Question

How is the peptide chain elongated during protein synthesis?

Short answer

Peptide chain elongation during protein synthesis involves three main steps: aminoacyl-tRNA binding, peptide bond formation, and translocation. A charged tRNA carrying an amino acid binds to the mRNA codon present at the ribosome's A-site via its anticodon. Then, a peptide bond forms between amino acids in the A and P sites, catalyzed by the peptidyl transferase activity of the large ribosomal subunit, elongating the chain. Finally, the ribosome translocates along the mRNA, moving tRNAs between sites and ultimately leading to chain termination upon encountering a stop codon. The completed peptide chain is then released and processed into a functional protein.

Step-by-step solution

Understand the Components Involved in Peptide Chain Elongation

In protein synthesis, peptide chain elongation involves mRNA (messenger RNA), ribosomes, tRNA (transfer RNA), and amino acids. Ribosomes have two subunits, namely a large and a small subunit. The small subunit reads the mRNA, while the large subunit is responsible for creating the peptide bond to elongate the peptide chain.

Initiation of Translation

Before the elongation process begins, translation initiation occurs. The small ribosomal subunit binds to the mRNA's 5' cap, together with a specialized initiator tRNA carrying the first amino acid, Methionine. The ribosome then scans the mRNA to find the start codon (AUG), where translation begins.

Elongation Process Overview

Peptide chain elongation consists of three main steps: aminoacyl-tRNA binding, peptide bond formation, and translocation. These steps are repeated multiple times to form a growing peptide chain.

Aminoacyl-tRNA Binding

During elongation, a charged tRNA carrying an amino acid binds to the ribosome in the appropriate site. The charged tRNA has an anticodon that is complementary to the mRNA codon present in the A-site (aminoacyl-tRNA binding site) of the ribosome. This pairing is essential for the correct amino acid to be incorporated into the growing peptide chain.

Peptide Bond Formation

Once the correct tRNA binds to the A-site, a peptide bond forms between the amino acid of the A-site tRNA and the amino acid of the tRNA in the P-site (peptidyl-tRNA binding site). This reaction is catalyzed by the peptidyl transferase activity of the large ribosomal subunit. As a result, the peptide chain is transferred from the tRNA in the P-site to the amino acid on the tRNA in the A-site, elongating the chain by one amino acid.

Translocation

After the peptide bond is formed, the ribosome moves one codon forward along the mRNA, resulting in the tRNA in the P-site being moved to the E-site (exit site), and the tRNA in the A-site being moved to the P-site. The tRNA in the E-site is then released from the ribosome, making room for the next charged tRNA to bind to the A-site. This process of translocation continues until a stop codon is encountered, and the ribosome reaches the end of the mRNA.

Termination and Protein Release

When the ribosome encounters a stop codon on the mRNA, a protein called release factor binds to the A-site, causing the termination of translation. The complete peptide chain is released from the tRNA in the P-site and the ribosome dissociates into its large and small subunits to recycle for new translation rounds. The newly synthesized peptide chain is then processed and folded into a functional protein.

Key concepts

Protein Synthesis

Protein synthesis is a fundamental biological process where living cells build proteins. It's vital for cell function, growth, and responding to the environment.

The process begins with transcription, where the DNA sequence is converted into messenger RNA (mRNA). The mRNA then travels to the ribosome, the cell's protein factory, carrying the genetic code that will define the protein's amino acid sequence. Understanding the mechanics of protein synthesis helps in areas ranging from genetics to medicine, such as drug design and gene therapy.

• Transcription of DNA to mRNA
• mRNA traveling to ribosomes
• Translation of genetic code into proteins

Ribosome Function

The ribosome plays a crucial role in protein synthesis, acting as the machinery where proteins are assembled. It's composed of two subunits, with the small one reading the mRNA, and the large one catalyzing peptide bond formation.

During translation, ribosomes go through a cycle of binding with tRNA and amino acids, facilitating their correct sequence order as per the mRNA instructions. It consists of the P-site, A-site, and E-site, which are critical for the process steps of initiation, elongation, and termination.

• Small ribosome subunit: Reads mRNA
• Large ribosome subunit: Forms peptide bonds
• Activity sites: P-site, A-site, and E-site

mRNA Translation

Translation is the phase in protein synthesis where mRNA's nucleotide code is converted into an amino acid sequence, forming the protein.

It begins with initiation at the start codon, usually 'AUG', leading to the binding of initiator tRNA with methionine. Elongation follows, where complementary tRNA anti-codons pair with mRNA codons, sequentially adding amino acids and forming a peptide chain. Eventually, a stop codon causes termination, releasing the newly formed protein. Key stages include:

• Initiation: Start codon recognition
• Elongation: tRNA pairing and peptide chain growth
• Termination: Stop codon recognition and protein release

tRNA Role

Transfer RNA (tRNA) is essential in translating the genetic code into a specific amino acid sequence.

Each tRNA molecule has an anticodon region that recognizes specific mRNA codons and an opposite end that carries the corresponding amino acid. During protein synthesis, tRNA delivers amino acids to the ribosome in the correct order. The match between a tRNA's anticodon and an mRNA codon ensures that proteins are built accurately, reflecting the mRNA template. This interaction is critical for maintaining the fidelity of protein synthesis.

• Anticodon matching with mRNA codon
• Amino acid delivery to the ribosome
• Ensuring correct protein sequence