Question

RECALL List several ways in which RNA is processed after transcription.

Short answer

The RNA is processed by adding a 5' cap, a poly-A tail, splicing out introns, RNA editing, and exporting from the nucleus.

Step-by-step solution

- Understand the Concept of RNA Processing

RNA processing is a series of steps that a primary RNA transcript undergoes to become a mature RNA molecule that can function in protein synthesis or other roles in the cell.

- Addition of 5' Cap

A modified guanine nucleotide is added to the 5' end of the RNA transcript. This cap protects the RNA from degradation and helps in ribosome attachment during translation.

- Addition of Poly-A Tail

A sequence of adenine nucleotides, known as the poly-A tail, is added to the 3' end of the RNA transcript. This tail also protects the RNA from degradation and assists in the export of the RNA from the nucleus.

- Splicing

Introns (non-coding regions) are removed from the RNA transcript, and exons (coding regions) are joined together. This process is carried out by the spliceosome complex.

- RNA Editing

In some cases, the nucleotide sequence of the RNA can be altered through processes like deamination, which changes one base to another.

- RNA Export

The mature mRNA is transported out of the nucleus through the nuclear pore complex to the cytoplasm where it can be translated into protein.

Key concepts

5' Cap Addition

In the initial stage of RNA processing, a special structure known as the 5' cap is added to the start of the RNA molecule. This is a modified guanine nucleotide.
This 5' cap serves several important functions:

• It protects the RNA from being degraded by enzymes that would otherwise shorten it.
• It helps the RNA molecule attach to ribosomes, which are the cellular machines that create proteins.

Without the 5' cap, the RNA could be destroyed before it gets a chance to produce proteins, making this step crucial in RNA maturation.

Poly-A Tail Addition

At the opposite end of the RNA molecule, a sequence of adenine nucleotides called the poly-A tail is added to the 3' end. This process is known as polyadenylation.
Just like the 5' cap, the poly-A tail also serves to protect the RNA from degradation. Additionally, it helps in:

• Transporting the RNA out of the nucleus to the cytoplasm, where it can be used in protein synthesis.
• Stabilizing the mRNA for efficient translation by the ribosomes.

The longer the poly-A tail, the more stable the RNA molecule usually is, extending its life in the cell.

RNA Splicing

Not all sections of an RNA transcript are needed for making proteins. The non-coding sections called introns are removed through a process called RNA splicing.
This process is performed by a large molecular machine known as the spliceosome, which cuts out the introns and joins the coding sections, or exons, together.
This is crucial because:

• Spliced RNA can then be accurately read by ribosomes to produce the correct proteins.
• It allows one gene to produce multiple different proteins through a process called alternative splicing, increasing genetic diversity and functionality.

RNA Editing

Sometimes after transcription, the RNA undergoes changes at the nucleotide level, a process known as RNA editing. This can involve the chemical alteration of a single nucleotide base.
A common type of RNA editing is deamination, where an Adenine (A) base is converted to Inosine (I).
This editing allows for:

• The production of proteins that may have different properties or functions from a single RNA sequence.
• Correcting mutations or creating new functional variants of proteins.

This added layer of control increases the complexity and adaptability of gene expression.

mRNA Transport

Once RNA has been fully processed, it must be transported out of the nucleus to the cytoplasm. This is facilitated by the nuclear pore complex, a large channel that spans the nuclear envelope.
This step is essential because:

• Translation, the process of protein synthesis, occurs in the cytoplasm at ribosomes.
• mRNA transport ensures that mature RNA can be used promptly for protein production.

Without this transport mechanism, RNA would remain in the nucleus and could not contribute to protein synthesis, leading to a breakdown in cellular functions.