Translation: tRNA, the ribosome and the polypeptide

A translator for the code

The mRNA carries the message in the language of nucleotides. The cell must put it into the language of amino acids. This is translation. It needs an adapter, a molecule that can read a codon on one side and carry the right amino acid on the other.

tRNA, the adapter

The adapter is the transfer RNA (tRNA). It was once called sRNA, soluble RNA. Look at it in the picture.

• At one end is the anticodon loop. It has three bases, the anticodon, that pair with a codon of the mRNA. The codon and the anticodon run in opposite directions. The codon AUG pairs with the anticodon 3′-UAC-5′.
• At the other end is the amino acid acceptor end, where the amino acid is joined.
• A tRNA is specific. There is a tRNA for each amino acid, and each carries only its own amino acid. There is a special initiator tRNA for the start codon. There are no tRNAs for the stop codons.

On paper the tRNA looks like a clover leaf. In the cell, it is folded into a compact molecule that looks like an inverted L.

Charging the tRNA

It takes energy to make a peptide bond. So first the amino acid is activated by ATP and joined to its own tRNA. This step is called charging of the tRNA, or aminoacylation. If two charged tRNAs lie close together, a peptide bond between their amino acids is easy to make.

The ribosome, the factory

The cell factory that makes proteins is the ribosome. It is made of rRNA and about 80 different proteins. When it is not working, it is in two parts, a small subunit and a large subunit. When the small subunit meets an mRNA, translation begins.

The large subunit has two sites that hold two charged tRNAs side by side. They are close enough for the peptide bond to form. A-Level books name them the P site (for the growing polypeptide) and the A site (where the new tRNA arrives).

The ribosome also acts as a catalyst. In bacteria, the 23S rRNA of the large subunit makes the peptide bond. It is an RNA that works as an enzyme, and so it is a ribozyme.

The three steps

1. Initiation. The ribosome binds to the mRNA at the start codon, AUG. Only the initiator tRNA can read this codon, and it carries methionine.
2. Elongation. A charged tRNA enters the empty site, a peptide bond is made, and the ribosome moves along the mRNA by one codon, from the 5′ end to the 3′ end. The next amino acid is added. This repeats for every codon.
3. Termination. The ribosome reaches a stop codon. No tRNA reads it. A release factor binds to it. The polypeptide is let go, and the ribosome comes off the mRNA.

The mRNA is longer than the part that is translated. At both ends there are untranslated regions (UTRs). One lies before the start codon, at the 5′ end, and the other lies after the stop codon, at the 3′ end. They are needed for efficient translation. The translational unit is the stretch from the start codon to the stop codon.

How long is the protein?

The coding region, from AUG to the end of the stop codon, has a whole number of codons. The stop codon has no amino acid. So

The average amino acid has a mass of about 110 daltons (Da), so a polypeptide of 300 amino acids has a mass of about 33 kDa.

You should find that the mass grows in a straight line with the length of the coding region, and that a ribosome takes seconds, not hours, to make a protein.

Try it in Python

The program reads an mRNA from its first AUG to a stop codon. In the program, M is methionine, A is alanine, F is phenylalanine and K is lysine.

Practise with new numbers

Common mistakes

• Saying that the tRNA carries the codon. It carries the anticodon, and the codon is on the mRNA.
• Saying that there is a tRNA for each stop codon. There is none. A release factor reads the stop codon.
• Counting the stop codon as an amino acid. It has none.
• Saying that the ribosome moves from the 3′ end to the 5′ end of the mRNA. It moves 5′ to 3′.
• Saying that the ribosome starts at the first base of the mRNA. It starts at AUG, after the 5′ UTR.

Remember these

• tRNA: anticodon loop at one end, amino acid acceptor end at the other. Clover leaf on paper, inverted L in the cell.
• Charging of the tRNA needs ATP. The ribosome has a small and a large subunit, rRNA and about 80 proteins.
• Initiation at AUG, elongation one codon at a time, termination at a stop codon with a release factor.
• UTRs lie before AUG and after the stop codon. 23S rRNA is a ribozyme in bacteria.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What does a tRNA do?

2. What is the anticodon for the codon GCU?

3. What happens in the charging of a tRNA?

4. How does the ribosome know where to stop?

5. An mRNA has a coding region of 603 nucleotides, from AUG to the end of the stop codon. How many amino acids does the polypeptide have?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 5, Molecular Basis of Inheritance.
• Clark M. A. et al. (2018). Biology 2e, OpenStax, chapter 15, section 15.5, Ribosomes and Protein Synthesis.
• Alberts B. et al. (2022). Molecular Biology of the Cell, 7th edition, W. W. Norton, chapter 6, How Cells Read the Genome.