Question

What are the three most common modifications by which primary transcripts are converted into mature mRNA?

Short answer

The three modifications are 5' capping, RNA splicing, and 3' polyadenylation.

Step-by-step solution

Understanding Transcription

Primary transcripts, also known as pre-mRNA, are the initial RNA molecules synthesized by transcription in eukaryotic cells. They undergo several modifications to become mature mRNA, which is then ready for translation.

5' Capping

The first modification is the addition of a 5' cap. This involves the attachment of a modified guanine nucleotide to the 5' end of the pre-mRNA. The 5' cap plays a crucial role in RNA stability, nuclear export, and initiation of translation.

RNA Splicing

The second modification is RNA splicing, where introns (non-coding regions) are removed from the pre-mRNA while exons (coding sequences) are joined together. This process is conducted by a complex known as the spliceosome.

3' Polyadenylation

The third common modification is polyadenylation of the 3' end, where a series of adenine bases (the poly-A tail) are added. This poly-A tail enhances the stability of the mRNA and aids in its export from the nucleus.

Key concepts

Transcription

Transcription is the first step in the process of converting genetic information from DNA into mRNA. This process occurs in the cell nucleus, where an enzyme called RNA polymerase binds to DNA at a specific region called the promoter.

The DNA strands unwind, and RNA polymerase uses one of the strands as a template to synthesize a single strand of RNA, known as the primary transcript or pre-mRNA. During transcription, nucleotides are added to form an RNA strand that is complementary to the DNA template. However, unlike DNA, RNA contains uracil instead of thymine.

Transcription concludes when RNA polymerase reaches a terminator sequence on the DNA, signaling the enzyme to detach, releasing the newly formed RNA molecule. This pre-mRNA requires several modifications before it can be translated into proteins.

5' Capping

5' Capping is the process by which a modified guanine nucleotide is added to the 5' end of the pre-mRNA molecule. This occurs soon after the commencement of transcription. The cap structure is actually a 7-methylguanylate triphosphate, which is important for several reasons:

• It protects the RNA from degradation by exonucleases.
• Facilitates the export of the mRNA from the nucleus to the cytoplasm.
• Plays a crucial role in the initiation of translation by helping ribosomes recognize the mRNA as being ready for protein synthesis.

The 5' capping is a critical step in mRNA processing, allowing the mRNA to be recognized and adequately protected as it journeys through the cell to its final destination.

RNA Splicing

RNA splicing is a mechanism that removes non-coding regions known as introns from the pre-mRNA and connects coding sequences called exons. This transformation is essential for the production of mature mRNA that can be translated into a functional protein.

The process of RNA splicing is carried out by a dynamic molecular machine known as the spliceosome. The spliceosome precisely excises the introns and ligates the exons, ensuring an accurate and continuous sequence of codons for translation.

• This process allows for alternative splicing, resulting in the generation of different protein variants from a single gene.
• It increases the diversity of proteins that a cell can produce.
• Errors in splicing can lead to genetic diseases, emphasizing its importance in gene expression regulation.

RNA splicing's role is crucial in the gene expression pathway, significantly impacting cellular function and genetic variability.

3' Polyadenylation

3' Polyadenylation is the addition of a poly-A tail to the 3' end of the pre-mRNA. This tail is a stretch of adenine nucleotides, typically around 200 bases long, and serves several key functions:

• Enhances the stability of the mRNA by protecting it from enzymatic degradation.
• Assists in the transport of the mRNA from the nucleus to the cytoplasm.
• Influences the translation efficiency of the mRNA.

The polyadenylation process starts with the cleavage of the 3' end of the pre-mRNA followed by the synthesis of the poly-A tail by the enzyme poly(A) polymerase.

This modification is essential for ensuring the longevity and proper functionality of the mRNA, providing the final touches to mature mRNA that is ready to be translated into proteins.