5' cap addition

Definition

5' cap addition is a crucial modification that occurs at the 5' end of eukaryotic mRNA molecules during the processing of pre-mRNA. This structure, consisting of a modified guanine nucleotide, serves several important functions such as enhancing mRNA stability, facilitating ribosome binding during translation, and assisting in mRNA export from the nucleus to the cytoplasm.

5 Must Know Facts For Your Next Test

1. The 5' cap is added co-transcriptionally, meaning it occurs while the RNA is still being synthesized by RNA polymerase II.
2. The cap structure is recognized by cap-binding proteins, which play a role in recruiting the ribosome for translation initiation.
3. Capping involves the addition of a 7-methylguanylate (m7G) residue to the 5' end of the transcript, which is linked via a unique 5'-5' triphosphate bridge.
4. The presence of a 5' cap protects mRNA from degradation by exonucleases, thus extending its lifespan in the cytoplasm.
5. In addition to stability and translation enhancement, the 5' cap also aids in splicing by facilitating interactions between the spliceosome and the mRNA.

Review Questions

• How does the process of 5' cap addition affect mRNA stability and its translation efficiency?
  • The addition of the 5' cap significantly enhances mRNA stability by protecting it from degradation by exonucleases. This protective feature allows the mRNA to have a longer lifespan within the cytoplasm, increasing its chances of being translated into protein. Additionally, the cap structure facilitates the binding of ribosomes during translation initiation, making it easier for the ribosome to recognize and efficiently translate the mRNA into an amino acid sequence.
• Discuss the role of cap-binding proteins in the process of translation initiation in relation to 5' cap addition.
  • Cap-binding proteins play a critical role in recognizing and binding to the 5' cap structure of eukaryotic mRNAs. Once bound, these proteins help recruit the ribosome to the mRNA molecule, positioning it correctly for translation initiation. This interaction ensures that translation begins at the correct starting point, enhancing overall translation efficiency and allowing for proper protein synthesis.
• Evaluate how defects in 5' cap addition could impact gene expression and overall cellular function.
  • Defects in 5' cap addition can have severe consequences on gene expression and cellular function. Without a proper cap structure, mRNAs may become unstable and susceptible to rapid degradation, leading to decreased levels of protein production. Additionally, impaired ribosome binding can hinder translation initiation, further reducing protein synthesis. Such disruptions can lead to various cellular dysfunctions and may contribute to disease states if essential proteins are not produced adequately.

Related terms

Pre-mRNA: The initial RNA transcript synthesized from DNA that undergoes various modifications, including capping, splicing, and polyadenylation, before becoming mature mRNA.

Polyadenylation: The addition of a poly(A) tail to the 3' end of mRNA molecules, which plays a key role in nuclear export, translation efficiency, and stability of the mRNA.

Ribosome: A cellular structure composed of RNA and proteins that serves as the site for protein synthesis by translating mRNA into amino acid sequences.