Protection and modification of pre-mRNA ends
The formation of the mature molecule begins with 5'-capping. This mechanism is triggered very early — when the length of the synthesized primary transcript is only about 30 nucleotides.
The essence of the process is the attachment of a guanosine triphosphate (GTP) residue to the pre-mRNA fragment. Notably, the GTP molecule attaches via its 5'-end to the 5'-end of the transcript, resulting in the formation of an atypical 5',5'-phosphodiester bond between them. The final stage of capping is the methylation of guanine within the GTP. The resulting "cap" structure has the appearance of 7-methyl-G(5')ppp(5')X..., where X denotes the first nucleotide of the pre-mRNA chain.
The second important modification is 3'-polyadenylation, which occurs after transcription is complete. The enzyme poly-A polymerase is responsible for carrying out this step. It catalyzes the synthesis of a poly-A sequence at the 3'-end of the primary transcript, building a chain of 100–200 adenylic acid residues. The presence of the poly-A tail performs two critically important functions:
- Significantly facilitates the export of the mRNA molecule from the nucleus.
- Slows down mRNA hydrolysis by enzymes in the cytoplasm.
It must be strictly remembered that such protective elements are not intrinsic to all types of nucleic acids: transfer and ribosomal RNAs completely lack a cap and a poly-A sequence.
Internal transcript structure and splicing
The primary transcript (pre-mRNA) is an exact complementary copy of the gene. It consists of two alternating types of sequences:
- Exons are coding regions that carry information about segments of the future protein molecule.
- Introns are non-coding sequences that the cell needs to remove.
The process of excising introns from the primary transcript and subsequently joining the exon ends together to form a "mature" mRNA is called splicing.
Small nuclear ribonucleoproteins (snRNPs) are key participants in the splicing mechanism. The structure of each snRNP includes a small nuclear RNA (snRNA) molecule and a protein core consisting of several protomers. Uniting together, these complexes form a large working structure — the spliceosome.
Stages of the splicing reaction:
- Individual snRNPs, using the principle of complementarity, accurately recognize specific sequences within introns.
- The spliceosome catalyzes the cleavage of the 3',5'-phosphodiester bond precisely at the junction between the exon and the intron.
- Subsequent precise joining of the ends of two adjacent exons takes place.
Post-transcriptional rRNA modifications and ribosome formation
Processing also affects ribosomal RNAs. rRNA maturation consists of the formation of final "mature" molecules from a single common high-molecular-weight precursor. As a result of such processing, types of mature rRNAs designated as 28S, 18S, and 5.8S appear in the cell.
All these matured molecules are part of the chemical composition of the ribosome — the most important cellular organelle that performs protein biosynthesis.
In addition to ribosomal RNAs, the ribosome contains various proteins. The organelle components work together, and the functions of rRNA and proteins within it are divided into three categories:
- Structural (formation of the complex architecture).
- Regulatory (control of polypeptide chain assembly stages).
- Catalytic (facilitation of biosynthesis chemical reactions).
Briefly: answers to topical questions
What is the mechanism of alternative splicing?
The mechanism of alternative splicing is that several exons of the primary transcript are spliced in various combinations. Sources also mention the combination of alternative splicing with polyadenylation. As a result, several different mature mRNAs are formed from the same primary transcript, allowing different proteins to be synthesized from a single gene.
Methods of implementation of the process:
- Use of different promoters — transcripts are formed with different 5'-ends and varying numbers of exons.
- Change in the position of the start site — the splice site.
- Cassette splicing — selection of different exons from identical mRNAs.
How is transfer RNA (tRNA) processing carried out?
Transfer RNA processing represents a sequence of structural and chemical changes in primary transcripts.
The process includes the following stages:
- End processing — shortening of the molecule by removing excess regions at the 5'- and 3'-ends.
- Splicing — excision of the intron located in the central region of the molecule (anticodon loop).
- Base modification — chemical alteration of 10–15% of nitrogenous bases (e.g., formation of dihydrouridine, ribothymidine, pseudouridine).
- Formation of the acceptor stem — attachment of the -CCA triplet to the 3'-end, necessary for amino acid binding.
To avoid confusing the terms describing pre-mRNA structure, remember: Exons — Expressed, they remain in the mRNA. Introns — In the trash (removed during splicing).
Class and exam questions
At what point is the capping reaction triggered?
The 5'-capping process begins very early — when the length of the synthesized primary transcript reaches approximately 30 nucleotides.
What functions does the poly-A sequence perform?
The poly-A tail, consisting of 100–200 adenylic acid residues, facilitates the export of messenger RNA from the nucleus and effectively slows down its hydrolysis in the cytoplasm.
What is the spliceosome made of?
The spliceosome is formed from small nuclear ribonucleoproteins (snRNPs). In turn, each snRNP consists of a small nuclear RNA (snRNA) and a protein core including several protomers.
Do ribosomal and transfer RNAs contain a cap and a poly-A tail?
No. The presence of a cap at the 5'-end and a poly-A sequence at the 3'-end is characteristic of mRNA, whereas tRNA and rRNA do not contain these structures.
What else to study on this topic
- The mechanism of 5',5'-phosphodiester bond formation during capping
- Principles of action of the poly-A polymerase enzyme
- Complementary recognition of introns by snRNP complexes
- Catalytic functions of rRNA molecules within the ribosome
Master the entire topic in the "Biochemistry" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "RNA Processing" and 3,616 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 115 video lessons on the subject
Other topics in the section "Template Biosyntheses"
Free video lessons: biochemistryShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.