A signaling pathway that stimulates cellular growth and division is positively regulated by a GTPase encoded by an autosomally located gene. During DNA replication in a cell within a large population of cells in culture, one of the two copies of this gene incurs a missense mutation that reduces the rate of hydrolysis of GTP to 1% of normal function.

Which of the following explanations best explains what effect this mutation might have on cells within this culture that are heterozygous for this mutation ?

Answers:

a. Because only one of the two copies of the GTPase encoding gene is affected, the cell will continue to grow and divide as normal.

b. Because the GTPase functions at a slower rate, signaling thought the pathway will occur with reduced efficiency and the cell's rate of growth and division will slow down.

c. Due to the reduced rate of hydrolysis of the GTP, signaling via the pathway will occur for a shorter period of time, so cells with the mutation will grow and divide more slowly.

d. Because the rate of hydrolysis is reduced, signaling will occur for a longer period of time and the cells will grow and divide more frequently.

The correct answer is: a.

Explanation:

A Diploid organism possess two copies ( also called alleles) of each  autosomal gene, of which one copy of the gene is obtained from one of its parents and the other copy from its other parent.During the process of replication, one of the alleles of the GTPase encoding gene undergoes a mis-sense mutation.A Mis-sense mutation can be defined as a non-synonymous mutation in which one of the nucleotide in the sequence of the gene gets altered such that it causes a change in the amino acid encoded by the codon (triplet nucleotide message encoding for a single amino acid)  formed by the mutated nucleotide. A single amino acid change in the protein, if occurs in the functional site of the protein, can render a protein non-functional. Hence, the mis-sense mutation in one of the alleles encoding for the GTPase gene causes the resulting protein produced from the mutated allele to have drastically reduced function (1%).A heterozygous cell is one which has one copy of the normal allele and the other copy of the mutated allele of the GTPase encoding gene.As one copy of the normal GTPase allele is present in such an individual, it can produce the normal GTPase protein that can assist in the process of cell growth and division.Hence, the phenotype(expressed character) of  the heterozygous cell will not be affected due to the mutation and it will be able to grow and divide normally.