Question

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

Short answer

Eukaryotic mRNA undergoes 5' capping, splicing, and polyadenylation before being transported to the cytoplasm for translation.

Step-by-step solution

Transcription

The first step in the process is transcription. The DNA sequence of a gene is copied into mRNA in the nucleus of eukaryotic cells.

5' Capping

Immediately after transcription begins, a modified guanine nucleotide is added to the 5' end of the mRNA. This is known as the 5' cap, and it protects the mRNA from degradation and assists in ribosome binding.

RNA Splicing

Introns, non-coding sequences in the mRNA, are removed, and exons, coding sequences, are spliced together. This process modifies the mRNA to be a continuous coding sequence.

Polyadenylation

A tail of adenine nucleotides, known as the poly-A tail, is added to the 3' end of the mRNA. This tail stabilizes the mRNA and regulates the export of the mRNA from the nucleus to the cytoplasm.

mRNA Transport

The processed and mature mRNA is then transported from the nucleus to the cytoplasm where it can be translated into a protein by ribosomes.

Key concepts

Transcription

Transcription is the first step in eukaryotic mRNA processing. It occurs in the nucleus, where the DNA sequence of a gene is used as a template to synthesize a complementary mRNA strand. This process involves an enzyme called RNA polymerase, which unwinds the DNA and assembles the mRNA by adding nucleotides in the 5' to 3' direction. The resulting mRNA strand is a copy of the gene's coding sequence, but it undergoes several modifications before it can be translated into a protein.

5' Capping

Right after transcription begins, the mRNA receives a 5' cap. This involves adding a modified guanine nucleotide to the 5' end of the mRNA.

The cap serves several important functions:

• It protects the mRNA from degradation by enzymes.
• It facilitates the export of mRNA out of the nucleus.
• It assists in the binding of mRNA to the ribosome during translation.

Without the 5' cap, the mRNA would be vulnerable and ineffective in guiding protein synthesis.

RNA Splicing

Before mRNA can be used in translation, it needs to undergo splicing. Eukaryotic genes often contain introns, which are non-coding sequences that interrupt the coding regions known as exons.

During RNA splicing:

• Introns are removed from the mRNA strand.
• Exons are joined together to form a continuous coding sequence.

This process ensures that the mRNA is correctly formatted to produce the intended protein. Splicing takes place within a complex called the spliceosome.

Polyadenylation

After splicing, the mRNA undergoes another modification known as polyadenylation. A string of adenine nucleotides, called the poly-A tail, is added to the 3' end of the mRNA.

The poly-A tail serves several roles:

• It increases the stability of the mRNA molecule.
• It aids in the export of the mRNA from the nucleus to the cytoplasm.
• It plays a role in the initiation of translation.

Polyadenylation ensures that the mRNA remains intact and functional during its journey to and within the cytoplasm.

mRNA Transport

Once all modifications are complete, the mature mRNA needs to be transported from the nucleus to the cytoplasm. This transport is vital because protein synthesis occurs in the cytoplasm, facilitated by ribosomes.

The mRNA passes through nuclear pores to enter the cytoplasm, where it is translated into a protein. This involves the mRNA binding to a ribosome, and then the ribosome reads the mRNA sequence to synthesize a polypeptide chain, which will fold into a functional protein.

Efficient mRNA transport is essential for proper gene expression and cellular function.