3.2.11Unit 3: Practical Skills in Biology I
Post-Transcriptional Changes to mRNA — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 13 of 27 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Splicing of Pre-mRNA
Gene expression can be regulated after the mRNA has been made, which is called post-transcriptional modification.
1
In eukaryotes, a gene contains coding regions called exons and non-coding regions called introns.
2
During transcription, RNA polymerase copies both exons and introns to make a long molecule called pre-mRNA.
3
Before leaving the nucleus the introns are removed in a process called splicing.
4
The exons are joined together to make mature mRNA, which contains only coding sequences.
5
The mature mRNA leaves the nucleus through a nuclear pore and is translated by a ribosome in the cytoplasm.
Part 2 of 5Alternative Splicing
Different combinations
- Different combinations The exons of a pre-mRNA can be joined in more than one way to give different mature mRNA molecules.
Exon included or not
- Exon included or not A particular exon may be kept in one version of the mRNA and left out of another.
Different proteins
- Different proteins The polypeptides from different mRNAs differ in amino acid sequence, structure and function.
One gene, many proteins
- One gene, many proteins This is how a single eukaryotic gene can code for several different proteins.
Proteome
- Proteome The proteome, which is all the proteins made, is therefore much larger than the number of genes in the genome.
Different tissues
- Different tissues Different cell types can splice the same pre-mRNA in different ways to produce different proteins.
Part 3 of 5Eukaryotes and Prokaryotes
Eukaryotes and Prokaryotes
Location
- Location In eukaryotes, transcription is in the nucleus and translation is in the cytoplasm, which allows time for modification.
Introns
- Introns Exons are expressed while introns are removed, and a handy aid to memory is that EXons are EXpressed.
Pre-mRNA
- Pre-mRNA The first transcript contains introns and exons and is therefore longer than the mature mRNA.
Regulation
- Regulation Splicing adds another stage at which cells can control which proteins are made.
| Feature | Pre-mRNA | Mature mRNA |
|---|---|---|
| Contains introns | Yes | No |
| Contains exons | Yes | Yes |
| Location | Nucleus | Leaves nucleus |
Part 4 of 5Worked Example
Question
- A pre-mRNA of 3000 nucleotides contains introns totalling 1800 nucleotides.
- Assuming the mature mRNA consists only of the exons and ends with a stop codon, calculate the number of amino acids in the polypeptide.
- Mature mRNA length = 3000 − 1800 = 1200 nucleotides.
- Number of codons = 1200 ÷ 3 = 400.
- One codon is the stop codon, so amino acids = 400 − 1 = 399.
Answer The polypeptide contains 399 amino acids.
Part 5 of 5Exam Checklist & Tips
3.2.11 – Key Points for the Exam
- Intron A non-coding section of a gene, removed from pre-mRNA.
- Exon A coding section of a gene that remains in the mature mRNA.
- Pre-mRNA The initial RNA transcript containing introns and exons.
- Splicing Removal of introns and joining of exons.
- Proteome The complete set of proteins made by a cell or organism.
Common Mistakes
- Saying introns are translated, when they are removed before translation.
- Confusing pre-mRNA, which contains introns, with mRNA, which does not.
- Stating that splicing happens in the cytoplasm, when it occurs in the nucleus.
- Writing that alternative splicing changes the DNA sequence, when it changes only how the mRNA is processed.
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- RNA: 5′ → 3′ 3′ → 5′.
- : AUG UAC .
- : UAA, UAG, UGA tRNA (Release Factor) .