Which of the following best defines mutation? (A) An alteration in the sugar–phosphate backbone of a DNA molecule. (B) The replication of a section of DNA. (C) The gain of an additional characteristic by an organism. (D) A change in the nucleotide base sequence of a DNA molecule. Or (E) a random change in the physical appearance of an organism.
In this question, we are being asked to select the option that best defines mutation. To answer this, let’s discuss what the word “mutation” means. As we know, DNA is the molecule that carries genetic information. DNA has two strands, which twist around one another to form a shape called the double helix. Each strand is a chain of individual units called nucleotides. Let’s unwind this section of DNA so that we can learn a little bit more about its structure.
In each strand, nucleotides join together to form a sugar–phosphate backbone, which we can see here in black. Here, we can see a single nucleotide circled. Each nucleotide consists of a nitrogenous base and a sugar–phosphate molecule. There are four types of nitrogenous bases in DNA: cytosine, shown here in blue; guanine, shown in orange; adenine, shown in green; and thymine, shown in pink.
The sequence of these nitrogenous bases in DNA is what provides the instructions to build different proteins. Mutations are changes in this nucleotide base sequence that can change the sequence of DNA and alter the function of the corresponding protein. Therefore, the option that best defines mutation is (D), a change in the nucleotide base sequence of a DNA molecule.