What is defined as a segment of DNA that encodes for a protein?

A segment of DNA that encodes for a protein is specifically referred to as a gene. Genes are the fundamental units of heredity and carry the instructions necessary for the synthesis of proteins, which perform a wide array of functions within an organism. Each gene consists of a specific sequence of nucleotides, which ultimately determines the structure and function of the proteins it encodes, impacting traits and biological processes.

While chromosomes contain many genes and are structures that house DNA within the cell, they are not defined solely by the segments that encode proteins. An allele refers to different versions of a gene that arise by mutation and exist at the same locus on homologous chromosomes. RNA, on the other hand, is a molecule that is transcribed from DNA and plays an essential role in translating the genetic information into proteins, but it is not DNA itself. Thus, the defining characteristic of a gene is its role in encoding proteins, making it the correct choice in this context.