Modification of molecule ends
For an mRNA molecule to function safely and participate in protein synthesis, its ends must be protected. This process involves two key chemical modifications.
The first stage is capping (capping). It takes place at the 5'-end of the molecule. The essence of the process is the addition of a modified nucleotide—7-methylguanosine. An unconventional bond is formed: guanine is attached to the first nucleotide of the chain via a specific triphosphate bridge (5'-5' linkage). The resulting "cap" performs two critical functions. First, it reliably protects the transcript from the destructive action of 5'-endonucleases in the cytoplasm. Second, this region is subsequently recognized by the small ribosomal subunit to initiate translation.
The second stage is polyadenylation. This modification affects the opposite, 3'-end of the mRNA, where the signaling nucleotide sequence (AAUAAA) is located. A long poly-A tail (poly-A tail)—a chain of 100–200 adenylyl residues—is attached to this region. The presence of such a tail is critically important: it determines the stability of the molecule and its lifetime, helps the mature mRNA exit the nucleus into the cytoplasm, and participates in the regulation of transcription.
Splicing mechanism
The primary transcript contains both coding regions and non-coding insertions. The process of removing non-coding sequences (introns) and subsequently joining coding fragments (exons) is called splicing (splicing). It occurs in the nucleus in parallel with RNA formation on the DNA template.
The main operating apparatus is the spliceosome. This is a large complex consisting of small nuclear RNAs (snRNAs) and dozens of enzymatic proteins that together form ribonucleoprotein particles.
The work of the spliceosome proceeds in several strictly sequential steps:
- Initiation: snRNA interacts with the 5'-end of the intron and precisely recognizes the intron-exon junction.
- Complex assembly: a fully functional spliceosome is formed. The ends of the intron to be removed are brought closer together, forming a loop.
- Lariat formation: the 5'-end of the intron is cleaved. This released end attaches to a specific internal region of the intron closer to its 3'-end. A loop structure, the "lariat" (lariat), arises.
- Removal and joining: cleavage occurs at the 3'-end. The intron in the form of a lariat is finally excised, and the spliceosome disassembles. The liberated exons are joined (ligated) together.
The result is a continuous coding chain—a functionally active mature mRNA.
Alternative splicing
There is a mechanism in cells whereby a single gene can serve as an instruction for creating different molecules. This phenomenon is called alternative splicing.
Its essence is that when introns are excised from the pre-mRNA, the remaining exons can be joined in various combinations. For example, exons can connect in order (1-2-3-4-5) or with a skipped fragment (1-2-3-5). As a result, different mature mRNAs are formed.
Biological significance of the process:
- The organism gains the ability to synthesize proteins differing in structure and properties based on a single gene.
- Families of closely related polypeptides (isoforms) are formed.
The cell can implement alternative splicing in several ways:
- Use of different promoters: transcripts are formed with different 5'-ends and a varying number of exons.
- Change of splicing site: shifting the starting point of the process.
- Cassette splicing: selective inclusion or exclusion of certain exons from identical primary transcripts.
Briefly: answers to topical questions
Which enzymes catalyze the polyadenylation process?
The polyadenylation process is catalyzed by the enzyme poly(A) polymerase. This enzyme synthesizes the poly(A) sequence upon completion of transcription, attaching from 100 to 200 adenylyl residues to the 3'-end of the primary transcript (pre-mRNA). The formation of the poly(A) tail performs several crucial functions:
- determines mRNA stability and its lifespan in the cell;
- slows down mRNA hydrolysis in the cytoplasm;
- promotes and facilitates the exit of the synthesized mRNA molecule from the nucleus.
What specific types of small nuclear RNAs (snRNAs) are part of the spliceosome?
The spliceosome includes small nuclear RNAs called U-RNAs. These are short, stable molecules that received their specific name due to the high content of uracil and its modified forms in them. In the nucleus, they bind to proteins to form small nuclear ribonucleoprotein particles (snRNPs), from which the spliceosome is then formed. U-RNAs are characterized by high evolutionary conservation: the nucleotide sequences of these molecules are 95% identical in all vertebrates.
"Cap on the head, tail at the back, and the excess in a lariat" — the 5'-cap protects the beginning (like a hat), the 3'-poly-A-tail stabilizes the end, and unnecessary introns are excised as a loop ("lariat") during splicing.
Class and exam questions
What is the main difference between the primary transcript and mature mRNA?
The primary transcript (pre-mRNA) is inactive and contains both exons and non-coding introns. Mature mRNA is modified at the ends and consists only of spliced exons, representing a continuous coding chain.
Why does an mRNA molecule need a 5'-cap?
The 7-methylguanosine cap protects mRNA from degradation by 5'-endonucleases in the cytoplasm. It is also required for recognition of the mRNA by the small ribosomal subunit at the initiation of translation.
Why is a single gene capable of coding for multiple different proteins?
This occurs due to alternative splicing. During this process, exons of the same primary transcript are spliced in different combinations, forming a family of protein isoforms.
What is a spliceosome?
This is a large enzymatic complex responsible for intron excision. The spliceosome consists of small nuclear RNAs (snRNAs) and specialized proteins forming ribonucleoprotein particles.
What else to study on this topic
- Structure and functions of small nuclear RNAs (snRNAs)
- Mechanisms of recognition of intron and exon boundaries by the spliceosome
- The role of the AAUAAA signaling sequence in polyadenylation
- Differences between protein isoforms generated as a result of cassette splicing
- The chemical nature of the 5'-5' triphosphate bridge during capping
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