Chapter 8 Microbial Genetics

Question 1

Two Methods of Genetic Change in Bacteria

Answer 1

1) Mutation

2) Horizontal Gene Transfer

Question 2

Mutation

Answer 2

• A change in the nucleotide sequence of DNA

- May cause a change in a protein encoded by a gene

Question 3

Horizontal Gene Transfer

Answer 3

Genetic info is passed from one living cell to another cell of the same generation
Example: Plasmid Transfer

Question 4

Base Substitution/ Point Mutation (Mutation)

Answer 4

• A single nucleotide is replaced by another nucleotide
• When DNA replicates - results in a substituted base pair
• When DNA is transcribed and translated can result in an incorrect amino acid in the protein - Missense mutation

Question 5

Frameshift Mutation (Mutation)

Answer 5

• Insertion - one or two nucleotide(s) added to the gene
• Deletion - one or two nucleotides removed from the gene
• Changes the reading frame of the mRNA
• Sequence of amino acids is changed “downstream” of mutation site
• Almost always results in a non-functional protein

Question 6

Spontaneous Mutation (How mutations occur)

Answer 6

Occur in absence of mutagens, due to occasional mistakes during DNA replication

Question 7

Induced Mutation (How mutations occur)

Answer 7

Occur when DNA damaging agents cause changes in DNA sequence - mutagens
-Example: Radiation, some chemicals

Question 8

Regardless of Origin, Mutations can result in

Answer 8

1) No effect on the protein (remains functional) - silent mutation
2) A protein with a different amino acid sequence that may have an altered function - missense mutation
3) Premature stop codon- Incomplete (truncated) protein, usually nonfunctional - nonsense mutation

Question 9

Plasmids

Answer 9

• Self-replicating, DNA molecules
• Contain non-essential genes
  Example. Genes for toxin production

Question 10

F Plasmids

Answer 10

• Fertility Factors
• Carry genes to make F pills (or sex pilus)
• Involved in conjugation (transfer of genetic material between bacteria)

Question 11

R Plasmids

Answer 11

• Resistance Factors

- Carry genes for antibiotic resistance

Question 12

Vir Plasmids

Answer 12

• Virulence factors

- Carry genes for toxin production

Question 13

Horizontal Gene Transfer in Bacteria (Basic Methods)

Answer 13

1) Transformation
2) Transduction
3) Conjugation

Question 14

Transformation

Answer 14

• Pieces of “naked” DNA are taken up by bacterial cells
• ie. From dead cells, or from released plasmids
• These pieces can be integrated into the chromosomes
  1) Recombination
  2) Can then be passed to progeny, and become a stable part of the genome

Question 15

Transduction (Part 1)

Answer 15

• Small fragments of DNA transferred between bacteria by bacteriophage (viruses that infect bacteria)
• Phage attaches to bacterial cell wall - injects its DNA into the cell
• Phage DNA is replicated inside bacterial cell
• Phage DNA also encodes enzymes that cut the bacterium’s DNA into fragments

Question 16

Transduction (Part 2)

Answer 16

• As new phages are being assembled, some accidentally receive a piece of bacteria DNA instead of phage DNA –> Transducing Particle
• The transducing particle can then carry bacterial genes to another cell
• Injection mechanism is still fully functional
• But, bacterial DNA is injected into the cell
• If the injected DNA recombines with the existing chromosome it becomes a stable genetic element (i.e. Will be passed to progeny)

Question 17

Conjugation (Bacterial Mating)

Answer 17

• Mediated by genes encoded on an F factor
• Transfer occurs when a:
  1) Donor cell (F+) - forms an F plus and uses it to attach to a recipient cell (F-)
  2) Pilus retracts bringing the cells together
  3) The donor cell replicates the F factor as a copy is passed to the recipient
  4) The recipient becomes an F+ cell