According to AbbreviationFinder, RNA stands for Ribonucleic Acid.
RNA types
Messenger RNA (mRNA) is the type of RNA that carries information from DNA to ribosomes, the site of protein synthesis. The nucleotide sequence of the mRNA determines the amino acid sequence of the protein. Therefore, the mRNA is called coding RNA.
However, many RNAs do not code for proteins, and are called noncoding RNAs; they originate from their own genes (RNA genes), or they are the introns rejected during the splicing process. Non-coding RNAs are transfer RNA (tRNA) and ribosomal RNA (rRNA), which are fundamental elements in the translation process, and various types of regulatory RNAs.
Certain non-coding RNAs, called ribozymes, are capable of catalyzing chemical reactions such as cutting and joining other RNA molecules, or forming peptide bonds between amino acids on the ribosome during protein synthesis.
RNA involved in protein synthesis
- Messenger RNA. Messenger RNA (mRNA or mRNA) carries information about the protein’s amino acid sequence from the DNA, where it is inscribed, to the ribosome, where the cell’s proteins are synthesized. It is, therefore, an intermediary molecule between DNA and protein and the name “messenger” is entirely descriptive. In eukaryotes, mRNA is synthesized in the nucleoplasm of the cell nucleus and from there it accesses the cytosol, where the ribosomes are found, through the pores of the nuclear envelope.
- Transfer RNA. Transfer RNAs (tRNAs or tRNAs) are short polymers of about 80 nucleotides that transfer a specific amino acid to the growing polypeptide; they bind to specific locations on the ribosome during translation. They have a specific site for the binding of the amino acid (3 ‘end) and an anticodon formed by a triplet of nucleotides that binds to the complementary codon of the mRNA by means of hydrogen bonds.
- Ribosomal RNA. Ribosomal RNA (rRNA or rRNA) is found combined with proteins to form ribosomes, where it represents about 2/3 of their parts. In prokaryotes, the major subunit of the ribosome contains two rRNA molecules and the minor subunit one. In eukaryotes, the largest subunit contains three rRNA molecules and the smallest one.
In both cases, specific proteins are associated on the framework constituted by the rRNAs. RRNA is very abundant and represents 80% of the RNA found in the cytoplasm of eukaryotic cells. Ribosomal RNAs are the catalytic component of ribosomes; They are responsible for creating peptide bonds between the amino acids of the polypeptide in formation during protein synthesis; thus they act as ribozymes.
Regulatory RNAs
Many types of RNA regulate gene expression because they are complementary to specific regions of mRNA or genes in DNA.
- RNA interference. Interfering RNAs (RNAi or iRNA) are RNA molecules that suppress the expression of specific genes through mechanisms known globally as ribointerference or RNA interference. Interfering RNAs are small molecules (20-25 nucleotides) that are generated by fragmentation of longer precursors. They can be classified into three large groups:
– Micro RNA. Micro RNAs (miRNAs or miRNAs) are short chains of 21 or 22 nucleotides found in eukaryotic cells that are generated from specific precursors encoded in the genome.
When transcribed, they fold into intramolecular hairpins and then bind to enzymes forming an effector complex that can block the translation of the mRNA or accelerate its degradation starting with the enzymatic removal of the poly A tail:
– Small interfering RNA. Small interfering RNAs (siRNAs or siRNAs), consisting of 20-25 nucleotides, are often produced by viral RNA cleavage, but can also be endogenous in origin. After transcription, it is assembled into a protein complex called RISC (RNA-induced silencing complex) that identifies the complementary mRNA that is cut into two halves that are degraded by the cellular machinery, thus blocking the expression of the gene.
– RNA associated with Pili.
The RNAs associated with Piwi are chains of 29-30 nucleotides, typical of animals; they are generated from long single-stranded precursors, in a process that is independent of Drosha and Dicer. These small RNAs associate with a subfamily of “Argonaut” proteins called the Piwi proteins. Germ line cells are active; they are believed to be a defensive system against transposons and to play a role in gametogenesis.
- Antisense RNA. An antisense RNA is the complementary (non-coding) strand of an mRNA (coding) strand.
Most inhibit genes, but a few activate transcription. Antisense RNA pairs with its complementary mRNA to form a double-stranded molecule that cannot be translated and is enzymatically degraded. Introduction of a transgene encoding an antisense mRNA is a technique used to block the expression of a gene of interest. A radioactively labeled antisense mRNA can be used to display the level of gene transcription in various cell types. Some antisense structural types are experimental, as they are used as antisense therapy.
- Long non-coding RNA. Many long noncoding RNAs (long cRNA or long ncRNA) regulate gene expression in eukaryotes; one of them is the Xist that covers one of the two X chromosomes in female mammals, inactivating it (Barr’s corpuscle).
- Riboswitch. A riboswitch is a part of the mRNA (messenger ribonucleic acid) to which small molecules can bind that affect the activity of the gene. Therefore, an mRNA containing a riboswitch is directly involved in the regulation of its own activity which depends on the presence or absence of the signaling molecule. Such riboswitchs are found in the 5 ‘untranslated region (5′-UTR), located before the start codon (AUG), and / or in the 3′ untranslated region (3’-UTR), also called trailing sequence., located between the stop codon (UAG, UAA or UGA) and the poly A tail.
