Biology · Gene Expression: RNA & Proteins

Ribosomes, translation and protein synthesis

The RNA fragment AUGCCUGAAUGA is only a row of letters until you specify direction, start and reading frame. With a chosen start, a ribosome can read AUG | CCU | GAA | UGA. Moving the grouping by one nucleotide changes the interpretation.

You need the ideas of DNA strands, complementary base pairing and cellular compartments. The DNA replication and repair chapter supplies that background. Here, direction labels and RNA base pairing are explained again before the worked examples; complete biochemical mechanisms need not be memorised.

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By the end, you can

Track codon reading, tRNA participation and chain growth while retaining the frame and direction labels.

A ribosome uses adaptors and activated building blocks

A ribosome contains RNA and proteins. Its machinery positions mRNA and tRNAs so codon–anticodon pairing contributes to selection of a suitable aminoacyl-tRNA: a tRNA with an amino acid attached. Aminoacyl-tRNA synthetases and related cellular machinery establish the link between tRNA and amino acid. Complementary anticodon pairing alone cannot correct every possible mistake in that attachment.

Translation requires energy-coupled preparation and movement. Amino acids are activated during tRNA charging, and nucleotide-dependent factors help with the translation cycle. The ribosome joins amino acids through peptide bonds; RNA contributes to the catalytic centre. Amino acids come from the cellular supply, not from converting the mRNA's bases into protein material.

Read the message and grow the chain in different molecular directions

Initiation establishes the start and reading frame with an initiator tRNA and other factors. AUG commonly specifies the initiating methionine, but its position and surrounding context matter: not every AUG in an RNA is an independent start. Initiation details differ between bacterial and eukaryotic systems; the diagram is a general elongation guide after a frame has been established.

During elongation, an incoming charged tRNA is selected, the growing chain is transferred to its amino acid, and ribosomal movement advances the message by a codon. The mRNA is read 5′ to 3′; the polypeptide grows from its amino, or N, end towards its carboxyl, or C, end. A direction arrow along mRNA is therefore not the same kind of label as the N-to-C arrow along the protein.

Under the standard code, UAA, UAG and UGA ordinarily signal termination. Release factors recognise the stop context and promote release of the polypeptide; a stop signal is not an extra amino acid carried by an ordinary tRNA. Special recoding mechanisms exist, but the worked examples use ordinary standard-code termination.

Codon reading and chain growth have distinct directions

Generalised elongation after initiation, not a complete bacterial or eukaryotic factor cycle. Ordinary standard-code termination is assumed.

Read the diagram in words

A ribosome is shown on an mRNA displayed 5′ to 3′ with codons AUG, CCU, GAA and UGA. A tRNA pairs antiparallel with a codon and carries an amino acid at a separate attachment site. Three step panels show selection of an incoming charged tRNA, chain transfer and movement by one codon. mRNA reading is 5′ to 3′; chain growth is N to C. The stop panel uses a release factor rather than a stop amino acid.

Established reading frame
Molecule or orientation	Left end	Bases from left to right	Right end
mRNA	5′	AUG CCU GAA UGA	3′
Paired codon	5′	AUG	3′
Anticodon aligned to the codon	3′	UAC	5′
Anticodon read from its own 5′ end	5′	CAU	3′

Separate orientation inset: the initiating AUG's tRNA is placed during initiation, rather than entering as an ordinary elongation tRNA

Established reading frame
Step	Description
Select	Incoming charged tRNA pairs in the established frame
Transfer	Growing chain transfers to the incoming amino acid
Advance	Ribosome moves by one codon
Terminate	Ordinary stop context recruits a release factor

Polypeptide growth: N → C
Illustrated product	N end	Amino-acid positions	C end	Amino-acid count
Terminate	N end	1 · 2 · 3	C end	3

Ribosome: RNA and proteins
tRNA adaptor
Attached amino acid
mRNA read: 5′ → 3′
Polypeptide growth: N → C
N end
C end
Established reading frame
Release factor at stop
Charging machinery attaches the appropriate amino acid

The chain is a product, not yet a complete function

Several ribosomes can translate one mRNA at once, each producing its own chain. An mRNA can therefore direct many polypeptides without being divided into protein pieces. RNA turnover, initiation and the availability of ribosomes and charged tRNAs all influence production.

The new chain may fold, join other subunits, receive modifications or reach a particular cellular destination before performing its function. Translation has produced a polypeptide sequence; it has not by itself demonstrated a correctly located, active protein. The next lesson follows that further work.

Worked example

Read a short message without adding a stop amino acid

Treat 5′-AUG CCU GAA UGA-3′ as a constructed coding fragment with initiation at its first AUG. In the standard code, AUG specifies methionine, CCU proline and GAA glutamate. UGA terminates this example. The resulting short chain contains three amino acids, not four.

An anticodon paired directly below an AUG codon written 5′ to 3′ is written antiparallel as 3′-UAC-5′. If you choose to report that anticodon in the usual 5′-to-3′ sequence format, it becomes 5′-CAU-3′. This is the same complementary sequence with a different writing direction, not a different tRNA. The diagram must label the orientation rather than displaying an unlabelled UAC.

Check your understanding

In the stated fragment AUG | CCU | GAA | UGA, how many amino acids are added before ordinary termination?
Show reasoning
Three. Methionine, proline and glutamate are added; UGA is the stop signal in this standard-code example.
A codon is displayed 5′-AUG-3′. Why must an anticodon displayed directly beneath it have its ends labelled?
Show reasoning
Pairing is antiparallel. The aligned anticodon reads 3′-UAC-5′, which is 5′-CAU-3′ when rewritten in the other direction.
Does translation use up one mRNA to make exactly one protein?
Show reasoning
No. An mRNA can be translated repeatedly and by several ribosomes, while RNA turnover is a distinct process.

What to carry forward

A defined frame connects codons to amino acids; correct orientation, adaptor charging and termination are essential parts of that connection.

References

Resources supporting this lesson’s explanations and examples.