state that a polypeptide is coded for by a gene and that a gene is a sequence of nucleotides that forms part of a DNA molecule

Protein Synthesis

Learning Objective

State that a polypeptide is coded for by a gene and that a gene is a sequence of nucleotides that forms part of a DNA molecule.

Key Concepts

• Gene: A specific segment of DNA that contains the instructions for making a functional product, usually a protein.
• DNA Structure: DNA is composed of two antiparallel strands forming a double helix. Each strand is a linear sequence of nucleotides (A, T, C, G).
• Polypeptide: A chain of amino acids linked by peptide bonds; the primary structure of a protein.

How a Gene Encodes a Polypeptide

1. Transcription – synthesis of messenger RNA (mRNA) from the DNA template.
2. RNA processing – addition of a 5’ cap, poly‑A tail, and removal of introns (in eukaryotes).
3. Translation – ribosome reads the mRNA codons and assembles the corresponding amino acids into a polypeptide.

DNA → mRNA → Polypeptide

Codons and the Genetic Code

The genetic code is read in sets of three nucleotides called codons. Each codon specifies one of the 20 standard amino acids or a stop signal.

Example: The mRNA codon \$AUG\$ codes for methionine and also serves as the start codon for translation.

Summary Statement

A gene is a defined sequence of nucleotides within a DNA molecule. The information contained in that nucleotide sequence is transcribed into mRNA, which is then translated by ribosomes to produce a specific polypeptide. Thus, the polypeptide is directly coded for by the gene.