome: Chromosome
- Extrachromosomal Genetic Material
- Bacterial DNA/Genome: Chromosome
Features: (4)
o Single copy per cell
o Circular, double stranded DNA
o 1 origin of replication
o Few million base pairs
- Extrachromosomal Genetic Material
Features: (2)
o Plasmids
o Transposons
What are plasmids? (5)
- Circular, double stranded DNA within the cytoplasm
- Not essential for life
- Carry advantageous genes which code for toxins or resistance or enhanced pathogenicity
- Number varies: 1-20 copies per cell
- Can be passed on to other bacteria within the same or different species
What are Transposons? (3)
- Jumping genes (can change their position within a bacterial genome)
- 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 - May carry advantageous genes that encode for antibiotic resistance, toxins or enhanced
pathogenicity.
Genetic variation occurs in bacteria through Genetic Transfer:
What are the 2 types?
- Vertical Genetic transfer
- Horizontal genetic transfer
What is Vertical Genetic transfer? (2)
o Genetic information passed on from parent to offspring
o Mutations
What is horizontal genetic transfer? (3)
o Genetic information passed on between bacterial cells
o Results in genetic variations
o These genetic variation are needed for evolution and sometimes for survival
o There are three ways in which bacteria transfer genomic information via horizontal gene transfer:(3)
▪ Transformation
▪ Conjugation
▪ Transduction
What is transformation in bacteria transfer? (8)
What is conjugation in bacteria transfer? (5)
What is transduction in bacteria transfer? (4)
- Following are the stages of transduction involving a lytic phage: (7)
- A lytic bacteriophage attaches to a susceptible bacterium
- The bacteriophage injects its DNA into the bacterial cell. The 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
What are mutations? (2)
- Heritable variations
- Random or induced
MUTATIONS:
- Caused by alterations in the nucleotide sequence due to: (3)
o Substitution
o Addition/Insertion
o Deletion
Types of Mutation: (4)
- Missense Mutations
- Nonsense Mutations
- Silent Mutations
- Frame Shift Mutations
- Missense Mutations: (3)
o Lead to changes in Amino Acid sequence - Changes protein products
o Point mutation
o Series of mutations
- Nonsense Mutations: (2)
o Lead to formation of Stop codon
o Terminates protein synthesis - Incomplete protein product
- Silent Mutations: (2)
o Single substitution in DNA sequence
o Results in the same amino acid = same protein product
- Frame Shift Mutations: (3)
o Addition or deletion of nucleotide
o Shifts the reading frame of the gene
o Wrong codon Amino acid protein
What are the outcomes of mutations?
- Deleterious
- Beneficial
- Random/Spontaneous:
- Deleterious: (3)
o Block or disrupts a gene
o Affect the vital function of the cell
o Lead to cell death
- Beneficial: (2)
o Added advantage for the cell
o Antibiotic Resistance
- Random/Spontaneous: (2)
o No obvious effect on the phenotype
o Silent mutations
Genetic basis of Antibiotic Resistance:
What is antimicrobial resistance?
- Antimicrobial resistance is 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? (3)
- There are two types of Resistance:
o Intrinsic resistance
o Acquired resistance
▪ Mutations
▪ Genetic transfer
▪ Biochemical mechanisms e g decrease permeability of the drug or produce enzymes to inactive drug
What is acquired resistance?
The Frequency of acquired resistance is affected by: (3)
- The amount of antibiotic being used
- The frequency with which the bacteria can undergoes 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: (3)
- 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!
