RNA is an essential component of the molecular genetic mechanisms of the cell. The content of ribonucleic acids is a few 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.
The mRNA molecule encodes the amino acid sequence of the protein read from the gene. Therefore, matrix ribonucleic acid is otherwise called informational (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 it has a secondary structure.
There are tens of thousands of mRNA species in a cell, each of which is represented by 10-15 molecules corresponding to a specific site in DNA. mRNA contains information about the structure of one or morebacteria) proteins. The amino acid sequence is presented as triplets of the coding region of the mRNA molecule.
Biological role
The main function of messenger RNA is to implement genetic information by transferring it from DNA to the site of protein synthesis. In this case, mRNA performs two tasks:
- rewrites information about the primary structure of the protein from the genome, which is carried out in the process of transcription;
- interacts with the protein-synthesizing apparatus (ribosomes) as a semantic matrix that determines the amino acid sequence.
Actually, transcription is the synthesis of RNA, in which DNA acts as a template. However, only in the case of messenger RNA, this process has the meaning of rewriting information about the protein from the gene.
It is mRNA that is the main mediator through which the path from genotype to phenotype (DNA-RNA-protein) is carried out.
The lifetime of mRNA in a cell
Messenger RNA lives in the cell for a very short time. The period of existence of one molecule is characterized by two parameters:
- Functional half-life is determined by the ability of mRNA to serve as a template and is measured by the reduction in the amount of protein synthesized per molecule. In prokaryotes, this figure is approximately 2 minutes. During this period, the amount of synthesized protein is halved.
- Chemical half-life is determined by the reduction of messenger RNA molecules capable of hybridization(complementary compound) with DNA, which characterizes the integrity of the primary structure.
Chemical half-life is usually longer than functional half-life, since a slight initial degradation of the molecule (for example, a single break in the ribonucleotide chain) does not yet prevent hybridization with DNA, but already prevents protein synthesis.
Half-life is a statistical concept, so the existence of a particular RNA molecule can be significantly higher or lower than this value. As a result, some mRNAs have time to be translated several times, while others are degraded before the synthesis of one protein molecule is completed.
In terms of degradation, eukaryotic mRNAs are much more stable than prokaryotic ones (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 in prokaryotes and eukaryotes.
The coding region starts with an initiation codon (AUG) and ends with one of the termination codons (UAG, UGA, UAA). Depending on the type of cell (nuclear or prokaryotic), messenger RNA may contain one or more translation regions. In the first case, it is called monocistronic, and in the second - polycistronic. The latter is characteristic only for bacteria and archaea.
Features of the structure and functioning of mRNA in prokaryotes
Transcription processes in prokaryotesand translations take place simultaneously, so messenger RNA has only a primary structure. Just like in 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 (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 about several proteins is encoded in one DNA transcripton, which is subsequently transferred to RNA. A small part of messenger RNA is monocistronic.
Untranslated regions of bacterial mRNA are represented by:
- leader sequence (located at the 5`-end);
- trailer (or end) sequence (located at the 3`-end);
- untranslated intercistronic regions (spacers) - located between the coding regions of polycistronic RNA.
The length of intercistronic sequences can be from 1-2 to 30 nucleotides.
Eukaryotic mRNA
Eukaryotic mRNA is always monocistronic and contains a more complex set of non-coding regions that include:
- cap;
- 5`-untranslated area (5`NTR);
- 3`-untranslated area (3`NTR);
- polyadenyl tail.
The generalized structure of messenger RNA in eukaryotes can be represented asschemes with the following sequence of elements: cap, 5`-UTR, AUG, translated region, stop codon, 3`UTR, poly-A-tail.
In eukaryotes, the processes of transcription and translation are separated both in time and space. The messenger RNA acquires a cap and a polyadenyl tail during maturation, which is called processing, and then is transported from the nucleus to the cytoplasm, where ribosomes are concentrated. Processing also cuts out introns 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 is carried out inside the nucleus, in which DNA is concentrated in the form of chromatin. At the same time, pre-mRNA is synthesized first, which undergoes a number of modifications and only after that 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 loops of bacterial chromatin.
Transcription mechanism
The synthesis of messenger RNA is based on the principle of complementarity of nucleic acids and is carried out by RNA polymerases that catalyze the closure of the phosphodiester bond between ribonucleoside triphosphates.
In prokaryotes, mRNA is synthesized by the same enzyme as other speciesribonucleotides, and in eukaryotes by RNA polymerase II.
Transcription includes 3 stages: initiation, elongation and termination. At the first stage, the polymerase attaches to the promoter, a specialized site that precedes the coding sequence. At the elongation stage, the enzyme builds up the RNA chain by adding nucleotides to the chain that interact complementarily with the template DNA chain.
