RNA: types and functions
Three main RNAs work together: mRNA carries the code, tRNA brings amino acids, and rRNA builds the ribosome, where amino acids are joined into protein.
Role in protein synthesis
| RNA | Structure | Role in protein synthesis |
|---|---|---|
| mRNA (messenger RNA) | Single linear strand. In eukaryotes it has a 5′ cap (7-methylguanosine) and a 3′ poly-A tail | Carries the genetic code from DNA in the nucleus to the ribosome as codons (three-base 'words'). Short-lived |
| tRNA (transfer RNA) | Small; cloverleaf shape in 2D, L-shape in 3D; amino acid binds at the 3′ CCA end; has an anticodon loop | Brings the correct amino acid. Its anticodon pairs with the codon on mRNA (the 'adapter' molecule) |
| rRNA (ribosomal RNA) | Joins with proteins to form ribosomes: 80S (60S + 40S) in eukaryotes, 70S (50S + 30S) in bacteria | Forms the ribosome, the site of protein synthesis. rRNA of the large subunit forms peptide bonds (peptidyl transferase), so it acts as a ribozyme |
tRNA cloverleaf: the arms
- Acceptor arm: ends in CCA at the 3′ end; the amino acid attaches here.
- Anticodon arm: carries the three-base anticodon that pairs with the mRNA codon.
- D arm (DHU arm): contains dihydrouridine; helps the enzyme that loads the amino acid (aminoacyl-tRNA synthetase) recognise the tRNA.
- TΨC arm: contains thymine, pseudouridine (Ψ) and cytosine; helps the tRNA bind to the ribosome.
- Variable (extra) arm: its length differs from one tRNA to another.
Other RNAs (in brief)
| RNA | Role |
|---|---|
| hnRNA (heterogeneous nuclear RNA) | The first copy of a gene (primary transcript); processed into mRNA |
| snRNA (small nuclear RNA) | With proteins forms the spliceosome, which cuts introns out of hnRNA (splicing) |
| miRNA (micro RNA) | Very small (about 22 nucleotides); blocks or breaks down target mRNAs, switching genes down |
Protein synthesis needs every essential amino acid at the same time. If one is short, such as lysine in cereals, tRNA cannot deliver it and the protein cannot be completed. This is why cereal–pulse combinations like idli, dosa and rice with dal improve protein quality.
2 marks: “What is an anticodon?” A three-base sequence on the anticodon loop of tRNA that pairs with the matching codon on mRNA.
5 marks: “Draw and explain the cloverleaf structure of tRNA.” Draw the cloverleaf with acceptor (CCA), D, anticodon, variable and TΨC arms, and give one line on each arm's job.
10 marks: “Describe the types of RNA and the role of each in protein synthesis.” Tabulate mRNA, tRNA and rRNA (structure and role), draw the tRNA cloverleaf, explain codon–anticodon pairing, add snRNA and miRNA.
mRNA = message, tRNA = transfer (the carrier), rRNA = ribosome (the factory).