Question

Processing of transfer RNA (tRNA) A. involves cleaving out several different tRNAs from one primary transcript. B. involves only exonucleases. C. cleaves excess bases from the \(3^{\prime}\) -end after the sequence CCA. D. cleaves excess bases from either end of the primary transcript but not in the interior of the molecule. E. necessitates modification of some of the bases by methylation.

Short answer

Choose all that apply. A. It involves cleavage of the tRNA precursor into several distinct tRNA molecules. B. It only employs exonucleases. C. A CCA sequence is added to the 3' end of the molecule followed by cleavage of excess bases. D. Cleavage can only occur at the ends of the molecule. E. Certain bases in tRNA are chemically modified, such as methylation, following transcription. Answer: A, E

Step-by-step solution

Understand the various aspects of tRNA processing

Review the information about tRNA processing. The process starts with the transcription of a precursor molecule which is then processed into a mature tRNA molecule through several steps like cleavage of precursor sequences, addition of CCA sequence at 3' end, and modification of certain bases.

Analyzing the options

Carefully read and understand the statements given in each option. Option A: tRNA processing involves the cleavage of the tRNA precursor into several distinct tRNA molecules, a fact well-known in molecular biology. Option B: tRNA processing employs two main categories of enzymes; endonucleases, which cleave within the molecule and exonucleases which cleave at the ends. Hence, this statement is not entirely accurate as it doesn't involve only exonucleases. Option C: During tRNA processing, a CCA sequence is indeed added to the 3' end of the molecule. However, this is not typically followed by the cleavage of excess bases. Instead, the addition of CCA happens after cleavage. Option D: Inaccurate, as during tRNA processing, cleavage can occur not only at the ends but also within the interior of the molecule to remove the introns. Option E: This is correct. Certain bases in tRNA are indeed chemically modified following transcription, including methylation.

Select the correct options

Based on the understanding of tRNA processing, it is clear that Options A and E are the correct answers to this exercise. tRNA processing indeed involves cleaving out several different tRNAs from one primary transcript (Option A) and necessitates modification of some of the bases by methylation (Option E).

Key concepts

tRNA Cleavage

Transfer RNA (tRNA) cleavage is a pivotal step in the maturation of tRNA molecules. As part of tRNA processing, a primary transcript known as pre-tRNA is subject to precise cutting to form individual tRNA molecules. This process is crucial as it removes unneeded sequences and helps in shaping the tRNA to its functional form. The cleavage is not limited to the ends of the transcript; it also takes place in the interior, to excise introns, which are non-coding sequences. Both exonucleases and endonucleases partake in this activity, with endonucleases making internal cuts and exonucleases trimming the ends.

Understanding this concept is essential to comprehend how tRNAs become functional. These mature tRNAs are ultimately necessary for protein synthesis, where their role is to bring specific amino acids to the ribosome, according to the genetic code.

tRNA Modification

Once tRNA molecules are cleaved into their characteristic cloverleaf structure, they undergo further modifications to become fully functional. tRNA modification includes the alteration of various bases and the sugar-phosphate backbone. These modifications, which can involve the addition of methyl groups, changing the chemical structure of bases, or adding complex groups, enhance the stability and functionality of tRNA molecules.

Chemical modifications are critical for the correct folding of tRNA, the accuracy of amino acid attachment, and the fidelity of protein synthesis. These changes are so significant that they can affect how well tRNA molecules recognize codons on mRNA during translation, which directly influences the synthesis of proteins.

RNA Methylation

RNA methylation is a form of tRNA modification where methyl groups are transferred to specific nitrogen or oxygen atoms on the RNA bases. This alteration is one of the most common modifications in tRNA processing and plays a fundamental role in the structure and function of the mature tRNA. Methylation can improve the stability of the tRNA, protect it from enzymatic degradation, and assist in the proper decoding of messenger RNA (mRNA) during protein synthesis.

One notable example of RNA methylation is the modification of adenosine to N1-methyladenosine. This type of methylation can affect the three-dimensional structure of tRNA and thus its interaction with ribosomes and other factors in the cell. Methylation modifications are essential for accurate translation of the genetic code and are often seen as markers of sophisticated cellular control over gene expression.

CCA Sequence Addition

The CCA sequence addition is the final step in tRNA processing. The sequence - Cytosine-Cytosine-Adenine (CCA) - is added post-transcriptionally at the 3' end of the tRNA molecule. This CCA tail is critical as it forms the active site where amino acids are attached to the tRNA. This attachment is necessary for the tRNA to carry the amino acid to the ribosome for incorporation into a growing protein chain.

The CCA sequence is not encoded by the DNA but is added enzymatically by an enzyme called tRNA nucleotidyltransferase. The presence of the CCA sequence is so crucial that if it is removed or damaged, the tRNA molecule cannot participate in protein synthesis. This highlights the CCA addition as a key regulatory point in the life cycle of tRNA.