From DNA to protein

TL;DR:

• Transcription copies a gene from DNA into mRNA. Translation reads the mRNA three letters at a time and builds a protein.

• Each three-letter codon names one amino acid or a stop. AUG is the usual start and codes for methionine.

• Read the mRNA from the start codon in the correct frame and stop at a stop codon, which adds no amino acid.

Your exam hands you a short stretch of DNA and asks for the protein it makes. The question looks like a puzzle, but it is only two copying steps and one lookup. If you keep the direction and the starting point straight, you can do it in under a minute.

The idea in plain words

A gene is a stretch of DNA that carries instructions for one protein. DNA stays in the nucleus of a eukaryotic cell, so the cell first makes a working copy. That copy is messenger RNA (mRNA), and the copying step is transcription.

The mRNA then meets a ribosome in the cytoplasm. The ribosome reads it in groups of three bases called codons, and brings in the matching amino acids one at a time. Linking amino acids into a chain is translation. The chain folds into a protein.

Why three letters at a time

RNA has four bases: A, U, G and C. One base per amino acid would only name 4 amino acids. Two bases would name 4 × 4 = 16. Three bases give 4 × 4 × 4 = 64 combinations, which is enough for the 20 amino acids used in proteins plus signals for start and stop.

That is why the genetic code has spare codons. Several codons can name the same amino acid, so the code is called redundant. It is not ambiguous: each codon names exactly one thing.

In RNA the base pairs are A with U and G with C. In DNA, A pairs with T. When the cell transcribes, it reads the template strand and builds an mRNA that matches the other strand, the coding strand, except that U replaces T.

Worked example: from a DNA coding strand to a four-amino-acid peptide

Say the coding strand of a tiny gene reads 5'-ATGGCTTGGAAATAA-3'. We want the template strand, the mRNA and the protein.

1. Template strand. Pair each base with its partner and read the new strand in the opposite direction: 3'-TACCGAACCTTTATT-5'.

2. Transcribe. The mRNA matches the coding strand with U in place of T: 5'-AUGGCUUGGAAAUAA-3'.

3. Split into codons from AUG. AUG | GCU | UGG | AAA | UAA. That is 15 bases, so 5 codons.

4. Look each one up. AUG is methionine (Met). GCU is alanine (Ala). UGG is tryptophan (Trp). AAA is lysine (Lys). UAA is a stop codon.

5. Write the protein. Met-Ala-Trp-Lys, four amino acids. The stop codon ends translation and adds nothing.

How to check your answer

• Count the bases. 15 ÷ 3 = 5 codons. If the count does not divide by 3, you dropped or added a base.

• Complement test. Pair your mRNA back with the template strand: U with A, G with C. Every position should match.

• Length of the protein. Number of codons before the stop: 5 − 1 = 4 amino acids, and you wrote four.

• Start and stop. The first codon should be AUG and the last one a stop. If your chain has no stop, say so, because the sequence may continue.

Common mistakes

• Writing T in mRNA. RNA has no thymine. Replace every T with U.

• Translating DNA directly. The code table is written for mRNA codons. Convert first.

• Reading the template strand as if it were the mRNA. If you are given the template, the mRNA is its complement, not a copy of it.

• Starting in the wrong place. Find the first AUG. Shifting by one base changes every codon after it.

• Counting the stop codon as an amino acid. It signals the end and has no amino acid.

In a real eukaryotic cell, the first mRNA is also edited before translation: non-coding pieces called introns are cut out and the coding pieces called exons are joined. Most school questions skip this step, but a question that mentions splicing is asking about it.

Two small details that cost marks

First, direction. Nucleic acids have a 5' end and a 3' end, and strands pair in opposite directions. When a question gives you the template strand written 3' to 5', the mRNA is built 5' to 3', so reverse your reading before you split into codons.

Second, the reading frame. Codons do not overlap and are never skipped. Once translation starts at AUG, the ribosome moves exactly three bases at a time. In our example the frame is fixed by the first AUG, so the second codon is GCU, not GGC.

Where you will use it next

The same pipeline is behind mutations (change one base and you may change one codon), protein structure, genetic engineering and inheritance. Once you can run DNA to protein by hand, questions about what a mutation does become a matter of redoing the lookup with one letter changed. Try that next in types of genetic mutations.

Frequently asked questions

Why does mRNA use U instead of T?

RNA's base is uracil. It pairs with adenine in the same way thymine does in DNA, so the information is the same.

Is there a start codon for every protein?

In the standard code, translation normally begins at AUG, which also codes for methionine inside the protein. Some cells use rarer start codons, so a question will tell you if it wants something different.

Do I need to memorize the codon table?

Usually no. Exams normally give it to you. What you need is practice reading it fast and keeping the frame.