Problem 10 - Biology

Question

During which stage of translation does the synthesis of a polypeptide occur? a. initiation b. elongation c. termination d. splicing

Step-by-Step Solution

Answer

The synthesis of a polypeptide occurs during the elongation stage of translation.

Step 1: Understand the stages of translation

In the initiation stage, the ribosome comes together with the mRNA and the first tRNA so translation can begin. This stage does not directly result in the synthesis of a polypeptide.

Step 2: Identification of the correct stage

In the elongation stage, amino acids are brought to the ribosome by tRNAs and linked together to form a polypeptide chain. This stage represents the actual synthesis of the protein.

Step 3: Elimination of incorrect stages

The termination stage marks the end of translation, signaled by a stop codon in the mRNA. This stage results in the release of the completed polypeptide from the ribosome. The splicing, which is a post-translational modification, happens after the polypeptide has already been synthesized. Hence, both these stages can be eliminated as they occur after the polypeptide is synthesized.

Key Concepts

Translation Initiation

The process of protein synthesis begins with translation initiation. This crucial first step involves the assembly of the translation machinery, which includes a small ribosomal subunit binding to the messenger RNA (mRNA). It scans the mRNA for the start codon, typically AUG, which signifies the beginning of the protein-coding region.

Once the start codon is recognized, a special initiator transfer RNA (tRNA) carrying the amino acid methionine binds to the start codon. Following this, the large ribosomal subunit joins to form a complete ribosome.

The formation of this initiation complex requires energy, which is provided by molecules called guanosine triphosphate (GTP). Various initiation factors also play a role, ensuring that each component is correctly positioned for the next stage.

Translation Elongation

After initiation, translation elongation occurs. This stage is where the polypeptide chain is synthesized, one amino acid at a time. The ribosome moves along the mRNA, reading the codons (three-nucleotide sequences) that dictate which amino acid is to be added next.

Each tRNA, carrying a specific amino acid, matches its anticodon with the corresponding codon on the mRNA strand. The ribosome facilitates the formation of a peptide bond between the amino acid on the tRNA in the A (aminoacyl) site and the growing polypeptide chain on the tRNA in the P (peptidyl) site.

Role of Elongation Factors

Proteins called elongation factors escort tRNAs to the ribosome and translocate the ribosome along the mRNA. This process continues, consuming GTP for energy, unless a stop codon is reached on the mRNA.

Translation Termination

Translation termination is the final stage in the synthesis of a polypeptide. It is triggered by the presence of one of the three stop codons (UAA, UAG, or UGA) on the mRNA. At this stage, no corresponding tRNA matches these codons, so the translation process comes to a halt.

Release factors recognize the stop codons and prompt the ribosome to cleave the completed polypeptide from the tRNA. Subsequently, the polypeptide is released into the cell, and the ribosome subunits disassemble, ready to start another round of translation.

The completed polypeptide often undergoes folding to achieve its functional three-dimensional shape. Sometimes, additional modifications such as cleaving or adding functional groups are needed for the protein to become fully functional.

Polypeptide Synthesis

Polypeptide synthesis refers to the process of linking amino acids together in a specific order, as directed by the mRNA, to form a polypeptide chain during translation elongation. A polypeptide is essentially a protein in its primary structure form and may consist of a single or multiple chains that need to fold into a functional protein.

The ribosome is the site of this synthesis, ensuring each amino acid addition is precise. This precision is vital to the protein's function, as the sequence of amino acids determines its structure and, as a result, its activity within the cell.

After synthesis, polypeptides will often undergo further processing, like folding, forming disulfide bonds, or specific cleavages, to become a mature protein—ready to perform a specific role in the cell or be secreted as required.