6.An antibiotic that resembles the 3′ end of a charged tRNA molecule is:
(1) Streptomycin(2) Sporsomycin
(3) Puromycin     (4) Tetracycline

Puromycin Resembles the 3′ End of a Charged tRNA

The correct answer is (3) Puromycin.

This is an important molecular biology concept because it connects the structure of tRNA, ribosomal protein synthesis, and the mechanism of translation inhibitors. The question asks which antibiotic resembles the 3′ end of a charged tRNA molecule. To solve it correctly, we need to understand what happens at the 3′ end of an aminoacyl-tRNA and how Puromycin interferes with translation.

During protein synthesis, an amino acid is attached to the 3′ end of a tRNA molecule. This produces what is called a charged tRNA or aminoacyl-tRNA. The amino acid attached to the tRNA is subsequently transferred to the growing polypeptide chain during translation. Puromycin has a structure that resembles the 3′ end of an aminoacyl-tRNA, allowing it to enter the ribosome and participate in the reaction normally performed by an aminoacyl-tRNA.

Why Is Puromycin the Correct Answer?

Puromycin is an aminonucleoside antibiotic that inhibits protein synthesis by mimicking the aminoacyl end of a charged tRNA. Its structural similarity allows it to interact with the ribosome, particularly at the aminoacyl (A) site.

Normally, an aminoacyl-tRNA enters the A site of the ribosome. The amino acid attached to its 3′ end is positioned close to the growing peptide chain. The ribosome then catalyzes formation of a peptide bond, transferring the growing polypeptide onto the amino acid attached to the incoming tRNA.

Puromycin takes advantage of this mechanism. Because its structure resembles the aminoacyl-tRNA acceptor end, the ribosome can accept Puromycin as though it were part of an aminoacyl-tRNA. The growing peptide chain can therefore be transferred to Puromycin.

However, Puromycin is not attached to a complete tRNA molecule. Consequently, once the peptide becomes attached to Puromycin, the resulting peptidyl-puromycin can dissociate from the ribosome. This causes premature termination of protein synthesis.

This is the central reason why Puromycin is associated with the phrase “mimics the 3′ end of charged tRNA.”

Role of the 3′ End of Charged tRNA in Translation

The 3′ end of tRNA is particularly important because it contains the conserved CCA sequence. The amino acid is covalently attached to the terminal adenosine of this CCA sequence.

For example, an aminoacyl-tRNA can be represented conceptually as:

tRNA–CCA–amino acid

The amino acid is linked to the terminal adenosine through an ester bond. During translation, this aminoacyl end is positioned in the ribosome so that the amino acid can participate in peptide-bond formation.

Puromycin structurally resembles this functional portion of aminoacyl-tRNA. Therefore, the ribosome can mistakenly use Puromycin as a substrate for peptide transfer.

How Puromycin Inhibits Protein Synthesis

The mechanism becomes easier to understand if we follow the normal translation process first.

An aminoacyl-tRNA enters the ribosomal A site and pairs its anticodon with the appropriate codon on mRNA. Its amino acid is positioned next to the growing peptide chain attached to the tRNA in the P site. The ribosome catalyzes peptide-bond formation, and the growing peptide is transferred to the amino acid on the A-site tRNA.

Puromycin can enter this process because it resembles the aminoacyl end of the incoming tRNA. The growing peptide chain is transferred to Puromycin, forming a peptidyl-puromycin molecule. Since Puromycin lacks the rest of the tRNA structure required for continued translation, the peptide cannot proceed normally through additional elongation cycles.

As a result, the newly synthesized polypeptide is released prematurely.

Therefore, the key relationship is:

Puromycin → mimics aminoacyl-tRNA 3′ end → enters ribosomal A site → accepts growing peptide → premature chain termination.

This mechanism makes Puromycin particularly useful not only as a translation inhibitor but also as an experimental tool for studying protein synthesis and translation.

Option 1: Streptomycin

Streptomycin is not the correct answer. It is an aminoglycoside antibiotic that primarily targets the 30S bacterial ribosomal subunit.

Streptomycin binds to the small ribosomal subunit and interferes with accurate decoding of mRNA. One important consequence is misreading of the genetic code, which can result in the incorporation of incorrect amino acids into newly synthesized proteins.

Therefore, Streptomycin is associated with errors in translation and inhibition of bacterial protein synthesis, rather than structural mimicry of the 3′ end of charged tRNA.

