RNA is an essential component of the molecular genetic mechanisms of a cell. The content of ribonucleic acids is several percent of its dry weight, and about 3-5% of this amount falls on messenger RNA (mRNA), which is directly involved in protein synthesis, contributing to the implementation of the genome.
An amino acid sequence of a protein read from the gene is encoded in the mRNA molecule. Therefore, matrix ribonucleic acid is otherwise called information (mRNA).
general characteristics
Like all ribonucleic acids, messenger RNA is a chain of ribonucleotides (adenine, guanine, cytosine and uracil) connected to each other by phosphodiester bonds. Most often, mRNA has only a primary structure, but in some cases a secondary one.
The cell contains tens of thousands of types of mRNA, each of which is represented by 10-15 molecules corresponding to a specific site in the DNA. The mRNA contains information about the structure of one or more (in bacteria) proteins. Amino acid sequence is presented in the form of triplets of the coding region of the mRNA molecule.
Biological role
The main function of messenger RNA is to realize genetic information by transferring it from DNA to the site of protein synthesis. At the same time, mRNA performs two tasks:
- transcribes information on the primary structure of the protein from the genome, which is carried out in the process of transcription;
- interacts with a protein synthesizing apparatus (ribosomes) as a semantic matrix that determines the sequence of amino acids.
Actually, transcription is an RNA synthesis in which DNA acts as a template. However, only in the case of messenger RNA does this process have the value of transcribing protein information from a gene.
It is mRNA that is the main mediator, with the help of which the path from the genotype to the phenotype (DNA-RNA protein) is carried out.
The lifetime of mRNA in the cell
Matrix RNA does not live in the cell for very long. The period of existence of one molecule is characterized by two parameters:
- The functional half-life is determined by the ability of mRNA to serve as a matrix and is measured by the decrease in the amount of protein synthesized from one molecule. In prokaryotes, this figure is about 2 minutes. During this period, the amount of synthesized protein is reduced by half.
- The chemical half-life is determined by a decrease in the molecules of messenger RNAs capable of hybridization (complementary connection) with DNA, which characterizes the integrity of the primary structure.
The chemical half-life is usually longer than the functional, since insignificant initial degradation of the molecule (for example, a single gap in the ribonucleotide chain) does not prevent hybridization with DNA, but already inhibits protein synthesis.
Half-life is a statistical concept; therefore, the existence of a specific RNA molecule may turn out to be significantly higher or lower than this value. As a result, some mRNAs manage to be translated several times, while others degrade before the synthesis of one protein molecule ends.
In terms of degradation, eukaryotic mRNAs are much more stable than prokaryotic (half-life is about 6 hours). For this reason, they are much easier to isolate from the cell intact.
MRNA structure
The nucleotide sequence of messenger RNA includes translated regions in which the primary structure of the protein is encoded, and non-informative regions, the composition of which differs between prokaryotes and eukaryotes.
The coding region begins with the initiating codon (AUG) and ends with one of the terminating codons (UAG, UGA, UAA). Depending on the type of cell (nuclear or prokaryotic), messenger RNA may contain one or more translational sites. In the first case, it is called monocistronic, and in the second - polycistronic. The latter is characteristic only of bacteria and archaea.
Features of the structure and functioning of mRNA in prokaryotes
In prokaryotes, transcription and translation processes occur simultaneously, therefore, messenger RNA has only a primary structure. Like eukaryotes, it is represented by a linear sequence of ribonucleotides, which contains informational and non-coding regions.
Most mRNAs of bacteria and archaea are polycistronic (they contain several coding regions), which is due to the peculiarity of the organization of the prokaryotic genome, which has an operon structure. This means that information on several proteins is encoded in one DNA transcripton, which is subsequently transferred to RNA. A small portion of messenger RNA is monocistronic.
The untranslated areas of bacterial mRNA are:
- leader sequence (located at the 5`-end);
- trailer (or trailer) sequence (located at the 3`-end);
- untranslated intercistronic regions (spacers) are located between the coding regions of polycistronic RNA.
The length of the intercistronic sequences may consist of 1-2 to 30 nucleotides.
Eukaryotic mRNA
Eukaryotic mRNA is always monocistronic and contains a more complex set of non-coding regions, which include:
- cap;
- 5`-untranslated region (5` NTO);
- 3`-untranslated region (3` NTO);
- polyadenyl tail.
The generalized structure of matrix RNA in eukaryotes can be represented in the form of a scheme with the following sequence of elements: cap, 5`-NTO, AUG, broadcast region, stop codon, 3` NTO, poly-A-tail.
In eukaryotes, the processes of transcription and translation are disconnected both in time and in space. Cap and polyadenyl tail, the messenger RNA acquires during maturation, which is called processing, and then is transported from the nucleus to the cytoplasm, where the ribosomes are concentrated. During processing, introns are also excised that are transferred to RNA from the eukaryotic genome.
Where ribonucleic acids are synthesized
All types of RNA are synthesized by special enzymes (RNA polymerases) based on DNA. Accordingly, the localization of this process in prokaryotic and eukaryotic cells is different.
In eukaryotes, transcription takes place inside the nucleus, in which DNA is concentrated in the form of chromatin. In this case, first pre-mRNA is synthesized, which undergoes a number of modifications and only after that it is transported to the cytoplasm.
In prokaryotes, the place where ribonucleic acids are synthesized is the region of the cytoplasm bordering the nucleoid. RNA synthesizing enzymes interact with despiralized bacterial chromatin loops.
Transcription mechanism
The synthesis of matrix RNA is based on the principle of complementarity of nucleic acids and is carried out by RNA polymerases, which catalyze the closure of the phosphodiester bond between ribonucleoside triphosphates.
In prokaryotes, mRNA is synthesized by the same enzyme as other types of ribonucleotides, and in eukaryotes by RNA polymerase II.
Transcription includes 3 stages: initiation, elongation and termination. At the first stage, polymerase joins the promoter - a specialized site that precedes the coding sequence. At the elongation stage, the enzyme builds up the RNA chain by attaching nucleotides to the chain, complementary interacting with the DNA template chain.