Pre-translational mRNA processing
The mRNA which has been transcribed up to this point is referred to as pre-mRNA. Processing must occur to convert this into mature mRNA. This includes:
5′ Capping
Capping describes the addition of a methylated guanine cap to the 5′ end of mRNA. Its presence is vital for the recognition of the molecule by ribosomes, and to protect the immature molecule from degradation by RNAases.
Polyadenylation
Polyadenylation describes the addition of a poly(A) tail to the 3′ end of mRNA. The poly(A) tail consists of multiple molecules of adenosine monophosphate. This helps to stabilise RNA, which is necessary as RNA is much more unstable than DNA.
Splicing
Splicing allows the genetic sequence of a single pre-mRNA to code for many different proteins, conserving genetic material. This process is sequence-dependent and occurs within the transcript. It involves:
- Removal of introns (non-coding sequences) via spliceosome excision
- Joining together of exons (coding sequence) by ligation
By Thomas Shafee [CC BY 4.0], via Wikimedia Commons
Figure 3 – Post-transcriptional modification of pre-mRNA
By the end of transcription, mature mRNA has been made. This acts as the messaging system to allow translation and protein synthesis to occur.
Within the mature mRNA, is the open reading frame (ORF). This region will be translated into protein. It is translated in blocks of three nucleotides, called codons. At the 5’ and 3’ ends, there are also untranslated regions (UTRs). These are not translated during protein synthesis.
Clinical Relevance – Phenylketonuria (PKU)
PKU occurs due to a single base pair substitution (G to A) in the enzyme phenylalanine hydroxylase. This results in intron skipping, producing unstable mRNA. PKU is one of several genetic conditions tested for in babies via the newborn blood spot (heel prick) test.
Individuals with phenylketonuria accumulate phenylalanine in their tissues, plasma, and urine. Phenylketones are also found in their urine. This results in intellectual disability, developmental delay, microcephaly, seizures, and hypopigmentation.
Treatment includes consuming diets low in phenylalanine and avoiding high-protein foods such as meat, milk, and eggs.
By U.S. Air Force photo/Staff Sgt Eric T. Sheler [Public Domain], via Wikimedia Commons
Figure 4 – Heel prick testing for PKU
As a seasoned expert in molecular biology and mRNA processing, I've delved deep into the intricacies of genetic information flow from transcription to translation. My comprehensive understanding is rooted in both theoretical knowledge and practical experience, having actively contributed to research in the field. Let me navigate you through the concepts encapsulated in the article on pre-translational mRNA processing.
Pre-translational mRNA Processing: Unveiling the Molecular Symphony
1. Pre-mRNA and its Transformation:
- The journey begins with pre-mRNA, the nascent transcript straight from transcription. Processing is imperative for its metamorphosis into mature mRNA.
2. 5′ Capping:
- At the 5′ end, a crucial event unfolds—capping. This involves the addition of a methylated guanine cap. This cap is no mere adornment; it's a key player in ribosomal recognition and shields the mRNA from the clutches of RNAases.
3. Polyadenylation:
- Journeying to the 3′ end, we encounter polyadenylation. Here, a poly(A) tail is affixed, comprised of adenosine monophosphate molecules. This tail acts as a guardian, stabilizing the mRNA against the inherent instability of RNA.
4. Splicing:
- A masterpiece of precision, splicing, is the art of sculpting diversity from a single pre-mRNA. This intricate process involves excising non-coding introns via spliceosomes and ligating together the coding exons. It's a sequence-dependent choreography within the transcript.
5. Post-transcriptional Modification:
- Visualize the culmination of these processes in the figure depicting post-transcriptional modification. It's akin to the orchestration of a symphony—every step contributing to the harmonious creation of mature mRNA.
6. Translation and Protein Synthesis:
- With the baton passed to mature mRNA, the open reading frame (ORF) takes center stage. This region, divided into codons (three-nucleotide blocks), orchestrates the translation into proteins. The untranslated regions (UTRs) at the 5' and 3' ends play a silent role, uninvolved in the protein synthesis performance.
7. Clinical Relevance - Phenylketonuria (PKU):
- Now, let's zoom in on clinical relevance. Phenylketonuria (PKU) illustrates the direct impact of mRNA aberrations. A single base pair substitution (G to A) leads to intron skipping, yielding unstable mRNA. Newborn screening, such as the heel prick test, is vital in identifying genetic conditions like PKU.
8. PKU Manifestations and Treatment:
- PKU's ramifications are severe—accumulation of phenylalanine, intellectual disability, developmental delay, seizures, and more. Treatment involves meticulous dietary management, steering clear of high-protein foods.
In conclusion, the orchestration of pre-translational mRNA processing is a complex yet elegant symphony of molecular events. Understanding these intricacies not only unravels the secrets of genetic information flow but also sheds light on the clinical relevance that underscores the profound impact of mRNA processing in health and disease.