This distinction is important in molecular biology questions. If a question describes an antibiotic that causes misreading of mRNA codons or interacts with the 30S subunit, Streptomycin becomes a strong candidate. But when the question specifically mentions resemblance to the aminoacyl end of charged tRNA, the answer is Puromycin.

Option 2: Sporsomycin

Sporsomycin is not the correct answer. Sporsomycin is a translation inhibitor that affects the ribosomal machinery, particularly the process associated with peptide-bond formation and translocation.

Although Sporsomycin is related to inhibition of protein synthesis, it does not have the characteristic mechanism in which an antibiotic structurally mimics the 3′ aminoacyl end of a charged tRNA.

Therefore, it should not be selected when the question specifically asks for the antibiotic that resembles the 3′ end of charged tRNA.

The important point is that not every antibiotic that inhibits translation is a tRNA mimic. Translation inhibitors can act at very different stages, including initiation, decoding, peptide-bond formation, translocation, and termination.

Option 3: Puromycin

Puromycin is the correct answer.

Puromycin is structurally similar to the 3′ end of an aminoacyl-tRNA, allowing it to interact with the ribosome and accept the growing polypeptide chain.

Once the peptide is transferred to Puromycin, the resulting peptidyl-puromycin is released from the ribosome. Because Puromycin does not possess the complete tRNA structure required for continued translation, the polypeptide chain cannot undergo normal elongation.

Thus, Puromycin causes premature termination of translation.

This is exactly what the question is testing: the relationship between charged tRNA structure and Puromycin-mediated translation inhibition.

Option 4: Tetracycline

Tetracycline is also not the correct answer. Tetracycline is a broad-spectrum antibiotic that inhibits bacterial protein synthesis by binding primarily to the 30S ribosomal subunit.

Its major effect is to prevent the binding of aminoacyl-tRNA to the A site of the bacterial ribosome. Consequently, the incoming amino acid cannot be properly positioned for peptide-bond formation, and elongation of the polypeptide chain is inhibited.

Notice the distinction between Tetracycline and Puromycin.

Tetracycline prevents aminoacyl-tRNA from entering the A site, whereas Puromycin mimics the aminoacyl end of tRNA and becomes incorporated into the translation reaction, causing premature termination.

Puromycin vs Tetracycline: The Key Difference

The two antibiotics can sometimes be confused because both interfere with translation, but their mechanisms are fundamentally different.

Therefore, when the wording specifically says “resembles the 3′ end of a charged tRNA”, Puromycin should immediately stand out.

Translation Inhibitors and Their Mechanisms

Understanding this question becomes much easier when antibiotics are grouped according to their effects on translation. Bacterial ribosomes consist of 30S and 50S subunits, and different antibiotics target different regions or stages of protein synthesis.

Streptomycin acts on the 30S subunit and affects decoding accuracy. Tetracycline also acts on the 30S subunit but primarily prevents aminoacyl-tRNA from binding to the A site. Other antibiotics target the 50S subunit and can interfere with peptide-bond formation or translocation.

Puromycin is different because it acts as a molecular mimic of the aminoacyl-tRNA acceptor end. This allows the ribosome to transfer the growing peptide onto the drug, resulting in premature release of the nascent protein.

Why the 3′ End of tRNA Is So Important

The 3′ end of tRNA is the functional site where the amino acid is attached. The conserved terminal sequence is CCA, and the amino acid is linked to the terminal adenosine.

This arrangement is essential for translation because the ribosome uses the aminoacyl group at this end to build the growing polypeptide chain.

The structural resemblance between Puromycin and this region allows the drug to exploit the ribosome’s normal catalytic machinery. In other words, Puromycin does not simply block the ribosome from outside; it acts as a substrate mimic within the translation process.

That is why this question is fundamentally testing your understanding of aminoacyl-tRNA structure, ribosomal catalysis, and translation termination.

Final Answer

The antibiotic that resembles the 3′ end of a charged tRNA molecule is Puromycin.

Correct option: (3) Puromycin

The key mechanism is:

Charged aminoacyl-tRNA → amino acid at 3′ CCA end → Puromycin mimics this aminoacyl end → peptide is transferred to Puromycin → peptidyl-puromycin is released → premature termination of protein synthesis.

This makes Puromycin the characteristic tRNA-mimicking translation inhibitor among the four options.