Define a gene.

A gene is defined as a segment of DNA that encodes for a specific protein or functional RNA. This encompasses the fundamental role that genes play in storing and transmitting genetic information, which ultimately directs the synthesis of proteins essential for various cellular functions. Each gene contains the instructions that are required to build and maintain an organism, influencing traits and biological processes through the proteins or RNA it produces.

The other choices focus on different biological components and processes that are important for cellular function but do not encapsulate the definition of a gene. For example, while RNA plays a crucial role in protein synthesis, it is not itself a gene but rather a product of gene expression. Structures that are involved in ATP production or cellular respiration, such as mitochondrial complexes, are essential for energy metabolism but are not genes per se. Thus, the definition of a gene is distinct and specific to the segment of DNA that encodes for proteins or functional RNA.