Explain how a gene codes for a protein through transcription and translation using the genetic code

What this dot point is asking

This outcome asks you to explain how a gene leads to a protein. The chain of logic is short and powerful: the order of bases in a gene is a code, read three at a time, that is copied into mRNA and then used to assemble a protein from amino acids in a specific order.

The answer

What a gene is

A gene is a section of DNA that carries the code for making one protein. The information is stored in the order of the bases (A, T, C, G) along the gene.

The triplet code

The bases are read in groups of three, called triplets or codons. Each triplet codes for one amino acid. Proteins are chains of amino acids, so the order of triplets in a gene sets the order of amino acids in the protein.

Transcription

The first stage of making a protein is transcription:

• The gene's base sequence is copied into a molecule of messenger RNA (mRNA) by complementary base pairing.
• The mRNA is a single strand that carries the code out of the nucleus to a ribosome.

Translation

The second stage is translation:

• The ribosome reads the mRNA three bases at a time.
• For each triplet, the matching amino acid is brought into place and joined to the growing chain.
• The completed chain of amino acids folds into the finished protein.

Why the base order matters

Because the base order sets the amino acid order, and the amino acid order determines how the protein folds and works, a change in even one base can change the protein. This is why genes control the characteristics of an organism, and why inserting a gene into another cell makes it produce that protein.

Examples in context

Example 1. Making human insulin. The human insulin gene is a base sequence that codes for the insulin protein. Put that gene into a bacterium and the bacterium transcribes and translates it, producing human insulin. The whole of recombinant protein production depends on this gene-to-protein pathway working in the new host.

Example 2. A single-base change. In some genetic conditions, changing one base in a gene changes one amino acid in the protein, which can stop it working. This shows directly how the order of bases controls the protein, and why precision matters in genetic engineering.

Try this

Q1. State what is meant by a gene. [1 mark]

• Cue. A section of DNA that codes for a protein.

Q2. Name the two stages by which a gene is used to make a protein, and state what each produces. [2 marks]

• Cue. Transcription, which produces messenger RNA from the gene; and translation, which produces the protein from the mRNA.

Q3. A coding sequence has $600$ bases. State the number of amino acids in the protein and explain why. [2 marks]

• Cue. $600 \div 3 = 200$ amino acids, because three bases (a triplet) code for one amino acid.