A mutation occurs in a section of DNA that codes for an amino acid sequence found in an enzyme’s active site. Which of the following best describes what could happen? (A) The protein will react differently and the enzyme will be killed. (B) The enzyme’s active site will have to be repaired by fixing enzymes. (C) The DNA will be transcribed into tRNA rather than mRNA. Or (D) the protein will fold differently and the active site will change shape.
Key knowledge required to select the correct solution option here includes mutations, protein synthesis, structure, and function, including that of enzymes. So let’s review what we need to know about each of these topics, starting with proteins.
Proteins are chains of amino acids that fold into specific shapes according to function. The sequence of amino acids in the protein is determined by the sequence of bases in the section of DNA that codes for the protein called a gene. To synthesize a protein, the sequence of DNA bases in the gene is used as a template to produce a molecule of messenger or mRNA. Then the messenger RNA delivers the information stored in its sequence of bases to one of the cell’s protein factories, a ribosome, where it is used to determine the sequence of amino acids in the protein.
The newly synthesized chain of amino acids can now fold into its specific shape, forming a protein with a particular function. Proteins have many different functions, including speeding up chemical reactions. Proteins that speed up chemical reactions are called enzymes. And their specific shape is responsible for this ability. Enzymes bind one or more reactants or substrates, speed the chemical reaction, and after releasing the product or products are free to catalyze more reactions.
The place on the enzyme where the substrate binds is called the active site, which is referenced in the question. And as we just saw, the shape of the active site needs to fit the substrate precisely. Keep that in mind as we review mutations.
Mutations are an accidental change in the sequence of bases in a section of DNA. And mutations that result in a different sequence of amino acids can affect the shape of a protein. Here is a diagram showing the normal base sequence of a very, very short gene. If the sequence of bases is changed such that one base is substituted for another, we have a substitution mutation. Or if a base is added, we have an insertion mutation that shifts the entire sequence of bases, changing the reading frame of the genetic code. This can result in many amino acid changes in the protein.
A deletion mutation can have the same frameshifting effect. So insertions and deletions are often called frameshift mutations. Other types of mutations are possible as well. But let’s try to connect what we know about proteins, specifically enzymes, to possible effects of mutations. Earlier, we noted that the sequence of amino acids in a protein, such as an enzyme, is responsible for its shape. And the shape of an enzyme determines its function, especially at its active site. And that’s just what this question is asking about. A mutation that occurs in a section of DNA that codes for an amino acid sequence found in an enzyme’s active site can modify the folded shapes of the active site and its affinity for its substrate.
Now let’s take another look at our solution options. We can eliminate option (A) because it doesn’t make any sense at all that the enzyme is going to be killed. Enzymes aren’t even alive. While option (B) is an improvement with its inference that there’s a problem in the active site, there’s no such thing as fixing enzymes. And we can eliminate option (C) because mutations do not cause DNA to be transcribed into tRNA.
Therefore, the answer to the question a mutation occurs in a section of DNA that codes for an amino acid sequence found in an enzyme’s active site is option (D). The protein will fold differently, and the active site will change shape.