5' cap

Definition

The 5' cap is a modified guanine nucleotide that is added to the 5' end of eukaryotic mRNA transcripts shortly after transcription begins. This structure plays a crucial role in RNA stability, nuclear export, and translation initiation, serving as a protective mechanism against degradation by exonucleases and facilitating ribosome binding for protein synthesis.

5 Must Know Facts For Your Next Test

1. The 5' cap consists of a 7-methylguanylate (7mG) residue linked to the mRNA via a unique 5'-5' triphosphate bridge.
2. This cap structure helps protect mRNA from degradation by exonucleases, extending its lifespan within the cell.
3. The 5' cap is also essential for the recruitment of translation initiation factors, ensuring proper ribosome assembly at the start codon.
4. Additionally, the cap facilitates the transport of mRNA from the nucleus to the cytoplasm, where translation occurs.
5. Capping occurs co-transcriptionally, meaning it happens concurrently with the transcription of mRNA, highlighting its importance in gene expression regulation.

Review Questions

• How does the 5' cap contribute to the stability and functionality of eukaryotic mRNA?
  • The 5' cap contributes to mRNA stability by protecting it from degradation by exonucleases. This protective function extends the lifespan of mRNA in the cytoplasm, allowing for more efficient translation into proteins. Additionally, the cap plays a key role in recruiting translation initiation factors, which are necessary for ribosome binding and proper protein synthesis.
• Discuss the process of capping in eukaryotic mRNA and its timing during transcription.
  • Capping occurs during transcription, shortly after RNA polymerase II begins synthesizing pre-mRNA. As soon as about 20-30 nucleotides are synthesized, a modified guanine nucleotide is added to the 5' end, forming the 5' cap. This co-transcriptional modification ensures that mRNA is protected and ready for processing and translation almost immediately after being transcribed.
• Evaluate the significance of the 5' cap in relation to gene expression and protein synthesis in eukaryotic cells.
  • The 5' cap is crucial for efficient gene expression and protein synthesis in eukaryotic cells. It not only protects mRNA from degradation but also facilitates its transport from the nucleus to the cytoplasm. By promoting ribosome binding during translation initiation, the cap enhances protein production. This process is vital for proper cellular function and regulation, underscoring how alterations in capping can impact gene expression and lead to various diseases.

Related terms

mRNA: Messenger RNA (mRNA) is the type of RNA that conveys genetic information from DNA to the ribosome, where it serves as a template for protein synthesis.

Polyadenylation: Polyadenylation is the addition of a poly(A) tail to the 3' end of mRNA, which enhances stability and aids in the export of the mRNA from the nucleus.

Ribosome: A ribosome is a molecular machine that synthesizes proteins by translating the sequence of mRNA into a polypeptide chain, consisting of ribosomal RNA and proteins.