Question

REFLECT AND APPLY List three molecular changes that take place in the processing of eukaryotic mRNA.

Short answer

5' capping, 3' polyadenylation, and splicing.

Step-by-step solution

5' Capping

The first molecular change in the processing of eukaryotic mRNA is the addition of a 5' cap. This involves attaching a modified guanine nucleotide to the 5' end of the mRNA. The 5' cap protects the mRNA from degradation and helps in ribosome binding during translation.

3' Polyadenylation

The second molecular change is the addition of a poly-A tail at the 3' end. This process, known as polyadenylation, involves adding a series of adenine nucleotides. The poly-A tail enhances the stability of the mRNA and aids in the export of the mRNA from the nucleus to the cytoplasm.

Splicing

The third molecular change is splicing, where introns (non-coding regions) are removed from the mRNA transcript, and exons (coding regions) are joined together. This process results in a mature mRNA molecule that can be translated into a protein.

Key concepts

5' Capping

During the initial stages of eukaryotic mRNA processing, a crucial modification occurs at the 5' end of the mRNA. This modification is known as 5' capping. The process involves the addition of a specially modified guanine nucleotide.
The 5' cap has multiple roles:

• It protects the mRNA from enzymatic degradation.
• It facilitates ribosome binding during the translation process.
• It helps in the export of mRNA from the nucleus to the cytoplasm.

The structure of the 5' cap is unique because it features a 7-methylguanosine linked via a 5'-to-5' triphosphate bridge. This distinct structure makes it easily recognizable by proteins involved in mRNA processing and translation.

3' Polyadenylation

Another essential modification in eukaryotic mRNA processing occurs at the 3' end, called 3' polyadenylation. This process entails adding a long sequence of adenine nucleotides, known as the poly-A tail, to the mRNA molecule.
Here's why the poly-A tail is important:

• It significantly enhances the stability of the mRNA.
• It plays a key role in the export of mRNA from the nucleus to the cytoplasm.
• It assists in the regulation of translation by interacting with certain proteins.

The poly-A tail can be several hundred nucleotides long and is added by an enzyme complex called polyadenylate polymerase. This complex ensures that the tail is successfully attached after the mRNA has been cleaved at a specific site.

mRNA Splicing

The final major modification of eukaryotic mRNA processing is splicing. During splicing, non-coding regions called introns are removed from the pre-mRNA, while coding regions called exons are joined together.
Splicing is crucial for several reasons:

• It ensures that only the necessary coding sequences (exons) are included in the mature mRNA.
• It allows for alternative splicing, which can produce diverse proteins from a single gene.

Splicing is carried out by a complex known as the spliceosome, which recognizes specific sequences at the intron-exon boundaries. The precise removal of introns and joining of exons results in a contiguous coding sequence that can be efficiently translated into a functional protein.