Genes code for polypeptides, the building blocks of proteins

Steps:

• A gene is a segment of DNA that contains instructions for protein synthesis.
• During transcription, the gene is copied into messenger RNA (mRNA).
• In translation, mRNA directs ribosomes to assemble amino acids into a polypeptide chain.
• The polypeptide is the direct molecular product of one gene's code.

Why D is correct:

• By the central dogma of molecular biology, one gene codes for one polypeptide, a chain of amino acids that forms functional proteins.

Why the others are wrong:

• A. Amino acids are individual units linked to form polypeptides, not the direct gene product.
• B. DNA is the genetic material containing genes, not produced by them.
• C. Nucleotides are the monomers of DNA and RNA, not the gene's output.

Final answer: D