Lesson 2 of 6 / DNA and gene expression
Three RNAs, three related jobs
Which RNA carries, matches and helps assemble?
I can describe RNA structure and the roles of mRNA, tRNA and rRNA.
I can describe DNA and RNA structure and the roles of mRNA, tRNA and rRNA.Syllabus
Syllabus 8876, Core 2(a). Describe DNA and RNA structure and the roles of mRNA, tRNA and rRNA.
About 5 min
Make a guess. You are not marked.
Is all RNA a straight, unfolded strand that carries a message?
Show the answer
tRNA and rRNA fold into working shapes.
tRNA folds to hold an anticodon and an amino acid. rRNA folds into ribosomes and helps catalyse bonds.
The key idea
RNA usually has one nucleotide strand with ribose and uracil. mRNA carries codons, tRNA matches codons to amino acids, and rRNA forms a functional part of ribosomes.
Explore the idea
Match the RNA to its job
The message carries codons. A ribosome reads them in the 5'-to-3' direction.
Original organisation models. RNA uses ribose and uracil and can fold through internal base pairing. Subunit shapes are illustrative.
Explanation
An RNA nucleotide contains ribose, phosphate and a base: A, U, G or C. RNA usually consists of one strand, although complementary regions of that strand can pair and fold. It differs from DNA in sugar, use of uracil instead of thymine, and usual strand organisation.
Messenger RNA carries a sequence of codons to a ribosome. A codon is a three-base sequence read in the 5'-to-3' direction. Its order directs the amino-acid sequence of a polypeptide.
Transfer RNA is a folded molecule with an anticodon and an amino-acid attachment site at different regions. Appropriate loading links the correct amino acid to a tRNA. Its anticodon pairs antiparallel with a complementary mRNA codon, acting as an adaptor rather than a template for the whole protein.
Ribosomal RNA associates with proteins to form ribosomes. It contributes to their structure and catalyses peptide-bond formation. Thus RNA can participate in both information transfer and catalysis; rRNA is not simply a disposable copy of a gene.
Step by step
- 1
Locate the message
mRNA codon order is read by the ribosome.
- 2
Match the adaptor
The tRNA anticodon pairs with a codon and delivers its attached amino acid.
- 3
Locate assembly
Ribosomal components, including catalytic rRNA, support peptide-bond formation.
Worked example
Matching AUG
An mRNA codon is 5'-AUG-3'. Give the aligned anticodon and the RNA that carries it.
One way to explain it
The aligned anticodon is 3'-UAC-5' on a tRNA. The tRNA carries the appropriate amino acid; it is not the mRNA itself.
Why this answer works
- Use RNA pairing rules A-U and G-C.
- Preserve antiparallel alignment.
- Distinguish codon from anticodon.
Watch out for this
A student says: "All RNA is an unfolded strand that only carries messages." What is wrong with this?
Show the answer
tRNA and rRNA fold into functional structures, and rRNA has a catalytic role in the ribosome.
Check your understanding
Which pairing is correct?
Show the answer
tRNA: anticodon and amino-acid attachment site.
These features let it act as an adaptor.
Quick recall
Which RNA carries, matches and helps assemble?