For protein synthesis, the amino acids are recognized and carried by:

Correct — D, tRNA. Transfer RNA is the adaptor that lets a language of nucleotides specify a language of amino acids. Each tRNA is a small folded molecule, around seventy-six nucleotides long, with two working ends. At one end sits the anticodon, a triplet that base-pairs with a matching codon on the messenger RNA held in the ribosome. At the other end, the CCA tail, the amino acid is attached. The matching of the right amino acid to the right tRNA is done beforehand by an aminoacyl-tRNA synthetase, one for each amino acid, and that enzyme is where the genetic code is physically enforced. So the tRNA recognises its amino acid through its synthetase, carries it to the ribosome, and hands it over only where its anticodon fits — recognition and transport in one molecule, which is exactly what the question describes.

• (a)mRNA — Messenger RNA carries the instruction, not the cargo. It is the transcript of a gene, read three bases at a time, and it never binds an amino acid.
• (b)snRNA — Small nuclear RNA works before translation begins. Combined with proteins into snRNPs it forms the spliceosome, which cuts introns out of the primary transcript inside the nucleus.
• (c)miRNA — MicroRNA is a regulator. Around twenty-two nucleotides long, it pairs with target messenger RNAs and either blocks their translation or triggers their degradation, so it reduces protein output rather than supplying its raw material.

Protein synthesis runs in two stages. Transcription copies a gene into messenger RNA in the nucleus; translation reads that message on a ribosome in the cytoplasm and builds the polypeptide. Four classes of RNA appear in the process. Messenger RNA carries the code, ribosomal RNA forms the catalytic core of the ribosome itself, transfer RNA delivers amino acids, and a family of small RNAs — snRNA, snoRNA, miRNA, siRNA — handles processing and regulation. The genetic code has sixty-four codons for twenty amino acids, so most amino acids answer to more than one codon.

Option lists in this topic are built from the RNA alphabet soup, and the reliable way through it is to hold one function per name rather than to guess from the letters. A useful check is the direction of travel: mRNA moves information out of the nucleus, tRNA moves amino acids into the ribosome, rRNA does not move at all. Note also that the ribosome's peptide-bond-forming site is made of RNA, not protein, which is why the ribosome is called a ribozyme — a detail that shows up as a stand-alone question often enough to be worth carrying.

• A transfer RNA is about seventy-six nucleotides long and folds into a cloverleaf that stacks into an L shape.
• The anticodon triplet pairs with the messenger RNA codon; the amino acid is carried on the 3' CCA end.
• Aminoacyl-tRNA synthetases charge each tRNA with its own amino acid, and there is one such enzyme per amino acid.
• Sixty-four codons cover twenty amino acids plus a start and three stop signals, so the code is degenerate but unambiguous.
• The peptide bond is formed by ribosomal RNA, which makes the ribosome a ribozyme.

• Treating mRNA as the carrier of amino acids because it is the molecule the question mentions most often elsewhere.
• Confusing the anticodon on the tRNA with the codon on the mRNA when a question asks which sits where.
• Assuming every RNA in the list is involved in translation; snRNA and miRNA act before and around it.

Asked as a one-line functional identification, with the four RNA classes offered as options and the whole mark resting on knowing which one is the adaptor.

• practice — not a real PYQ

  The enzyme that attaches an amino acid to its corresponding transfer RNA is

  • (a)RNA polymerase
  • (b)aminoacyl-tRNA synthetase
  • (c)reverse transcriptase
  • (d)DNA ligase

  Answer(b) aminoacyl-tRNA synthetase — there is one for each amino acid, and its accuracy is what keeps translation faithful.

• practice — not a real PYQ

  Which one of the following RNAs forms the catalytic core of the ribosome?

  • (a)mRNA
  • (b)tRNA
  • (c)rRNA
  • (d)miRNA

  Answer(c) rRNA — ribosomal RNA catalyses peptide-bond formation, which is why the ribosome is described as a ribozyme.