Question

Explain briefly what happens to eukaryotic mRNA before it can be translated to protein.

Short answer

Eukaryotic mRNA is capped, spliced, polyadenylated, and exported to the cytoplasm before translation.

Step-by-step solution

- Transcription

Eukaryotic mRNA is first transcribed from DNA in the nucleus. This process involves the enzyme RNA polymerase, which synthesizes a complementary RNA strand from the DNA template.

- 5' Capping

Immediately after the mRNA is transcribed, a 5' cap is added to the mRNA's 5' end. This cap protects the mRNA from degradation and helps in ribosome binding during translation.

- Splicing

The pre-mRNA undergoes splicing where introns (non-coding regions) are removed and exons (coding regions) are joined together. This process is conducted by a complex called the spliceosome.

- 3' Polyadenylation

A poly-A tail is added to the 3' end of the mRNA. This tail also protects the mRNA from degradation and assists in the export of the mRNA from the nucleus to the cytoplasm.

- Export to Cytoplasm

The processed mRNA is transported from the nucleus to the cytoplasm. Here, it can be recognized by the ribosome for translation into protein.

Key concepts

transcription

Transcription is the first step in eukaryotic mRNA processing. During this stage, RNA polymerase binds to the DNA at a specific region called the promoter. RNA polymerase then moves along the DNA strand, synthesizing a complementary RNA strand from the DNA template. This RNA strand is known as pre-mRNA. Transcription occurs in the cell nucleus and is a crucial first step because it converts the genetic information stored in DNA into a format that can be used to make proteins.

5' capping

5' capping is one of the earliest modifications that the newly synthesized pre-mRNA undergoes. A specially modified guanine nucleotide, known as a 5' cap, is added to the 5' end of the mRNA. This cap is important for several reasons:

• It protects the mRNA from enzymatic degradation.
• It assists in mRNA export from the nucleus.
• It is crucial for ribosome binding during translation.

Without the 5' cap, the mRNA would be highly unstable and could not efficiently be translated into protein.

splicing

Splicing is a process that removes introns (non-coding regions) from the pre-mRNA. This leaves behind only exons (coding regions) that will be expressed in the final protein. The splicing process is carried out by a complex called the spliceosome. The spliceosome cuts out the introns and joins the exons together to form the mature mRNA. This step is essential because introns do not code for proteins and need to be removed for the mRNA to be functional in protein synthesis.

3' polyadenylation

In the 3' polyadenylation step, a string of adenine nucleotides, known as a poly-A tail, is added to the 3' end of the mRNA. This poly-A tail serves several key functions:

• It enhances the stability of the mRNA molecule.
• It aids in the export of the mRNA from the nucleus to the cytoplasm.
• It helps in the initiation of translation by ribosomes.

The addition of the poly-A tail is crucial for the longevity and functionality of the mRNA in the cytoplasm.

mRNA export

Once transcription, 5' capping, splicing, and 3' polyadenylation are complete, the mature mRNA is ready for export to the cytoplasm. Transport proteins recognize signals on the processed mRNA and facilitate its movement through the nuclear pore complex and into the cytoplasm. This exported mRNA can now be translated by ribosomes to synthesize proteins. The efficiency of mRNA export is crucial as it ensures that the genetic information is accurately and promptly turned into functional proteins.