1.6. Bacterial genetics

Question 1

list the sources of genetic information

Answer 1

1. Bacterial DNA/Genome: Chromosome
2. Extrachromosomal Genetic Material
   - plasmids
   - transposons

Question 2

Bacterial DNA/Genome: Chromosome

Answer 2

• Single copy per cell
• Circular, double stranded DNA
• 1 origin of replication
• Few million base pairs

Question 3

Plasmids

Answer 3

• 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

Question 4

Transposons

Answer 4

• 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

Question 5

name the two types of gene transfer

Answer 5

1. vertical gene transfer

2. horizontal gene transfer

Question 6

Vertical Genetic transfer

Answer 6

• Genetic information passed on from parent to offspring

* Mutations

Question 7

Horizontal genetic transfer

Answer 7

•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

Question 8

Transformation

Answer 8

• 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.

Question 9

Conjugation

Answer 9

• 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.

Question 10

Transduction

Answer 10

• 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.

Question 11

list the stages of transduction involving a lytic phage

Answer 11

1. lytic bacteriophage attaches to a susceptible bacterium
2. bacteriophage injects its DNA into the bacterial cell –> phage DNA directs the bacteria’s metabolic machinery to manufacture bacteriophage components and enzymes
3. occasionally during maturation, a bacteriophage capsid incorporates a fragment of the bacteria’s chromosome or a plasmid instead of a phage genome by mistake
4. the bacteriophages are released with the lysis of bacterium
5. the bacteriophage carrying bacterial DNA attaches to another recipient bacterium
6. the bacteriophage injects the bacteria’s DNA it is carrying into the new recipient bacterium
7. the bacteria DNA from the phage is exchanged by recombination for some of the recipient’s DNA

Question 12

Mutations

Answer 12

•Heritable variations
•Random or induced
•Caused by alterations in the nucleotide sequence due to:
-Substitution
-Addition/Insertion
-Deletion

Question 13

types of mutation

Answer 13

1. missense mutations
2. nonsense mutations
3. silent mutations
4. frame shift mutations

Question 14

Missense Mutations

Answer 14

• Lead to changes in Amino Acid sequence -changes Protein products
• Point mutation
• Series of mutations

Question 15

Nonsense Mutations

Answer 15

• Lead to formation of Stop codon

* Terminates protein synthesis = incomplete protein product

Question 16

Silent Mutations

Answer 16

• Single substitution in DNA sequence

* Results in the same amino acid = same protein product

Question 17

Frame Shift Mutations

Answer 17

• Addition or deletion of nucleotide
• Shifts the reading frame of the gene
• Wrong codon –Amino acid –protein

Question 18

list the mutation outcomes

Answer 18

1. deleterious
2. beneficial
3. random/ spontaneous

Question 19

Deleterious

Answer 19

• Block or disrupts a gene
• Affect the vital function of the cell
• Lead to cell death

Question 20

Beneficial

Answer 20

• Added advantage for the cell

* Antibiotic Resistance

Question 21

Random/Spontaneous

Answer 21

• No obvious effect on the phenotype

* Silent mutations

Question 22

genetic basis of antibiotic resistance

Answer 22

the ability of a microbe to resist the effects of an antibiotic that was once effective in killing that particular microbe

Question 23

what is intrinsic resistance?

Answer 23

• 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

Question 24

Acquired Resistance

Answer 24

•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!

Question 25

what affects the frequency of acquired resistance?

Answer 25

1. The amount of antibiotic being used
2. The frequency with which the bacteria can undergo spontaneous mutations and become resistant
3. The prevalence of plasmids able to transfer resistance from one bacterium to another

Question 26

Causes of spread of genes conferring antibiotic resistance:

Answer 26

• 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!