Eukaryotic transcription 4

What is the purpose of post-transcriptional modification?

To convert hnRNA (pre-mRNA) into functional mRNA and increase accuracy, regulation, and protein diversity 🧬.

What is hnRNA?

Heterogeneous nuclear RNA — the newly transcribed pre-mRNA before processing 🧵.

What are the major RNA rearrangements after transcription?

Splicing ✂️, self-splicing 🔁, alternative splicing 🔀, and RNA editing ✏️.

What is the spliceosome?

A large RNA–protein complex that removes introns and joins exons 🏗️.

What RNAs make up the spliceosome?

U1, U2, U4, U5, and U6 snRNAs 🧩.

What is an snRNP?

A complex of one snRNA plus proteins (small nuclear ribonucleoprotein) 🔗.

How many proteins are found in the spliceosome?

About 150 proteins 🧠.

Does spliceosomal splicing require energy?

Yes — it requires ATP ⚡.

When does spliceosome assembly occur?

Co-transcriptionally, as RNA is being made ⏱️.

Why is co-transcriptional splicing important?

It links transcription to RNA processing for speed and regulation 🔄.

What happens in the commitment (early) complex?

U1 binds the 5′ splice site,

BBP binds the branch point,

and U2AF binds the polypyrimidine tract and 3′ splice site 📍.

What is BBP (SF1)?

Branch point binding protein that recognises the branch point A 🔺.

What is the role of U2AF?

U2AF65 binds the polypyrimidine tract and U2AF35 binds the 3′ splice site 🧲.

What are SR proteins?

Serine-arginine rich proteins that enhance splicing efficiency 🚀.

What happens during pre-spliceosome (A complex) formation?

U2 snRNP binds the branch site, BBP is displaced, and the branch point A bulges out 🪢.

Why is the bulged branch point A important?

It is essential for the first cleavage reaction ✂️.

What happens when the spliceosome is fully assembled?

U4, U5, and U6 bind, splice sites are aligned, and catalysis begins ⚙️.

What are the catalytic steps of spliceosomal splicing?

Two cleavage reactions remove introns and ligate exons ✂️➕.

What is self-splicing?

A rare process where introns act as ribozymes and remove themselves 🔁.

Do self-splicing introns require proteins?

No — RNA itself acts as the catalyst 🧪.

Why is self-splicing evolutionarily important?

It suggests RNA once acted as both genetic material and enzyme 🧠.

What is alternative splicing?

A process where one gene produces multiple mRNA transcripts and protein isoforms 🔀.

Why is alternative splicing important in humans?

It explains how ~25,000 genes produce huge protein diversity 📊.

What classic gene demonstrates alternative splicing?

The calcitonin / CGRP gene 🧬.

How is the calcitonin/CGRP gene spliced in thyroid cells?

Exons 1–4 are used to produce calcitonin 🦋.

How is the calcitonin/CGRP gene spliced in brain cells?

Exons 1–3 and 5–6 are used to produce CGRP 🧠.

What is the key message of alternative splicing?

One gene can produce multiple proteins with different functions 🎯.

What is RNA editing?

A process that changes RNA sequence after transcription without altering DNA ✏️.

What is the classic example of RNA editing?

Apolipoprotein-B (ApoB) 🧬.

What RNA editing occurs in ApoB?

CAA (Gln) is changed to UAA (STOP codon) ⛔.

What are the tissue-specific outcomes of ApoB editing?

Liver makes full-length protein; intestine makes truncated protein 🍔.

Why is ApoB editing not considered an error?

It is a controlled and regulated process ✔️.

What are the two main types of RNA editing?

Substitution editing 🔄 and insertion/deletion editing ➕➖.

What substitutions occur in RNA editing?

C → U (cytidine deaminase) and A → I (ADAR enzymes) ✏️.

How does the ribosome read inosine (I)?

As guanine (G) 🔁.

Where is RNA editing especially important?

In the nervous system 🧠.

What receptors are regulated by RNA editing?

Glutamate receptors 🧲.

What enzyme edits glutamate receptor RNA?

ADAR (adenosine deaminase acting on RNA) 🧪.

What is the functional effect of glutamate receptor editing?

Alters Ca²⁺ permeability of ion channels ⚡.

What happens in ADAR knockout mice?

Epilepsy and early death ⚠️.

What is the overall importance of post-transcriptional control?

It adds multiple regulatory layers and fine-tunes gene expression 🎛️.