Types and Functions of RNA

The role of RNA and the central dogma of molecular biology

In both prokaryotic and eukaryotic organisms, all hereditary information is securely encoded in DNA molecules. However, DNA itself is unable to directly participate in the creation of protein structures. For instructions to be implemented, the cell requires mediator molecules. It is RNA that assumes this function, copying the necessary data and transporting them from the nucleus to the cytoplasm.

This mechanism is described by the central dogma of molecular biology. The essence of the dogma lies in the strict direction of the flow of genetic information: from DNA to RNA, and then to protein. The only known exception to this fundamental rule is the so-called RNA viruses. In these specific intracellular parasites, the ribonucleic acid molecule assumes the role of a full-fledged carrier of the genetic code.

RNAs ensuring translation (protein synthesis)

The first major group of ribonucleic acids is directly involved in the assembly of protein molecules. This category includes the most prominent representatives:

• Messenger RNAs (mRNA). These are large molecules whose size can reach up to 100,000 nucleotides. Their main task is to serve as a precise template from which the sequence for the synthesis of a specific protein is read.
• Ribosomal RNAs (rRNA). In size, they are comparable to mRNA (also up to 100,000 nucleotides). They form the physical framework of ribosomes, performing both structural and crucial catalytic functions during the assembly of the polypeptide chain.
• Transfer RNAs (tRNA). These are relatively small molecules up to 100 nucleotides in size. Their mission is to capture free amino acids and deliver them directly to the active ribosomes.

In addition, there is a rare type — transfer-messenger RNA (tmRNA). Their size ranges from 350 to 450 nucleotides. These molecules are found in many bacteria as well as in plant plastids. They perform a quality control function: they recognize and initiate the destruction of defective messenger RNAs lacking stop codons in their structure.

Participants in RNA interference

The next group unites molecules that specialize in suppressing (inhibiting) gene activity. This process is called RNA interference.

1. MicroRNAs (miRNA). These are very short endogenous molecules only 21–22 nucleotides long. They are single-stranded but possess self-complementary properties. The main function of microRNAs is to block translation or to targetedly remove the poly(A) tail in messenger RNAs.
2. Small interfering RNAs (siRNA). Their size is slightly larger — from 21 to 25 nucleotides, and unlike the previous type, they are represented by double-stranded structures. These molecules effectively suppress gene activity both directly during transcription and at the post-transcriptional stage.

RNAs modifying other molecules

The third functional group is responsible for the maturation, structural modification, and degradation of other ribonucleic acids. Without their work, the formation of functional molecules would be impossible.

• Small nuclear RNAs (snRNA). They are up to 400 nucleotides long. They are key participants in splicing — a complex mechanism in which non-coding regions (introns) are excised from the primary transcript and coding fragments (exons) are joined together.
• Small nucleolar RNAs (snoRNA). They take responsibility for the fine chemical modification of other types, including rRNA, tRNA, and small nuclear RNAs.
• Ribozymes (catalytic RNAs). The size of these structures varies from several dozens to hundreds of nucleotides. They are unique in that they possess enzymatic (catalytic) activity and are capable of cleaving any type of ribonucleic acid in the cell.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanism of recognizing defective mRNAs lacking stop codons via tmRNA
• The splicing process: excision of introns and joining of exons
• Mechanism of RNA interference and gene activity suppression
• Chemical modification of ribosomal and transfer RNAs in the nucleolus
• Catalytic mechanism of ribozyme action

Other topics in "Molecular Genetics"

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