Correct option is A
Key Concepts:
Aminoacyl-tRNA Synthetases (AARSs) are enzymes that attach the appropriate amino acid to its cognate tRNA molecule during protein synthesis.
tRNA molecules have a CCA sequence at their 3' end, which is where the amino acid is covalently attached.
The amino acid is typically attached to the hydroxyl group of the adenosine at the 3'-OH position of the CCA end.
Reviewing the Options:
Option 1: "Either at the 3'-OH or 2'-OH positions of the adenosine at the CCA end"
Correct: This option is correct because, in some tRNA molecules, the amino acid can be attached to either the 3'-OH or 2'-OH positions of the adenosine in the CCA end.
In most cases, amino acids are attached to the 3'-OH position, but there are cases, such as with certain class I tRNA synthetases, where the 2'-OH position is used for attachment. This is referred to as 2'-O-aminoacylation (though it is less common than the 3'-O-aminoacylation).
For example:
Class I tRNA synthetases typically attach the amino acid to the 2'-OH group (called 2'-O-aminoacylation).
Class II tRNA synthetases usually attach the amino acid to the 3'-OH group (called 3'-O-aminoacylation).
Thus, Option 1 is correct, as both the 3'-OH and 2'-OH positions can serve as attachment sites for the amino acid.
Option 2: "Only at the 3'-OH position of the adenosine at the CCA end"
Incorrect: While 3'-OH is the most common position for amino acid attachment, there are exceptions where the amino acid is attached to the 2'-OH group. Therefore, this option is too restrictive and does not account for the full variability of aminoacylation.
Option 3: "Only at the 2'-OH position of the adenosine at the CCA end"
Incorrect: While 2'-O-aminoacylation does occur in some cases, 3'-O-aminoacylation is more common. This option is incorrect because it excludes the well-established 3'-OH attachment that occurs in most cases.
Option 4: "Only at the C1' position of the adenosine at the CCA end"
Incorrect: The C1' position is part of the sugar backbone of the adenosine and is not involved in aminoacylation. The amino acid attaches to the hydroxyl groups of the adenosine at the 2'-OH or 3'-OH positions, not the C1' position.
Conclusion:
The correct answer is Option 1, which allows for both 3'-OH and 2'-OH positions to be involved in aminoacylation.
