Explain transcription, translation and DNA strand roles
RNA polymerase transcribes a DNA template into mRNA; ribosomes read mRNA codons as tRNA anticodons deliver amino acids.
Section 6.2 · Protein synthesis
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain DNA, RNA and protein synthesis. Written practice is self-checked; it does not replace teacher review or practical assessment.
Before you begin
- Recognise that DNA stores genetic information and that proteins are made from amino acids.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which enzyme builds an RNA strand during transcription?
What you will learn
- Describe the roles of RNA polymerase, mRNA, codons, tRNA, anticodons and ribosomes
- Distinguish the transcribed/template strand from the non-transcribed strand
- Outline transcription and translation in sequence
Transcription makes an RNA copy from a template
During transcription, RNA polymerase uses the transcribed, or template, DNA strand to build a complementary RNA molecule. The other DNA strand is the non-transcribed strand. RNA polymerase joins RNA nucleotides using complementary pairing; RNA contains U instead of T.
The mRNA sequence is complementary to the template strand and has the same base sequence as the non-transcribed strand except that U replaces T. Keep strand labels and 5′/3′ directions aligned when comparing sequences.
Translation reads mRNA at a ribosome
During translation, a ribosome binds mRNA and reads codons. A tRNA carries a specific amino acid and has an anticodon complementary to an mRNA codon. Matching codon–anticodon pairs position amino acids in the order specified by the mRNA.
The ribosome forms peptide bonds as the polypeptide grows and moves along the mRNA. Translation starts at a start codon and ends at a stop codon. The DNA template is used in transcription; tRNA anticodons pair with mRNA codons in translation.
Pause and explain
Where does a tRNA anticodon pair during translation?
Worked example
The template strand is 3′-T A C-5′. Write the complementary mRNA codon and state which molecule carries the anticodon.
Show the worked solution
- Pair DNA T with RNA A, A with U, and C with G.
- tRNA carries the complementary anticodon.
Answer The mRNA codon is 5′-A U G-3′. A tRNA molecule carries an anticodon complementary to this mRNA codon.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Distinguish the transcribed/template DNA strand from the non-transcribed strand.
Give me a hint
State which is read by RNA polymerase.
Compare my reasoning
- RNA polymerase uses the template strand.
- The other DNA strand is non-transcribed.
The transcribed or template strand is the DNA strand used by RNA polymerase to make RNA. The other strand is called the non-transcribed strand.
Look for these in your work
- Template role correct
- Other strand named
Outline how tRNA contributes to translation.
Give me a hint
Include anticodon, amino acid and the mRNA codon.
Compare my reasoning
- tRNA carries an amino acid.
- Its anticodon pairs with a complementary mRNA codon at the ribosome.
A tRNA carries a specific amino acid to a ribosome. Its anticodon pairs with a complementary codon on mRNA, positioning the amino acid in the polypeptide sequence.
Look for these in your work
- Amino acid transport stated
- Codon–anticodon pairing stated
A DNA template triplet is 3′-C A T-5′. Write the mRNA codon and explain how you determined it.
Give me a hint
Use RNA complementary pairing and write the RNA 5′ to 3′.
Compare my reasoning
- C pairs with G, A with U and T with A.
- The complementary mRNA is 5′-G U A-3′.
The mRNA codon is 5′-G U A-3′. It is complementary to the template, with U used in RNA instead of T, and is written antiparallel to the 3′-to-5′ template.
Look for these in your work
- Complementary bases correct
- RNA and strand direction correct
Common mistakes
- Calling the non-transcribed strand the template
- Putting T in an RNA sequence
- Saying tRNA anticodons pair directly with DNA
What can you explain now?
Which enzyme builds an RNA strand during transcription?
Compare with the explanation
RNA polymerase
RNA polymerase joins RNA nucleotides.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How does information move from a DNA template through mRNA and tRNA to a polypeptide?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.