Steps of Translation: Videos & Practice Problems

Translation consists of three key steps: initiation, elongation, and termination. During initiation, the small ribosomal subunit binds to messenger RNA (mRNA) and transfer RNA (tRNA), with the start codon AUG signaling the beginning of the polypeptide chain, which starts with methionine (MET). In elongation, amino acids are sequentially added to the growing chain through peptide bonds, facilitated by tRNAs entering the ribosome's A site. Finally, termination occurs when a stop codon triggers a release factor, cleaving the polypeptide and disassembling the translation complex, concluding the process.

1) Initiation of Translation

The process of translation, essential for protein synthesis, unfolds in three key stages: initiation, elongation, and termination. Each of these stages mirrors the steps found in transcription, beginning with the initiation phase, which marks the start of translation.

During the initiation of translation, the small ribosomal subunit first binds to the messenger RNA (mRNA). This mRNA contains codons, which are sequences of three nucleotides that specify particular amino acids. The first codon, known as the start codon, is AUG, which codes for the amino acid methionine (abbreviated as MET). Following the binding of the small ribosomal subunit to the mRNA, a transfer RNA (tRNA) carrying methionine attaches to the start codon. The tRNA has an anticodon that is complementary to the mRNA codon, ensuring the correct amino acid is added to the growing polypeptide chain.

After the small ribosomal subunit and tRNA are in place, the large ribosomal subunit joins the complex, completing the formation of the ribosome. This assembly is facilitated by several proteins known as initiation factors, which play a crucial role in the process. Additionally, the initiation of translation requires energy, highlighting the complexity of this initial step.

At the conclusion of the initiation phase, a fully assembled ribosome is ready to begin elongation, with the first tRNA positioned at the start codon, ready to initiate the synthesis of the polypeptide chain. Understanding this process is vital as it sets the stage for the subsequent elongation and termination phases of translation.

2) Elongation of Translation

Elongation in translation is a crucial step where the polypeptide chain, composed of amino acids, is extended. This process occurs in the ribosome, which reads the messenger RNA (mRNA) in codons, sequences of three nucleotides, from the 5' to the 3' end. Each codon corresponds to a specific amino acid, which is brought to the ribosome by transfer RNA (tRNA) molecules that have the appropriate anticodons.

During elongation, charged tRNAs, which are tRNAs linked to their respective amino acids, enter the ribosome at the A site. Once in position, a peptide bond is formed between the amino acid on the incoming tRNA and the growing polypeptide chain, which is attached to the tRNA in the P site. This covalent bond links the amino acids together, facilitating the growth of the polypeptide chain from the N-terminus to the C-terminus.

As the ribosome moves along the mRNA, the tRNA in the P site shifts to the E site, where it exits the ribosome, while the newly charged tRNA moves from the A site to the P site. This cycle of tRNA entry, peptide bond formation, and tRNA exit continues, allowing for the sequential addition of amino acids to the growing chain. The process of elongation is vital for synthesizing proteins, as it ensures that the correct sequence of amino acids is assembled according to the genetic code carried by the mRNA.

3) Termination of Translation

Termination of translation is the final step in the process of protein synthesis, marking the conclusion of the translation phase. This process begins when a stop codon, such as UGA, reaches the A site of the ribosome. Unlike regular codons, stop codons do not correspond to tRNA molecules; instead, they signal the binding of a release factor protein.

Upon the release factor's binding, several critical events occur. First, the growing polypeptide chain is cleaved from the tRNA, resulting in the release of the completed polypeptide. This marks the transition from the ribosome's active site to the final product, which is now free. Additionally, the entire translation assembly disassembles. The mRNA strand is released, and the small and large subunits of the ribosome separate from one another, effectively ending the translation process.

This termination step is crucial as it ensures that the newly synthesized protein is released and can subsequently fold into its functional form or undergo further modifications. Understanding this process is essential for grasping how proteins are synthesized and regulated within the cell, laying the groundwork for further exploration of molecular biology concepts.

The tRNA for which amino acid is the first to enter the ribosome?

