Final answer:
A gene is a specific segment of DNA on a chromosome that codes for a specific polypeptide chain or RNA molecule. It contains the genetic code which, during transcription, is used to create mRNA and ultimately translates into a protein.
Step-by-step explanation:
The best definition of a gene out of the given options is 5) A specific segment of DNA on a chromosome. A gene is the fundamental, physical, and functional unit of heredity which is made up of DNA. Genes are responsible for the genetic instructions that lead to the synthesis of proteins, which in turn play a crucial role in the expression of traits. Each gene codes for a specific polypeptide chain or RNA molecule, and the human genome has thousands of genes that determine an individual's characteristics.
During the process of transcription, a segment of DNA within a gene is copied into mRNA, which then undergoes translation to create the corresponding polypeptide chain. This polypeptide can either function alone or combine with others to form a complete protein. The genetic code contained within a gene is read in a series of three-letter codes, known as codons, each of which specifies a particular amino acid.