What best defines a gene?

• A. A section of DNA that codes for a protein
• B. A section of RNA that codes for a protein
• C. A chromosome
• D. A sequence of DNA that codes for a protein

Answer: (A)

A gene is a specific segment of DNA that carries the instructions to build a protein (the functional product). This framing fits how genes are studied: they are discrete pieces of the genome on chromosomes, and through transcription and translation the information in that DNA segment directs the synthesis of a polypeptide. The option that describes it as a section of DNA that codes for a protein correctly highlights that a gene is a defined part of the genome with a specific coding role.

The alternative that mentions RNA would blur the origin of the genetic information, since genes are defined by their DNA sequence even though the final product may be RNA in some cases. Describing a gene as a chromosome is too broad, because a chromosome contains many genes and other sequences. Describing it as a DNA sequence that codes for a protein is very close, but the common way textbooks express the unit is as a distinct section or segment of DNA that provides the code for making a protein.