Question

Which of the following RNA molecules or proteins is NOT found in the spliceosome during intron excision? (A) snRNA (B) hnRNA (C) shRNA (D) snRNPs

Short answer

shRNA (C) is NOT found in the spliceosome.

Step-by-step solution

- Understanding the Spliceosome

The spliceosome is a complex of RNA and proteins that removes introns from a transcribed pre-mRNA (hnRNA) segment.

- Identify Components of the Spliceosome

The main components of the spliceosome include snRNA (small nuclear RNA), snRNPs (small nuclear ribonucleoproteins), and proteins.

- Review Each Option

Evaluate each option: (A) snRNA - essential component of the spliceosome.(B) hnRNA - precursor mRNA involved in splicing.(C) shRNA - small hairpin RNA, used in RNA interference, not involved in splicing.(D) snRNPs - essential component, combining snRNA with proteins.

- Identify the Incorrect Component

Based on the review, shRNA (C) is used in RNA interference and not a part of the spliceosome during intron excision.

Key concepts

RNA Splicing

RNA splicing is a crucial process in gene expression. It occurs in eukaryotes where introns, non-coding regions of RNA, are removed, and exons, coding regions, are joined together to form a mature mRNA molecule. This mature mRNA is then used for protein synthesis.

The process of RNA splicing is carried out by a complex called the spliceosome. This spliceosome ensures that the introns are accurately removed, and exons are joined seamlessly. Mistakes in RNA splicing can lead to diseases, illustrating the importance of this process.

Key steps in RNA splicing include:

• Recognition of splice sites
• Branch point formation
• Excision of introns and ligation of exons

Proper RNA splicing ensures that the resultant mRNA is functional and competent for translation.

hnRNA

Heterogeneous nuclear RNA (hnRNA) is the primary transcript that is produced during transcription in eukaryotic cells. It includes both introns and exons and is not yet processed.

hnRNA undergoes several modifications before becoming mature mRNA:

• Addition of a 5' cap
• Addition of a poly-A tail
• Splicing to remove introns

These modifications are essential because they protect the RNA from degradation and aid in its export from the nucleus to the cytoplasm. Once hnRNA is fully processed, it is called mRNA and can be translated into a protein.

snRNA

Small nuclear RNA (snRNA) are short RNA molecules essential for the splicing of pre-mRNA. They are found within the spliceosome and work together with proteins to form small nuclear ribonucleoproteins (snRNPs).

There are five main snRNAs named U1, U2, U4, U5, and U6 that play a direct role in recognizing splice sites and catalyzing the removal of introns.

The roles of snRNAs include:

• Identifying splicing sites
• Facilitating catalytic steps of splicing
• Stabilizing the structure of the spliceosome

Without snRNAs, the splicing machinery would not be able to function properly, leading to faulty mRNA production.

shRNA

Small hairpin RNA (shRNA) is not involved in the process of splicing. Instead, it is used for RNA interference (RNAi) which is a mechanism for regulating gene expression.

shRNA forms a tight hairpin loop and is processed into small interfering RNA (siRNA) by an enzyme called Dicer. These siRNAs are incorporated into the RNA-induced silencing complex (RISC) and guide the complex to target mRNA molecules for degradation or translational repression.

Key points about shRNA:

• Used in gene silencing
• Forms hairpin structures
• Processed into siRNA

In contrast to hnRNA and snRNA, shRNA is not a part of the spliceosome and does not play a role in mRNA splicing.