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6.2.7Topic 6 · Immunity, Infection and Forensics

Post-transcriptional Modification — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

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📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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6.2.7 Post-transcriptional Modification — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe how pre-mRNA is processed to mature mRNA, including splicing, capping and tailing, and explain how alternative splicing allows one gene to code for several proteins.In eukaryotes the g…

  • poly-A tail
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3
  • Spliceosome
  • Unlike prokaryote mRNA, this molecule cannot be used at once

Section 1 of 9From gene to protein in eukaryotes

In eukaryotes the genetic code is transcribed in the nucleus by RNA polymerase to make a primary transcript, called pre-mRNA.

1
Unlike prokaryote mRNA, this molecule cannot be used at once.
2Exons
Eukaryote genes are made of coding regions called exons interrupted by non-coding regions called introns.
3
Pre-mRNA contains both, and it is modified in the nucleus before export to the cytoplasm for translation.
4Post-transcriptional modification
These changes are called post-transcriptional modification.

Section 2 of 9Splicing

The introns are removed and the exons are joined together in a process called splicing.

Splicing
It is carried out by a complex of RNA and proteins called the spliceosome,
  • which recognises specific base sequences at the intron boundaries, cuts out the intron and joins the neighbouring exons.
The result is a shorter mature mRNA that contains only the coding sequence.
If the introns were not removed the ribosome would translate them and produce a faulty protein.

Section 3 of 9Capping and tailing

Two other modifications protect the mRNA and help its use.

5′ cap
A modified guanine nucleotide, the 5′ cap, is added to the start of the mRNA.
  • It protects the molecule from degradation by enzymes and helps the ribosome to bind.
Poly-A tail
A chain of adenine nucleotides, the poly-A tail, is added to the 3′ end.
  • It protects the mRNA from exonucleases and helps export from the nucleus.
  • The length of the tail influences how long the mRNA survives in the cytoplasm.

The mature mRNA then passes through a nuclear pore into the cytoplasm, where it is translated by ribosomes. After translation, the polypeptide may be modified again, for example by folding, cutting or addition of carbohydrate, which is called post-translational modification.

Section 4 of 9Alternative splicing and protein diversity

In alternative splicing, different combinations of exons from the same pre-mRNA are joined.

1
One gene can therefore produce several different mature mRNAs, and so several different proteins (isoforms) with different functions.
2
This explains how humans, with about 20 000 protein-coding genes, can make many more than 20 000 different proteins.
3
The splicing pattern may differ between tissues, so the same gene gives different products in different cell types.
4
Errors in splicing, caused for example by mutations in the intron boundary, can lead to disease.
FeatureProkaryotesEukaryotes
Introns in genesRareCommon
Transcription and translationOccur together in cytoplasmSeparate, in nucleus and cytoplasm
mRNA processingNot neededSplicing, cap and tail

Section 5 of 9Link to immunity

Alternative splicing helps the immune system.

Link to immunity
A B cell can produce both a membrane-bound form of an antibody (a receptor) and a secreted form from the same gene by splicing the final exons differently; the secreted form lacks the part that anchors it in the membrane.
Wider diversity of antibodies and T cell receptors also comes from the rearrangement of gene segments during lymphocyte development,
  • so that each cell has a unique specificity.
Questions on this topic often require applying knowledge of splicing to a novel gene and predicting the number of different mRNA molecules.

Section 6 of 9Gene expression and regulation in context

Post-transcriptional modification is one of several levels at which gene expression is controlled.

Gene expression and regulation in context
Transcription can be switched on or off by transcription factors that bind to promoters; the stability of mRNA can be adjusted by the poly-A tail length and by small interfering RNAs that bind to complementary mRNA and cause its breakdown;
  • and translation can be regulated at the ribosome.
Together these processes explain why cells of different types, which contain the same genes, have different proteins.

You should be able to follow a worked example: pre-mRNA with exons 1 to 4 and three introns is spliced to give mRNA with exons 1-2-3-4, but in another tissue exon 3 is omitted, giving 1-2-4. The second protein would be shorter by the number of amino acids that exon 3 coded, and its function may be changed, for example if it lacked a domain required for binding.

Gene expression and regulation in context
A mutation at an intron-exon boundary may cause an exon to be skipped or an intron to be kept, producing a non-functional protein.
Always connect the change in mRNA sequence to the change in amino acid sequence and then to function.

Section 7 of 9Exam-style questions

  1. Describe the post-transcriptional modification of pre-mRNA in a eukaryotic cell. [4]4 marks

  2. A gene has five exons. Explain how alternative splicing allows more than one protein to be made from this gene. [3]3 marks

  3. A pre-mRNA of 3000 nucleotides contains introns that make up 60% of its length. Calculate the length of the mature mRNA coding region, ignoring the cap and tail, and the maximum number of amino acids coded. [3]3 marks

Section 8 of 9Key terms

TermMeaning
Pre-mRNAThe primary transcript copied from DNA before processing.
ExonA part of a gene that is retained in mature mRNA.
IntronA non-coding part of a gene removed from pre-mRNA.
SplicingRemoval of introns and joining of exons.
5′ capA modified nucleotide added to the start of mRNA.
Poly-A tailA chain of adenine nucleotides added to the end of mRNA.
Alternative splicingJoining exons in different combinations to give different mRNAs.
SpliceosomeA complex of RNA and protein that carries out splicing.

Section 9 of 9Exam technique

1
Saying introns are coding. Exons are retained and usually code; introns are removed.
2
Thinking that splicing happens in the cytoplasm. It occurs in the nucleus.
3
Confusing post-transcriptional with post-translational modification.
4
Saying prokaryotes show the same splicing. It is mainly a eukaryotic feature.

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