30. The natural primer for reverse transcriptase in RNA tumor viruses is:
1. Oligo-dT
2. rRNA
3. 5S RNA
4. tRNA

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What Is the Natural Primer for Reverse Transcriptase in RNA Tumor Viruses?

Retroviruses, including RNA tumor viruses, rely on a unique enzyme called reverse transcriptase to convert their RNA genome into DNA. But for this process to begin, reverse transcriptase needs a primer—a short nucleic acid sequence that provides a starting point for DNA synthesis.

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Correct Answer:

(4) tRNA

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Detailed Explanation

How Reverse Transcriptase Works

• Reverse transcriptase is an enzyme that synthesizes complementary DNA (cDNA) from an RNA template.

• Unlike DNA polymerases, reverse transcriptase cannot initiate DNA synthesis de novo; it requires a primer with a free 3’-OH group to start DNA elongation.

Role of tRNA as Primer

• Retroviruses package a host-derived tRNA molecule within their viral particles.

• This tRNA acts as the natural primer by binding to a specific sequence on the viral RNA genome called the primer-binding site (PBS).

• Reverse transcriptase uses this annealed tRNA as a starting point to synthesize the first strand of viral DNA.

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Why Not Other Options?

1. Oligo-dT:

   • Used in laboratory settings to prime reverse transcription of eukaryotic mRNAs (by binding to the poly-A tail).

   • Not used naturally by retroviruses.

2. rRNA and 5S RNA:

   • These are structural RNAs involved in ribosomes.

   • They are not involved in reverse transcription or priming retroviral replication.

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Importance in Molecular Biology

Understanding the role of tRNA as a primer:

• Offers insights into viral replication mechanisms.

• Provides a foundation for designing antiviral drugs targeting early stages of viral life cycles.

• Helps researchers engineer reverse transcriptases for lab techniques like cDNA synthesis and RT-PCR.

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Summary

The natural primer for reverse transcriptase in RNA tumor viruses is tRNA, not synthetic primers like oligo-dT or unrelated RNAs like rRNA or 5S RNA. This tRNA is hijacked from the host cell and is critical for initiating the viral replication cycle.

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Final Answer:
(4) tRNA