Mutation Questions and Answers

Answer: C

The true statements are P, R.

P is true: Barbara McClintock discovered transposable elements in maize, including the autonomous Activator (Ac) and the non-autonomous Dissociation (Ds). R is true: Ac is 4563 bp long and ends in 11 bp terminal inverted repeats.

Q is false: flower colour variegation in Antirrhinum (snapdragon) is caused by its own transposons, the Tam elements, not by the maize Ac and Ds. S is false the other way round: Ac encodes the transposase and mobilises Ds, which cannot move by itself.

Answer: D

The function of an essential gene is studied with a Temperature sensitive mutant.

A mutation that destroys an essential gene kills the cell, so it cannot be kept as a strain. A temperature-sensitive (conditional) mutant makes a protein that works at the permissive temperature but fails at the restrictive temperature. The strain is grown normally, then shifted up, and the effect of losing the gene's function is watched as it happens.

Knock-out, deletion and insertion mutants all remove the gene product permanently, which is lethal for an essential gene, so they cannot be propagated for analysis.

Answer: D

The mutagen that converts adenine to hypoxanthine is nitrous acid.

Nitrous acid (HNO₂) oxidatively deaminates bases with amino groups: adenine becomes hypoxanthine, cytosine becomes uracil and guanine becomes xanthine. Hypoxanthine pairs with cytosine, so an A:T pair becomes G:C after replication, a transition.

Ultraviolet light forms pyrimidine dimers between adjacent thymines or cytosines. Mitomycin C cross-links the two strands of DNA. Methyl methanesulfonate is an alkylating agent that adds methyl groups, chiefly at N7 of guanine and N3 of adenine; it does not deaminate.

Answer: B

A nonsense suppressor mutation is found in tRNA.

A nonsense mutation creates a premature stop codon (UAG, UAA or UGA) in an mRNA. A suppressor mutation in a tRNA gene, usually in the anticodon, lets that tRNA read the stop codon as a sense codon and insert an amino acid, so translation continues and a full-length protein can be made. Amber suppressors such as supE and supF of E. coli are examples.

The stop codon in the mRNA is where the original nonsense mutation lies, not the suppressor. rRNA and the start codon are not where nonsense suppression occurs.

Answer: A

The Ds element is located on the 9th chromosome.

Barbara McClintock discovered the Activator-Dissociation (Ac-Ds) system in maize in the 1940s. She named Ds (Dissociation) because it caused chromosome breakage at a particular site on the short arm of chromosome 9, a breakage that she could follow through the loss of the kernel colour markers (such as C) on that arm. Ds breaks or moves only when the autonomous Ac element is present.

Maize has ten chromosome pairs, so a 13th chromosome does not exist, and chromosomes 5 and 7 are not where the Ds breakage site was mapped.

Answer: C

Genetic variation in pure lines is possible only through mutation.

A pure line is the progeny of a single homozygous self-fertilised plant, so all its members are genetically identical. Johannsen showed with beans that selection within a pure line has no effect, because the variation within it is environmental, not heritable. New heritable variation can arise only when a gene changes, which is a mutation.

Selfing maintains homozygosity and is what creates pure lines in the first place. Cloning and vegetative propagation copy the same genotype, so none of them can introduce genetic variation.

Answer: D

Replacing A with G is a transition mutation.

A transition substitutes one purine for another purine (A and G) or one pyrimidine for another (C and T). A and G are both purines, so A to G is a transition. A transversion swaps a purine for a pyrimidine or the reverse, such as A to T.

A frameshift requires an insertion or deletion that is not a multiple of three, not a substitution. 'Missense' describes an effect on the protein, an amino acid change, which depends on where the substitution falls in a codon; the question asks for the type of base change, which is fixed by the bases themselves.

Answer: B

The mutagens incorporated by DNA polymerase are base analogues.

Base analogues such as 5-bromouracil (a thymine analogue) and 2-aminopurine (an adenine analogue) resemble the normal bases closely enough to be taken up as nucleotides during replication. Once in the DNA they mispair more often than the bases they replace, for example 5-bromouracil pairing with guanine in its rarer tautomer, causing transitions in later rounds.

Alkylating agents such as EMS and deaminating agents such as nitrous acid modify bases already in the DNA. Intercalating agents such as acridines slip between stacked base pairs and cause frameshifts; none of these is built into the chain by the polymerase.

Answer: B

New alleles arise by Mutation.

An allele is a variant DNA sequence of a gene, and the only way a new variant sequence appears is a change in the DNA itself: a base substitution, insertion, deletion or rearrangement. Mutation is therefore the ultimate source of all genetic variation; selection, drift and recombination then act on what mutation provides.

Karyokinesis and cytokinesis are the division of the nucleus and of the cytoplasm during cell division; they copy and distribute existing alleles. Heredity is the transmission of existing alleles from parents to offspring, not the creation of new ones.