Mutation in Microorganisms Practice Questions

Correct answer: Transition

A transition is a point mutation where a nitrogenous base is replaced by another of the same chemical class (e.g., Adenine to Guanine). These occur more frequently in nature than transversions.

Correct answer: Missense mutation

Missense mutations result in a single amino acid change in the synthesized protein. Depending on the location and nature of the new amino acid, this can significantly alter or destroy protein function.

Correct answer: A mutation that changes an amino acid codon into a stop codon

Nonsense mutations create a premature termination signal (UAG, UAA, or UGA) in the mRNA. This leads to the production of a truncated, usually non-functional, protein.

Correct answer: The entire protein sequence downstream of the mutation is altered

Frameshift mutations occur when nucleotides (not in multiples of three) are inserted or deleted. This shifts the triplet reading frame, changing every subsequent codon and usually resulting in a non-functional protein.

Correct answer: 5-bromouracil

5-bromouracil is structurally similar to thymine and can be incorporated into DNA in its place. Because it frequently undergoes tautomeric shifts, it can pair with Guanine, causing transition mutations.

Correct answer: Deamination of Adenine and Cytosine

Nitrous acid removes amino groups from nitrogenous bases. For example, it converts Cytosine to Uracil, which then pairs with Adenine during the next round of replication, leading to a C-G to T-A transition.

Correct answer: Pyrimidine dimers

UV light promotes the formation of covalent bonds between adjacent pyrimidines (usually thymines) on the same DNA strand. These dimers distort the DNA helix and block replication and transcription.

Correct answer: Potential carcinogens based on their mutagenicity

The Ames test utilizes Salmonella typhimurium auxotrophs to detect if a chemical can cause revertant mutations. Since most carcinogens are also mutagens, it serves as an effective preliminary screen for cancer-causing agents.

Correct answer: RecA

When DNA damage is extensive, RecA protein binds to single-stranded DNA and becomes activated. Activated RecA facilitates the self-cleavage of LexA, allowing the expression of SOS response genes.

Correct answer: Photoreactivation

Photoreactivation involves the enzyme photolyase, which captures energy from blue light to break the covalent bonds of thymine dimers. This process directly reverses the damage without removing any nucleotides.

Correct answer: Repairing mismatched bases after replication

The MutHLS system performs mismatch repair by identifying errors missed by DNA polymerase proofreading. It distinguishes the newly synthesized strand from the template strand by checking for lack of methylation.

Correct answer: Errors in DNA replication and tautomeric shifts

Spontaneous mutations occur naturally without the influence of external mutagens. The most common causes are transient tautomeric shifts of nitrogenous bases and 'slippage' by DNA polymerase during replication.

Correct answer: Requires an additional nutrient not needed by the parent strain

Auxotrophs have mutations in metabolic pathways that prevent them from synthesizing essential compounds like amino acids or vitamins. They can only grow if the specific nutrient is added to the culture medium.

Correct answer: Screen for auxotrophic mutants by negative selection

Replica plating allows researchers to compare the growth of bacterial colonies on different media types. Colonies that grow on enriched media but fail to grow on minimal media are identified as auxotrophic mutants.

Correct answer: A mutation in the third position of a redundant codon

Silent mutations change the DNA sequence but do not change the amino acid sequence due to the degeneracy of the genetic code. Because the protein remains identical, there is no change in the organism's phenotype.

Correct answer: A second mutation that restores the original wild-type phenotype

Reversion occurs when a second mutation either restores the original DNA sequence or compensates for the first mutation elsewhere in the gene. This returns the mutant organism to the wild-type phenotype.

Correct answer: Frameshift mutations

Intercalating agents wedge themselves between the stacked base pairs of the DNA double helix. This distortion causes DNA polymerase to insert or skip one or more nucleotides, resulting in frameshifts.

Correct answer: Forming reactive oxygen species and double-strand breaks

Ionizing radiation has enough energy to dislodge electrons, creating free radicals. These radicals react with the DNA backbone to cause single and double-strand breaks, which are difficult for the cell to repair accurately.

Correct answer: Bacterial mutations are spontaneous, not directed by the environment

The fluctuation test proved that mutations for antibiotic resistance occur randomly before exposure to the antibiotic. This disproved the Lamarckian idea that environmental stress induces specific, directed mutations.

Correct answer: It utilizes specialized polymerases that lack proofreading ability

Under extreme stress (SOS response), bacteria use translesion polymerases (like Pol IV or V). These enzymes can bypass DNA damage that would stop standard polymerases, but they tend to insert incorrect bases, increasing the mutation rate.