General molecular structure
RNA is a single-stranded linear heteropolymer. The structural unit is a ribonucleotide, which consists of three mandatory components:
- Carbohydrate: pentose (ribose).
- Nitrogenous bases: purines (adenine and guanine) or pyrimidines (cytosine and uracil).
- Phosphoric acid residue.
Nucleotides are linked together into a single chain in the exact same way as in a DNA molecule—via the carbon atom of the sugar and the phosphate group. In addition to protein translation, the molecule is capable of performing specialized endonuclease functions and regulating stages of gene expression.
Main types of RNA (prokaryotes and eukaryotes)
Three main classes of RNA are stably present in the cells of all living organisms:
- Ribosomal RNA (rRNA). Makes up the vast majority (about 85%) of all cellular RNA. These are the largest molecules with a mass of 1.0–1.5 million Daltons and a length of up to 5 thousand nucleotides. It is synthesized in the nucleolus on specialized genes (in humans, they are located on the 13th, 14th, 15th, 21st, and 22nd chromosome pairs). The main function is the formation of the active center of the ribosome and ensuring its interaction with tRNA. At the same time, eukaryotic cytoplasmic ribosomes (80S) are larger than the structures of prokaryotes, mitochondria, and plastids (70S).
- Transfer RNA (tRNA). Accounts for about 10%. These are the shortest chains (70–90 nucleotides, mass 25–30 thousand Daltons). The main task is the transport of amino acids in the cytoplasm to the site of polypeptide assembly.
- Messenger RNA (mRNA). Makes up about 5%, but its amount heavily depends on the cell cycle stage and the intensity of protein synthesis. The size varies greatly (300–3000 nucleotides) depending on the volume of copied information. It is synthesized in the nucleus during transcription and carries the protein structure code to the ribosomes.
Specific types of eukaryotic RNA
In eukaryotic cells, molecules involved in the maturation of messenger chains are additionally distinguished:
- Heterogeneous nuclear RNA (hnRNA). Located in the nucleus and represents a mixture of transcripts from many nuclear genes. It includes primary copies (corresponding to the length of the gene) and partially processed chains that lose introns during splicing and become mature mRNA.
- Small nuclear RNA (snRNA). Short but extremely stable molecules that are part of the nucleoprotein particles of the nucleus. In mammals, they form spliceosomes—structures for excising non-coding regions. Due to their high uracil content, they are called U-RNA. They are remarkably conservative: their sequences are 95% identical across all vertebrates.
Structure and function of tRNA
The transfer RNA molecule has a specific secondary structure resembling a "cloverleaf" (trefoil) shape. Several key centers are distinguished in its configuration:
- Acceptor stem. Located at the ends of the chain (3' and 5' ends); this is where the required amino acid attaches.
- T-loop and D-loop.
- Anticodon loop. Contains a triplet of nucleotides (anticodon) that is strictly complementary to the codon on the mRNA.
The process of amino acid attachment occurs outside the ribosome with the participation of enzymes—aminoacyl-tRNA synthetases (codases). The resulting complex is called aminoacyl-tRNA. Due to the complementarity of the anticodon and the mRNA codon, the delivered amino acid takes a strictly defined position in the protein being built.
Comparative characteristics of DNA and RNA
Despite their chemical affinity, the molecules have fundamental structural and functional differences.
| Characteristic | DNA | RNA |
|---|---|---|
| Number of chains | Two | One |
| Carbohydrate (pentose) | Deoxyribose | Ribose |
| Nitrogenous bases | Adenine (A), Thymine (T), Guanine (G), Cytosine (C) | Adenine (A), Uracil (U), Guanine (G), Cytosine (C) |
| Localization (prokaryotes) | Cytoplasm | Cytoplasm, ribosomes |
| Localization (eukaryotes) | Nucleus, organelles (plastids, mitochondria) | Nucleus, nucleolus, cytoplasm, ribosomes, organelles |
Briefly: answers to topical questions
What steps, besides splicing, does pre-mRNA processing include?
The processing of the primary transcript (pre-mRNA) in the nucleus, in addition to excising introns and joining exons, includes two stages of molecular end modification:
- Capping — attachment of a 5'-cap to the 5'-end of the molecule.
- Polyadenylation — attachment of a 3'-poly(A) tail to the 3'-end.
These biochemical reactions transform the inactive primary transcript into a mature and functionally active messenger RNA, which is ready for export through nuclear pores and subsequent translation.
Which types of RNA polymerases are responsible for the synthesis of rRNA, mRNA, and tRNA in eukaryotes?
In eukaryotes, three different types of nuclear RNA polymerases with a complex quaternary structure are responsible for the synthesis of the main types of RNA:
- RNA polymerase I — transcribes ribosomal RNA (rRNA) genes.
- RNA polymerase II — transcribes protein-coding genes (synthesizes mRNA) as well as small nuclear RNA genes.
- RNA polymerase III — is responsible for the transcription of transfer RNA (tRNA) genes, 5S ribosomal RNA, and certain types of snRNA.
To remember the difference in bases, use the rule: RNA does not contain Thymine, but Uracil instead. And the structure of tRNA is easy to picture as a clover flower, where the stem "holds" the amino acid, and the opposite leaf (anticodon) "reads" the template.
Class and exam questions
Where in the cell does ribosomal RNA synthesis occur?
rRNA synthesis takes place in the nucleus, in the nucleolar region, at special regions of the genome (nucleolar organizers).
What is the characteristic feature of viral RNA?
In many RNA-containing viruses (retroviruses), this molecule acts as the primary keeper of genetic information, and its structure can be not only single-stranded but also double-stranded.
What is splicing and which RNA is involved in it?
Splicing is the process of mRNA maturation in which non-coding regions (introns) are removed from it. Small nuclear RNAs (snRNAs), which form spliceosomes, are actively involved in this process.
What else to study on this topic
- Organization of the nucleolar organizer in human chromosomes
- The mechanism of action of aminoacyl-tRNA synthetase enzymes (codases)
- Differences in the structure of 70S and 80S ribosomal subunits
- Regulatory and endonuclease functions of ribonucleic acids
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