# From DNA to Protein > How a gene stored in DNA is copied into RNA and then read by a ribosome to build a protein. Level: High school | Reading time: 14 min | Last reviewed: 2026-10-01 Web page: https://cellnaut.com/learn/dna-to-protein **Quick answer:** A gene is a stretch of DNA that codes for a protein or a functional RNA. In transcription, RNA polymerase copies the gene into mRNA; in translation, a ribosome reads the mRNA three bases (one codon) at a time and joins amino acids into a protein. ## What you will learn - Describe the structure of DNA and what a gene is. - Explain transcription and where it takes place. - Use a codon table idea to read a short mRNA message. - Explain why all the cells in your body can have the same DNA but different proteins. ## Key takeaways - DNA is a double helix built from four bases (A, T, G, C); RNA uses U in place of T. - The central dogma runs DNA to RNA to protein. - Each codon of three bases specifies one amino acid or a stop signal. - Cells differ because they switch on different genes, not because they contain different DNA. ## The cell's instruction manual Everything a cell does depends on proteins: they build structures, speed up reactions, pump ions, and send signals. The instructions for every protein are stored in DNA. DNA is a double helix, like a twisted ladder. Each rung is a pair of chemical bases: **A** always pairs with **T**, and **G** always pairs with **C**. The order of bases along one strand is a code, much like the order of letters in a sentence. A gene is a stretch of this code that contains the recipe for a particular RNA or protein. A human cell holds around 20,000 protein-coding genes spread across 46 chromosomes. ![Stylized 3D illustration of a DNA replication fork with the double helix unwinding.](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/dna-replication-fork-teaching.jpg) *A stylized illustration of DNA being unwound and copied. Replication happens before cell division; the same double helix is what gets read during transcription. Credit: Project-generated teaching illustration (AI-generated; simplified)* [Open the interactive, labelled version](https://cellnaut.com/topics/dna-replication-fork) ## The central dogma Information moves in one main direction inside cells: > DNA → RNA → protein The two steps are called transcription and translation. ## Step 1: transcription Transcription copies a gene into a molecule of RNA. An enzyme called RNA polymerase attaches at the start of the gene, unzips a short section of the double helix, and builds a matching RNA strand using one DNA strand as a template. RNA differs from DNA in three ways: it is single-stranded, it uses the sugar ribose, and it uses the base **U** (uracil) in place of T. The result is a messenger RNA (mRNA), a portable copy of the gene. In eukaryotic cells transcription takes place in the nucleus. The new mRNA is processed (non-coding sections are cut out, a protective cap and a tail are added) and then leaves through a nuclear pore. Interactive 3D model: [Animal Cell](https://cellnaut.com/studio/animal/nucleus) - Transcription happens in the nucleus; the mRNA then travels out to the cytoplasm. ## Step 2: translation In translation, a ribosome grabs the mRNA and reads it three letters at a time. Each three-letter group is a codon, and each codon specifies one amino acid. Adapter molecules called transfer RNAs (tRNAs) bring in the right amino acid for each codon. The ribosome joins the amino acids into a chain, which folds into a finished protein. The genetic code has a few special codons: - **AUG** is the start codon and codes for the amino acid methionine. - **UAA, UAG, and UGA** are stop codons. They signal the ribosome to release the finished chain. There are 64 possible codons but only 20 amino acids, so several codons mean the same thing. The code is nearly universal: bacteria, plants, and humans all read it the same way. ### A tiny worked example Suppose a gene is transcribed into this message: > AUG GCU UAC UAA Reading it three letters at a time, the ribosome starts at the start codon AUG (methionine), adds the amino acids that GCU (alanine) and UAC (tyrosine) specify, and stops at UAA. The result is a very short chain: Met – Ala – Tyr. > **Note** > > In bacteria there is no nucleus, so translation can begin while the mRNA is still being made. In eukaryotic cells the two steps are separated in space and time, which gives the cell an extra chance to process the message. Interactive 3D model: [Bacteria Cell](https://cellnaut.com/studio/bacteria/nucleoid) - In a bacterium, DNA lies in the open cytoplasm, so ribosomes can start reading the message as soon as it appears. **Check your understanding:** What is the product of transcription? - A. A protein - B. A messenger RNA - C. A new DNA molecule - D. A ribosome **Answer:** B. A messenger RNA Transcription copies a gene's DNA sequence into messenger RNA. Translation then turns that mRNA into a protein. **Check your understanding:** How many nucleotides make up one codon? - A. 1 - B. 2 - C. 3 - D. 4 **Answer:** C. 3 A codon is a group of three nucleotides, and each codon specifies one amino acid or a stop signal. ## One genome, many cell types > **Common misconception** > > **"A muscle cell and a neuron have different DNA."** Nearly every cell in your body carries the same genome. What differs is **which genes are transcribed**. A neuron makes proteins for neurotransmitter receptors, and a muscle cell makes large amounts of myosin, using the same instruction manual with different pages opened. This idea, selective gene expression, is the key to understanding how a single fertilized egg can build hundreds of cell types. You will return to it in lesson 10. **Check your understanding:** Where does translation take place? - A. On the ribosome - B. Inside the nucleolus only - C. On the cell wall - D. Inside the Golgi apparatus **Answer:** A. On the ribosome Ribosomes read mRNA and assemble amino acids into proteins. ## Sources - [Biology 2e, Chapter 14: DNA Structure and Function (OpenStax)](https://openstax.org/books/biology-2e/pages/14-introduction) - CC BY-NC-SA 4.0. Concepts adapted and rewritten; no text reproduced verbatim. - [Biology 2e, Chapter 15: Genes and Proteins (OpenStax)](https://openstax.org/books/biology-2e/pages/15-introduction) - CC BY-NC-SA 4.0. Concepts adapted and rewritten; no text reproduced verbatim. - [Project teaching illustration: DNA replication fork (AI-generated, stylized)](https://github.com/cclank/cell-architecture-studio/blob/main/docs/ASSETS.md) - MIT (project asset)