During gene expression, a protein is actively being assembled at the ribosome when which process is taking place?

Introduction / Context:
The central dogma of molecular biology describes how information flows from DNA to RNA to protein. Protein synthesis occurs in a specific step of this flow, and exam questions often check whether you can correctly identify which process corresponds to the actual assembly of amino acids into a polypeptide chain. This question asks you to connect protein assembly at ribosomes with the appropriate molecular process.

Given Data / Assumptions:

• The focus is on the step when a protein is actively being assembled.
• Options mention translation of RNA, translation of DNA, and transcription of DNA or RNA.
• We assume knowledge of transcription and translation definitions.

Concept / Approach:
Transcription is the process in which DNA is used as a template to synthesize RNA, usually messenger RNA for protein coding genes. Translation is the process in which the sequence of nucleotides in messenger RNA is decoded to assemble a specific sequence of amino acids, forming a polypeptide. Translation occurs on ribosomes in the cytoplasm or on rough endoplasmic reticulum. DNA is never directly translated into protein; there is always an RNA intermediate. Therefore, when a protein is being assembled at a ribosome, messenger RNA is being translated.

Step-by-Step Solution:
Step 1: Recall the central dogma: DNA is transcribed into RNA, and RNA is translated into protein. Step 2: Recognise that transcription refers to copying information from DNA to RNA, usually in the nucleus in eukaryotes. Step 3: Understand that translation is the process at ribosomes in which transfer RNA molecules bring amino acids to match codons on messenger RNA. Step 4: Identify that the physical assembly of a polypeptide chain from amino acids occurs during translation of messenger RNA. Step 5: Choose the option stating that messenger RNA is being translated into a polypeptide chain.

Verification / Alternative check:
Diagrams of protein synthesis show mRNA passing through a ribosome, where tRNA molecules align amino acids according to codon anticodon pairing. The growing polypeptide emerges from the ribosome as translation proceeds. In contrast, transcription diagrams show RNA polymerase moving along DNA, building an RNA strand, without any direct protein assembly. These representations and experimental observations confirm that protein assembly is associated with translation of mRNA, not with transcription or direct DNA translation.

Why Other Options Are Wrong:

• DNA is being directly translated into protein: There is no direct translation of DNA into protein in normal cells; RNA is always the intermediary.
• RNA is being transcribed from a DNA template: This describes transcription, which produces RNA but does not assemble proteins.
• DNA is being transcribed into RNA inside the nucleus only: Again, this is transcription and does not involve polypeptide assembly at the ribosome.

Common Pitfalls:
Students sometimes mix up transcription and translation because both terms describe transfers of information. A useful memory aid is that transcription stays in the same language of nucleotides, while translation converts from nucleotides to the amino acid language. Another pitfall is forgetting that ribosomes are the site of translation in the cytoplasm or rough endoplasmic reticulum, while transcription occurs in the nucleus of eukaryotic cells.

Final Answer:
A protein is being actively assembled at the ribosome when messenger RNA is being translated into a polypeptide chain.