• A. Arginine.
• B. Lysine.
• C. Methionine.
• D. Histidine.

Answer: (C)

Which of the following processes is the first event to take place in translation?

• A. Base pairing of charged methionine-tRNA to AUG of the messenger RNA.
• B. Binding of the larger ribosomal subunit to smaller ribosomal subunits.
• C. The ribosome reaches a stop codon.
• D. The small subunit of the ribosome recognizes and attaches to the mRNA.

Answer: (D)

When is a peptide bond formed during the process of translation?

• A. During the elongation phase, just after a tRNA charged with the next amino acid binds to the A site.
• B. During the termination phase, just after the release factor binds to the A site.
• C. During the elongation phase, just after a tRNA that has lost its amino acid and exits the E site.

Answer: (A)

Which of the following does not occur during translation's termination step?

• A. The first tRNA brings the amino acid methionine to the ribosome.
• B. The small and large ribosomal subunits separate from each other.
• C. The polypeptide is released from the ribosome.
• D. A "stop" codon is reached by the ribosome along the mRNA.

Answer: (A)

What is the function of the release factor during translation?

• A. It binds to the stop codon in the A site in place of a tRNA.
• B. It releases the amino acid from its tRNA to allow the amino acid to be added to the growing polypeptide.
• C. It supplies a source of energy for termination of translation and the release of the polypeptide.
• D. It releases the ribosome from the rough endoplasmic reticulum and allows the polypeptide to enter the cytosol.

Answer: (A)

Here’s what students ask on this topic:

What are the three main steps of translation in protein synthesis?

The three main steps of translation in protein synthesis are initiation, elongation, and termination. During initiation, the small ribosomal subunit binds to the messenger RNA (mRNA) and transfer RNA (tRNA), with the start codon AUG signaling the beginning of the polypeptide chain, which starts with methionine (MET). In elongation, amino acids are sequentially added to the growing chain through peptide bonds, facilitated by tRNAs entering the ribosome's A site. Finally, termination occurs when a stop codon triggers a release factor, cleaving the polypeptide and disassembling the translation complex, concluding the process.

What happens during the initiation step of translation?

During the initiation step of translation, the small ribosomal subunit first binds to the messenger RNA (mRNA) and a transfer RNA (tRNA) that carries the amino acid methionine (MET). The start codon AUG on the mRNA signals the beginning of the polypeptide chain. The large ribosomal subunit then joins to form a complete ribosome. This assembly positions the tRNA carrying methionine in the P site of the ribosome, setting the stage for the elongation phase where amino acids will be added to the growing polypeptide chain.

How does elongation occur during translation?

Elongation during translation involves the sequential addition of amino acids to the growing polypeptide chain. The ribosome reads the mRNA codons from the 5' to the 3' end. Charged tRNAs carrying specific amino acids enter the ribosome's A site, where their anticodons pair with the mRNA codons. A peptide bond forms between the amino acid in the A site and the growing chain in the P site. The ribosome then shifts, moving the tRNA with the growing chain to the P site and the empty tRNA to the E site, where it exits the ribosome. This process repeats, elongating the polypeptide chain.

What triggers the termination of translation?

Termination of translation is triggered when a stop codon (UAA, UAG, or UGA) reaches the ribosome's A site. These stop codons do not code for any amino acids and instead signal the binding of a release factor protein. The release factor promotes the cleavage of the polypeptide chain from the tRNA in the P site, releasing the newly synthesized protein. Subsequently, the ribosomal subunits, mRNA, and tRNA dissociate, effectively ending the translation process.

What role do tRNAs play in the translation process?

Transfer RNAs (tRNAs) play a crucial role in translation by delivering specific amino acids to the ribosome, where they are added to the growing polypeptide chain. Each tRNA has an anticodon that is complementary to an mRNA codon, ensuring the correct amino acid is incorporated. During elongation, charged tRNAs enter the ribosome's A site, where their anticodons pair with the mRNA codons. The amino acid they carry is then linked to the growing chain through a peptide bond. The tRNA then shifts to the P site and eventually exits through the E site, allowing the process to repeat.