1. Which RNA carries amino acids to a ribosome?
- tRNA
- DNA
- Cellulose
Check the answer and reasoning
tRNA
A tRNA pairs its anticodon with a codon and brings the corresponding amino acid.
Biology / AS level
Nucleic acids and protein synthesis
DNA stores sequence information in nucleotide bases.
Learning objectives
- Describe DNA replication and complementary pairing.
- Distinguish transcription from translation.
- Interpret the triplet genetic code.
The big idea
DNA stores sequence information in nucleotide bases. Complementary pairing supports semi-conservative replication, in which each new DNA molecule contains one old strand and one newly synthesised strand.
Transcription produces RNA from a DNA template. During translation, ribosomes read mRNA codons and tRNAs deliver amino acids. The code is degenerate: different codons can specify the same amino acid, so some base substitutions do not change a protein’s amino-acid sequence.
Worked example
An mRNA coding region has 90 bases specifying amino acids, with no stop codon included. How many amino acids does it encode?
Follow the solution
Three bases form one codon: 90/3=30 amino-acid codons under the stated assumption.
Investigate and explain
Using the worked example as supplied data, predict the effect of changing one condition. Draw a labelled model, record your reasoning and identify a variable that would need controlling. No human, animal or microbial experiment is required.
Related simplified PhET model. Explain what it represents and what the living system does differently.
Check your understanding
2. An mRNA coding region has 90 bases specifying amino acids, with no stop codon included. How many amino acids does it encode?
- 90
- 30
- 270
Check the answer and reasoning
30
Three bases form one codon: 90/3=30 amino-acid codons under the stated assumption.
3. Does every single-base substitution change the encoded amino acid?
- Yes
- Only in a long gene
- No
Check the answer and reasoning
No
Degeneracy means more than one codon may specify the same amino acid; effects depend on position and context.