mRNA Processing

Summary

This transcript explains the intricate process of eukaryotic gene expression, from transcription to translation. It covers the key steps in pre-mRNA processing, including capping, polyadenylation, and splicing, which are crucial for creating mature mRNA. The failure of any of these processes, such as improper capping or splicing errors, can lead to instability or degradation of mRNA, hindering gene expression. The sequence of events ensures the mRNA is properly processed and ready for translation in the cytoplasm, emphasizing the delicate regulation required for successful gene expression.

Takeaways

• 😀 Eukaryotic genes consist of upstream sequences (e.g., promoter), coding regions (exons), and non-coding regions (introns), along with a 3' consensus sequence.
• 😀 Pre-mRNA undergoes extensive processing before it can be translated into protein, which includes capping, polyadenylation, and splicing.
• 😀 The 5' cap is added to the pre-mRNA shortly after transcription begins, providing stability, protection from degradation, and assistance in splicing.
• 😀 Polyadenylation involves cleaving the pre-mRNA at the 3' consensus sequence and adding 50-250 adenine nucleotides, ensuring mRNA stability and preventing degradation.
• 😀 Splicing removes non-coding introns from the pre-mRNA. It involves three critical consensus sequences: 5' splice site, branch point, and 3' splice site.
• 😀 During splicing, the intron is excised in a lariat structure, and the exons are joined together to form a continuous coding sequence.
• 😀 The final processed mRNA consists of the 5' untranslated region (UTR), the protein-coding region, and the 3' UTR.
• 😀 Once mature, the mRNA is transported from the nucleus to the cytoplasm, where it can be translated by ribosomes into a protein.
• 😀 If the 5' cap is missing, the mRNA is less stable, splicing is impaired, and translation cannot occur.
• 😀 Mutations that disrupt splicing can prevent mRNA from exiting the nucleus, blocking translation and gene expression.
• 😀 Failure in polyadenylation results in mRNA lacking a poly-A tail, causing instability and rapid degradation, thus preventing translation.

Q & A

What are the main components of a eukaryotic gene?

A eukaryotic gene is composed of upstream sequences such as the promoter, the RNA coding region, and downstream sequences. The RNA coding regions include exons (coding) and introns (non-coding).

What is the function of the 5' cap in RNA processing?

The 5' cap is added to the pre-mRNA shortly after transcription begins. It plays a critical role in protecting the mRNA from degradation, facilitating splicing, and aiding in translation by helping the ribosome recognize the mRNA.

What is polyadenylation, and what does it involve?

Polyadenylation is the process where 50 to 250 adenine nucleotides are added to the 3' end of the pre-mRNA after cleavage at the 3' consensus sequence. This process helps stabilize the mRNA and assists in its export from the nucleus.

How do introns and exons differ in eukaryotic genes?

Exons are the coding regions of the gene, whereas introns are non-coding regions that are spliced out during RNA processing. Exons contain the information that is translated into protein.

What are the key consensus sequences required for splicing?

The key consensus sequences for splicing include the 5' splice site, the branch point, and the 3' splice site, which are essential for the removal of introns during RNA processing.

What happens during the first step of RNA splicing?

In the first step of splicing, the 5' end of the intron is removed and attached to the branch point, forming a lariat structure. This step simultaneously removes the 5' end of the upstream exon.

What occurs if splicing fails during RNA processing?

If splicing fails, the mRNA will not be properly processed and cannot be exported from the nucleus, preventing translation. However, 5' capping and polyadenylation may still occur normally.

What is the consequence of lacking a 5' cap on mRNA?

The absence of a 5' cap on mRNA affects several processes: splicing is impaired, the mRNA becomes less stable and degrades rapidly, and translation cannot occur because the ribosome cannot recognize the mRNA.

How does the 3' consensus sequence impact mRNA processing?

The 3' consensus sequence is crucial for the cleavage and polyadenylation of the pre-mRNA. Without a functional 3' consensus sequence, polyadenylation cannot occur, resulting in an unstable mRNA that is rapidly degraded.

Why is the stability of mRNA important for gene expression?

The stability of mRNA is critical because unstable mRNA degrades quickly and cannot be translated into protein. Proper processing steps, such as capping and polyadenylation, help ensure that mRNA remains stable and is available for translation.