What are the functions of the 5’ cap and poly (A) tail of mature mRNAs in eukaryotes?
The correct answer and explanation is :
In eukaryotic cells, mature mRNAs have two key modifications: the 5′ cap and the poly(A) tail, both of which play crucial roles in mRNA stability, translation, and transport.
Functions of the 5′ Cap
The 5′ cap is a modified guanine nucleotide (7-methylguanosine) added to the 5′ end of the mRNA via an unusual 5′ to 5′ triphosphate linkage. Its functions include:
- Protection from degradation – The 5′ cap prevents exonucleases from degrading the mRNA, enhancing its stability.
- Facilitating translation initiation – It is recognized by the eukaryotic initiation factor 4E (eIF4E), which helps recruit the ribosome for translation.
- Aiding nuclear export – The cap is recognized by cap-binding proteins that assist in transporting the mRNA from the nucleus to the cytoplasm.
- Regulation of splicing – It influences proper splicing of the first intron in some pre-mRNAs.
Functions of the Poly(A) Tail
The poly(A) tail is a string of adenine (A) nucleotides added to the 3′ end of the mRNA by poly(A) polymerase. It serves multiple functions:
- mRNA stability – The tail protects the mRNA from rapid degradation by exonucleases, as longer tails typically correlate with longer mRNA half-lives.
- Translation efficiency – It interacts with the poly(A)-binding proteins (PABPs), which enhance ribosome recruitment and circularization of mRNA for efficient translation.
- Nuclear export – The poly(A) tail is recognized by nuclear export proteins, ensuring efficient transport to the cytoplasm.
Together, the 5′ cap and poly(A) tail ensure that mRNA is stable, properly translated, and efficiently exported from the nucleus, playing essential roles in gene expression regulation in eukaryotic cells.