Process eukaryotic primary transcripts
In eukaryotes, introns are removed from a primary transcript and exons are joined to produce mature mRNA.
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 sequences are removed from a eukaryotic primary transcript?
What you will learn
- Describe removal of introns from a eukaryotic primary transcript
- Describe joining of exons to form mRNA
Transcription produces a primary transcript
In eukaryotes, the RNA molecule first produced by transcription is a primary transcript. It contains sequences that will not remain in the mature mRNA as well as sequences that will be retained.
Non-coding sequences called introns are removed. Coding sequences called exons are joined together to form mRNA. This processing occurs before the mature mRNA is used for translation.
Keep the sequence of events clear
The syllabus model is: transcription produces a primary transcript; introns are removed; exons are joined; mature mRNA is available for translation.
Do not say introns are removed from DNA. The RNA transcript is processed; the gene remains in the DNA. Avoid adding complex processing steps when the question asks only about introns and exons.
Pause and explain
What happens to exons during this processing?
Worked example
A primary transcript contains exon 1, intron 1, exon 2 and intron 2. Describe how it becomes mature mRNA.
Show the worked solution
- Remove both introns from the transcript.
- Join exon 1 and exon 2.
Answer Introns 1 and 2 are removed from the primary transcript, and exons 1 and 2 are joined to form mature mRNA.
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Test this topic →From guided practice to a new situation
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.
Outline how a eukaryotic primary transcript is modified to form mRNA.
Give me a hint
Include the two named sequence types and both actions.
Compare my reasoning
- Introns are removed.
- Exons are joined together.
Introns are removed from the primary transcript, and the remaining exons are joined together to form mature mRNA.
Look for these in your work
- Introns removed
- Exons joined
A student says “RNA processing deletes the introns from the gene.” Correct this description.
Give me a hint
Identify which molecule is processed.
Compare my reasoning
- The primary RNA transcript is processed.
- The DNA gene is not cut during this step.
Introns are removed from the eukaryotic primary RNA transcript, not from the DNA gene. The exons are joined to form mRNA.
Look for these in your work
- RNA transcript identified
- DNA distinction made
A transcript is shown as E1–I1–E2–I2–E3. Write the order of the retained regions in mature mRNA.
Give me a hint
Remove I1 and I2, then join the exons in their original order.
Compare my reasoning
- The introns are removed.
- The exons remain in order.
The mature mRNA contains E1–E2–E3, with introns I1 and I2 removed and the exons joined in their original order.
Look for these in your work
- Introns omitted
- Exon order preserved
Common mistakes
- Removing introns from DNA rather than RNA
- Joining introns and removing exons
- Placing translation before the RNA processing described
What can you explain now?
Which sequences are removed from a eukaryotic primary transcript?
Compare with the explanation
Introns
Introns are removed during RNA processing.
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Make it stick
What is removed from a eukaryotic primary transcript, what is retained, and what molecule is ready for translation?
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.