Number of t-RNA present in cells is
**Core Concept**
The number of transfer RNA (t-RNA) molecules present in a cell is a fundamental aspect of protein synthesis. t-RNA molecules are responsible for transferring specific amino acids to the ribosome during translation, where they are assembled into polypeptide chains. The diversity of t-RNA molecules is essential for encoding the 20 standard amino acids.
**Why the Correct Answer is Right**
Each t-RNA molecule is responsible for recognizing a specific codon sequence on the messenger RNA (mRNA) and binding to a corresponding amino acid. With 20 standard amino acids, theoretically, 20 t-RNA molecules would be sufficient. However, the genetic code is degenerate, meaning that more than one codon can encode the same amino acid. As a result, the actual number of t-RNA molecules required to read the genetic code is higher than 20. Research has shown that there are approximately 23 t-RNA molecules in a typical eukaryotic cell, which is consistent with the degeneracy of the genetic code. These 23 t-RNA molecules can recognize all 61 codons, ensuring accurate protein synthesis.
**Why Each Wrong Option is Incorrect**
**Option B:** This option is incorrect because the actual number of t-RNA molecules is not 25. While some t-RNA molecules may have multiple functions, the total number of unique t-RNA molecules in a cell is not 25.
**Option C:** This option is incorrect because the actual number of t-RNA molecules is not 28. This number does not account for the degeneracy of the genetic code and the resulting need for multiple t-RNA molecules to recognize different codons.
**Option D:** This option is incorrect because the actual number of t-RNA molecules is not 30. This number is higher than the actual number and does not accurately reflect the complexity of the genetic code.
**Clinical Pearl / High-Yield Fact**
It is essential for medical students to understand the concept of genetic code degeneracy and its impact on the number of t-RNA molecules required for accurate protein synthesis. This knowledge is critical in understanding various genetic disorders and their effects on protein function.
**✓ Correct Answer: A. 23**