How is "gene expression" best defined?

Gene expression is best defined as the process of synthesizing a functional gene product, which is typically a protein. This definition encompasses the entire journey from the initial transcription of DNA to RNA, followed by the translation of that RNA into a polypeptide chain, and finally the folding and possible post-translational modifications needed to create a functional protein.

Understanding gene expression is crucial because it highlights how genes, which contain the instructions for building proteins, are ultimately responsible for the phenotypic traits of an organism. Every step in the gene expression process is highly regulated, ensuring that genes are expressed at the right time and in the right amounts according to the needs of the cell or organism.

The other options do not capture the full essence of gene expression. Although the synthesis of DNA from RNA is related to reverse transcription, it does not pertain to typical gene expression processes where RNA is ultimately translated into protein. Transcription of non-coding regions is a specific aspect of gene regulation that doesn’t lead to a functional protein product, while the modification of genome sequences refers to genetic changes rather than the expression of existing genes.