eukaryotic transcription 3

Why does post-transcriptional regulation exist in eukaryotes?

Because transcription occurs in the nucleus and translation in the cytoplasm, allowing extra regulation and quality control 🧪➡️🏭.

What must happen to eukaryotic RNA before it is exported?

It must be modified, processed, and quality-checked ✔️.

What is the C-terminal domain (CTD) of RNA polymerase II?

A tail region made of 52 repeats of a 7-amino-acid sequence that coordinates RNA processing 🧵.

Which amino acids in the CTD are phosphorylated?

Serine 2 and Serine 5 (Ser2 & Ser5) 🔋.

Why is CTD phosphorylation important?

It recruits capping enzymes, splicing factors, and 3′ processing machinery during elongation 🧲.

What are the five major post-transcriptional modifications of eukaryotic mRNA?

5′ capping 🎩, splicing ✂️, 3′ processing & polyadenylation 🧬, RNA editing ✏️ (rare), and export 🚪.

What is the structure of the 5′ cap?

A 7-methylguanosine (m⁷G) linked by a unique 5′–5′ triphosphate bond 🔗.

When is the 5′ cap added?

When the RNA is ~25–30 nucleotides long ⏱️.

Why is the 5′ cap essential?

It protects mRNA from degradation, enables nuclear export, allows translation, and marks RNA as mRNA 🛡️.

What enzymes are involved in 5′ capping?

RNA triphosphatase, guanylyltransferase, and methyltransferase 🧪.

Describe the mechanism of 5′ capping.

Step 1: Triphosphatase removes γ-phosphate ➖;

Picture:
PP——RNA

Step 2: Guanylyltransferase adds GMP (guanine) via 5′–5′ bond 🔗

G–PP——RNA

Step 3: Methyltransferase methylates guanine → m⁷G (7-methylguanosine)🎩.
m⁷G–PP——RNA 🎩

How is the 5′ capping process coordinated?

Capping enzymes bind the phosphorylated RNA Pol II CTD during transcription 🧲.

What binds the 5′ cap after it is added?

Cap-binding complexes (CBCs) 🧢.

What are the functions of cap-binding complexes?

Protect mRNA, promote export, enable pioneer translation, and allow nonsense-mediated decay 🔍.

What happens to improperly capped RNA?

It is recognised and destroyed as part of quality control 🚮.

How is faulty capping handled in yeast?

The Rai1–Rat1 complex removes faulty caps and degrades RNA 🧬.

How is faulty capping handled in mammals?

DXO removes defective caps and degrades the RNA 🧪.

Why can influenza virus not cap its own RNA?

It is a negative-sense ssRNA virus lacking capping enzymes ❌.

What is cap snatching?

Influenza steals capped RNA fragments from host mRNA to prime viral transcription 🦠.

What happens to the host mRNA during cap snatching?

It is decapped and degraded 📉.

Which drug inhibits influenza cap snatching?

Baloxavir marboxil 💊.

What type of genes do eukaryotes have?

Monocistronic genes (one gene → one mRNA) 🧬.

Why do eukaryotic genes contain introns?

To increase regulatory potential and allow multiple protein products 🔀.

On average, how many introns does a human gene contain?

About 8–9 introns 📊.

What are the three types of RNA splicing?

Nuclear pre-mRNA splicing ✂️, Group II self-splicing 🔁, Group I self-splicing 🔂.

Which type of splicing is most common?

Nuclear pre-mRNA splicing using the spliceosome 🧠.

What is the branch point nucleotide in nuclear splicing?

Adenine (A) 🔺.

What are Group II introns?

Rare self-splicing introns in organelles where RNA acts as a ribozyme 🧬.

What are Group I introns?

Very rare introns in some viruses with guanine (G) as branch point 🧪.

What are the conserved splice site sequences?

5′ splice site = GU, 3′ splice site = AG, branch point = A 📍.

What is the polypyrimidine tract?

A pyrimidine-rich sequence just upstream of the 3′ splice site 🧵.

Describe the mechanism of splicing.

Step 1: 2′-OH of branch point A attacks 5′ splice site → lariat 🔁;

Step 2: 3′-OH of exon 1 attacks 3′ splice site → exons ligated ✂️.

What happens to the intron after splicing?

It is released as a lariat and degraded 🚮.

What is the key takeaway about post-transcriptional regulation?

RNA processing is tightly coupled to transcription and ensures stability, accuracy, and correct gene expression 🧠.