The eukaryotic mRNA shown schematically in the diagram below has five ribosomes carrying out translation.

Which one of the following statements about this polyribosome complex is true?

Correct option is C

Explanation: In a polyribosome (or polysome), multiple ribosomes simultaneously translate an mRNA molecule. The ribosomes move along the mRNA from the 5' to the 3' end. The first ribosome initiates translation at the start codon, and as translation progresses, ribosomes further along the mRNA chain begin translating the same message. The distance between the ribosomes is determined by the rate of translation.

• Ribosome 4 is farther along the mRNA than Ribosome 3. As translation continues, the polypeptide chain attached to ribosome 4 will be longer than the one on ribosome 3, since ribosome 4 is translating a longer portion of the mRNA.

Information Booster:

1. Polyribosomes allow for efficient protein synthesis by enabling multiple ribosomes to translate a single mRNA molecule simultaneously.

2. The first ribosome starts translation at the 5' end of the mRNA, and subsequent ribosomes join as the mRNA moves through the translation process.

3. The ribosome closest to the 3' end of the mRNA is translating the most mature part of the polypeptide.

4. The mRNA is translated in the direction from 5' to 3', with the polypeptide chain growing from the amino (N)-terminus to the carboxyl (C)-terminus.

5. The longer the ribosome is translating the mRNA, the longer the polypeptide chain attached to it.

6. The polypeptide attached to ribosome 4 is longer than that on ribosome 3 due to ribosome 4 being farther along in translation.

Additional Information:

• Statement 1 (The mRNA was being transcribed when the first ribosome started translation): mRNA transcription happens before translation. Translation begins after the mRNA has been fully transcribed.

• Statement 2 (Ribosome 5 is nearest to the initiation codon): Ribosome 5 is closest to the 3' end of the mRNA and thus farthest from the initiation codon at the 5' end.

• Statement 4 (All the ribosomes have incorporated the carboxyl-terminal amino acid): Only the ribosome closest to the 3' end (ribosome 5) would have incorporated the carboxyl-terminal amino acid, as translation progresses along the mRNA.