Post-Transcriptional and Post-Translational Regulation

Transcription is only the beginning. A gene can be transcribed perfectly well and still never produce a functional protein if the mRNA is not correctly processed, stable, translated efficiently and the protein modified correctly. Eukaryotes in particular have many points of control after transcription, and these let the cell fine-tune protein levels in response to signals without having to re-start transcription every time. OCR A-Level Biology A specification module 6.1.1(b)(iii) requires you to understand post-transcriptional regulation (RNA splicing) and post-translational regulation (protein modification, activation by phosphorylation).

Key Definitions:

• Intron — a non-coding sequence within a gene that is transcribed but removed from the pre-mRNA before translation.
• Exon — a coding sequence that is retained in the mature mRNA.
• RNA splicing — the removal of introns and joining of exons to form mature mRNA.
• Spliceosome — a large ribonucleoprotein complex that carries out splicing.
• Alternative splicing — combining exons in different ways to make different proteins from one gene.
• Post-translational modification (PTM) — a chemical change made to a protein after it has been synthesised.
• Phosphorylation — the addition of a phosphate group, usually to serine, threonine or tyrosine residues.

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The Pre-mRNA is not the Finished Product

In eukaryotes, genes are rarely a continuous coding sequence. Most contain introns (non-coding) interrupting exons (coding). The initial RNA transcript is called pre-mRNA and contains both. Three major processing steps turn pre-mRNA into mature mRNA ready for translation:

1. 5' capping — addition of a modified guanine (7-methylguanosine) cap to the 5' end. Protects the mRNA from 5' exonucleases and serves as a landing pad for ribosomes.
2. 3' polyadenylation — addition of a tail of about 100–250 adenine residues to the 3' end. Stabilises the mRNA and helps export from the nucleus.
3. Splicing — removal of introns and ligation of exons.

flowchart TB
    DNA[Gene: exon1 intron1 exon2 intron2 exon3] --> TX[Transcription]
    TX --> PRE[Pre-mRNA with introns and exons]
    PRE --> CAP[5 cap added]
    CAP --> TAIL[3 poly-A tail added]
    TAIL --> SPL[Spliceosome removes introns]
    SPL --> M[Mature mRNA: exon1 exon2 exon3]
    M --> EX[Exported to cytoplasm for translation]

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RNA Splicing

Splicing is carried out by the spliceosome, an enormous molecular machine made of several small nuclear ribonucleoproteins (snRNPs) plus dozens of other proteins. Each intron starts with GU and ends with AG (the "GU-AG rule"); the spliceosome recognises these sequences, loops the intron out, cuts it at both ends and ligates the flanking exons together. The excised intron is released in a lariat shape and degraded.

Why bother with introns at all?

It might seem wasteful to transcribe sequences only to throw them away, but introns allow two extremely useful things:

• Alternative splicing, producing several different proteins from one gene.
• Exon shuffling during evolution — new genes can be built by mixing and matching existing exon-encoded protein domains.

Alternative Splicing

Different exon combinations can be selected from the same pre-mRNA to produce different proteins. A famous example is the gene for troponin T in muscles: depending on which exons are retained, the cell makes a version suited to either fast-twitch or slow-twitch muscle. Humans have roughly 20,000 protein-coding genes but produce well over 100,000 distinct proteins — the extra diversity comes largely from alternative splicing.

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Other Post-Transcriptional Mechanisms

• mRNA stability — regulatory proteins and small non-coding RNAs (microRNAs) can bind the 3' untranslated region (UTR) of an mRNA and mark it for rapid degradation.
• RNA interference — small interfering RNAs (siRNAs) pair with complementary mRNAs and trigger their destruction by the RISC complex. This is a natural defence against viruses and is also used as a laboratory tool to "knock down" specific genes.
• Regulation of translation initiation — eIF factors can be phosphorylated to block ribosome binding under stress, giving an almost instantaneous global decrease in protein synthesis.

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