From gene to protein: transcription & translation

Your cells store instructions in DNA, a long molecule made of four chemical letters called bases: A, T, C, and G. A gene is just one meaningful stretch of that DNA, like one recipe in a giant cookbook. But a recipe is not a meal, so the cell uses genes to build proteins, the tiny machines that do the actual work, like digesting food or carrying oxygen.

The central dogma

The path from gene to protein has a grand-sounding name, the central dogma, but it is only two steps: DNA to mRNA to protein. First, transcription copies a gene from DNA into a messenger molecule called mRNA (messenger RNA). mRNA uses the same letters as DNA except that T (thymine) is replaced by U (uracil), so wherever the DNA said T, the mRNA says U.

Codons and reading frame

The second step is translation, which happens on a ribosome, the cell's protein-building machine. The ribosome reads the mRNA three letters at a time. Each group of three bases is called a codon, and each codon names one amino acid, the building block of a protein. Chain amino acids together and you have a protein.

Because letters are read in groups of three, where you start matters. The starting point sets the reading frame, the way the sequence is chopped into triplets, so AUGCAU read as AUG then CAU means something different than starting one letter over. The ribosome finds the frame using the start codon AUG, which also codes for the amino acid Methionine, and it halts at any of three stop codons, UAA, UAG, and UGA, which name no amino acid and simply mean stop.

A worked example

Mini codon table (codon -> amino acid)
AUG -> Methionine (also the START signal)
GCU -> Alanine
GGU -> Glycine
UUU -> Phenylalanine
UAA -> STOP

Read the mRNA left to right in groups of three, then look each codon up:

mRNA:   AUG GCU GGU UAA
codons: AUG | GCU | GGU | UAA

AUG -> Methionine (start)
GCU -> Alanine
GGU -> Glycine
UAA -> STOP (translation ends)

Protein: Methionine - Alanine - Glycine

# Tiny Python version of what the ribosome does: read 3 bases at a time,
# look each codon up in a table, and stop at a stop codon.

table = {
    "AUG": "Methionine",
    "GCU": "Alanine",
    "GGU": "Glycine",
    "UUU": "Phenylalanine",
    "UAA": "STOP",
}

mrna = "AUGGCUGGUUAA"
protein = []

for i in range(0, len(mrna), 3):   # jump forward 3 letters each loop
    codon = mrna[i:i+3]            # grab the next 3 bases
    amino_acid = table[codon]     # look it up
    if amino_acid == "STOP":
        break                     # stop building the protein
    protein.append(amino_acid)

print(" - ".join(protein))

Methionine - Alanine - Glycine