Final answer:
A gene is a segment of DNA that carries the code for making a specific polypeptide chain. It is transcribed into mRNA which is then translated to synthesize a protein. The genetic code in mRNA determines the sequence of amino acids in a protein.
Step-by-step explanation:
A gene is fundamentally a segment of DNA that carries the code for making a specific polypeptide chain. During the process of transcription, a gene is 'read' and an RNA molecule is produced. This RNA, specifically messenger RNA (mRNA), provides the information that guides the synthesis of a polypeptide chain through the process called translation. The genetic code is composed of three-letter sequences called codons. Each codon specifies an amino acid, the building blocks of proteins.
Regulation of gene expression determines when and how much protein or RNA a cell produces. Certain genes produce structural components or enzymes required constantly by the cell, whereas others are regulated based on the cell's needs, with regulatory genes controlling the expression levels.