Translation

Essential Question

How does a ribosome turn an mRNA message into a protein?

Translation is the step where the message in an mRNA is used to build a protein. A ribosome reads the mRNA three bases at a time and matches each group to an amino acid using the genetic code.

The Genetic Code

• A codon is a group of 3 mRNA bases. There are 64 possible codons.
• 61 codons code for amino acids. There are 20 different amino acids. The other 3 codons (UAA, UAG, UGA) are stop codons.
• AUG codes for methionine (Met) and also serves as the start codon.
• Most amino acids have more than one codon, but each codon codes for only one amino acid.
• To read the chart, find the first base on the left, the second base across the top, and the third base on the right.

The genetic code is nearly the same in almost all living things, which is evidence that they share common ancestry.

The Players in Translation

• The ribosome is made of rRNA and proteins, with a large and a small subunit. It holds the mRNA and the tRNAs.
• mRNA carries the codons.
• Each tRNA carries one amino acid and has an anticodon that pairs with a matching codon. The anticodon is complementary to the codon.
• The ribosome holds tRNAs matched to two neighboring codons at a time. It joins their amino acids with a bond and then moves along the mRNA.

Base pairing between codons and anticodons makes sure that the right amino acid is added for each codon.

Three Stages of Translation

• Initiation: the small ribosomal subunit attaches to the mRNA and finds the start codon AUG. A tRNA carrying Met pairs with it, and the large subunit joins.
• Elongation: a tRNA with the matching anticodon brings the next amino acid. The ribosome joins it to the chain and moves one codon along the mRNA. This repeats for each codon.
• Termination: when the ribosome reaches a stop codon, no tRNA matches it. The chain of amino acids is released, and the ribosome comes apart.
• The finished chain is a polypeptide. It folds into a protein, and its shape determines its function.

Translation starts at AUG, adds one amino acid per codon, and ends at a stop codon.

Reading Frame

• The ribosome reads the mRNA in groups of three, starting at AUG. The groups do not overlap.
• The first AUG sets the reading frame. A different starting point gives different codons.
• The same mRNA read in a different frame gives different amino acids and may hit a stop codon early.
• Number of amino acids = number of codons – 1 (for the stop codon).

Only the frame that starts at AUG gives the protein. This idea is important for understanding mutations in the next lesson.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Translate an mRNA

Use the genetic code chart to translate this mRNA, starting at the start codon: AUG GCU GAG UUU AAA CGU UGG UAA

Show the solution

1. Divide the mRNA into codons: AUG GCU GAG UUU AAA CGU UGG UAA.
2. AUG codes for Met.
3. GCU codes for Ala.
4. GAG codes for Glu.
5. Continue for each codon: UUU = Phe, AAA = Lys, CGU = Arg, UGG = Trp, UAA = Stop.
6. The ribosome stops at the stop codon UAA.
7. Answer: Met-Ala-Glu-Phe-Lys-Arg-Trp (then stop). The protein has 7 amino acids.

Example 2: How Many Nucleotides?

A protein has 150 amino acids. How many nucleotides are in the mRNA from the start codon through the stop codon?

Show the solution

1. Each amino acid needs one codon, so 150 amino acids need 150 codons.
2. Add one stop codon: 150 + 1 = 151 codons.
3. Each codon is 3 nucleotides: 151 x 3 = 453 nucleotides.
4. Answer: 453 nucleotides.

Example 3: Find the Anticodons

Write the tRNA anticodon that pairs with each of the first four codons of the mRNA: AUG, GCU, GAG, UUU.

Show the solution

1. The anticodon is complementary to the codon: A pairs with U, U with A, G with C, C with G.
2. AUG pairs with UAC.
3. GCU pairs with CGA.
4. GAG pairs with CUC.
5. UUU pairs with AAA.
6. Answer: AUG: UAC, GCU: CGA, GAG: CUC, UUU: AAA.

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Practice

Use the practice worksheet to read the genetic code chart, translate mRNA, find anticodons, and calculate message and protein lengths.

• Translation Practice Worksheet (PDF)
• Translation Practice Answer Key (PDF)

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Lesson Slides

• 9.6 Translation Slides (PDF)

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Big Ideas

• Translation builds a protein from the codons of an mRNA.
• A codon is 3 bases. AUG is the start codon, and UAA, UAG, and UGA are stop codons.
• tRNA anticodons pair with codons and bring the matching amino acid to the ribosome.
• Translation has three stages: initiation, elongation, and termination.
• The reading frame is set by the start codon, so shifting it changes every amino acid after the shift.

Back to the essential question: A ribosome reads the mRNA three bases at a time. Each codon is matched by a tRNA anticodon that brings the correct amino acid, and the chain grows until a stop codon is reached. The sequence of the gene therefore sets the sequence of the protein.

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