RNA: Structure, Types and Functions in Cells

Ribonucleic Acid (RNA)

Ribonucleic acid (RNA) is a nucleic acid formed by a chain of ribonucleotides. It is present in both prokaryotes and eukaryotes and is the genetic material of certain viruses (RNA viruses). In cells, RNA is usually a single-stranded molecule, although in some viruses the genome is formed by a double-stranded RNA molecule.

Cellular RNAs perform a variety of functions. Some of these molecules direct the synthesis of proteins, since DNA cannot act alone. RNA transfers vital information for protein synthesis, and protein production requires the cell to use its RNA molecules. Several types of RNA regulate gene expression, while others have catalytic activity. RNA is, therefore, a very versatile molecule.

Chemical Composition of RNA

The sugar component of RNA nucleotides is ribose, instead of the deoxyribose found in DNA. Also, RNA contains the nitrogenous bases adenine (A), guanine (G), cytosine (C) and uracil (U), which replaces thymine (T) found in DNA.

cDNA is a short DNA sequence derived from RNA. Due to biological and chemical limitations, a cDNA molecule has a structure similar to that of RNA, but it is formed by DNA and has an identical phosphodiester bond.

RNA is almost always a single-stranded (monostranded) molecule, although in certain situations, such as tRNA and rRNA, it can form complex structures. While DNA contains genetic information, RNA expresses that information, from a linear sequence of nucleotides to a linear sequence of amino acids in a protein.

Structure of RNA and Chemistry

RNA is formed by a chain of repetitive monomers called nucleotides. Nucleotides are linked one after another through negatively charged phosphodiester bonds.

Each nucleotide is formed by three molecules: a five-carbon monosaccharide (pentose) called ribose (deoxyribose in DNA), one phosphate group, and one of four possible nitrogenous bases: adenine, guanine, uracil (thymine in DNA) and cytosine. The carbons of ribose are numbered from 1' to 5' in a clockwise direction. The nitrogenous base is attached to the 1' carbon; the phosphate group is attached to the 5' carbon, and the 3' carbon of the ribose sugar is the point where the next nucleotide is linked.

The negative charge at physiological pH gives RNA its polyanionic character. Purine bases (adenine and guanine) can form hydrogen bridges with pyrimidine bases (uracil and cytosine) according to the pairing rules G≡C and A=U. Additionally, other interactions are possible, such as base stacking or triplets, with the sugar-phosphate backbone.

RNA contains multiple nucleotides, including common nucleotides and modified nucleotides, which originate from typical nucleotides after transformation. Characteristic examples include transfer RNA (tRNA) and ribosomal RNA (rRNA), and methylated nucleotides are also found in eukaryotic messenger RNA.

Secondary Structure

Unlike DNA, RNA molecules are usually single-stranded and do not form extensive double helices. However, RNA folds as a result of the presence of short regions with intramolecular complementary base pairing, that is, base pairs formed between complementary sequences within the same strand. tRNA, for example, has approximately 60% of its bases paired, forming four arms with a double helix structure.

An important structural characteristic of RNA that distinguishes it from DNA is the presence of a hydroxyl group in the 2' position of ribose. Because of this, RNA double helices adopt an A-form structure, which differs from the B-form common in DNA.

A second consequence of the hydroxyl group is that the phosphodiester bonds of RNA in double-helical regions are susceptible to chemical hydrolysis. RNA phosphodiester bonds are rapidly hydrolyzed in alkaline solution, while DNA bonds are stable. The average life of RNA molecules is much shorter than that of DNA: minutes for bacterial RNA, and days for human RNA.

Tertiary Structure

The tertiary structure of RNA results from the stacking of bases and different types of hydrogen bonds between parts of the molecule. tRNA is a good example. In solution, its chains are folded into a compact structure stabilized by conventional Watson-Crick pairings (A=U, C≡G) and by interactions between nucleotide bases, such as triplets. Oxygen atoms can donate hydrogen bonds to the phosphodiester backbone, and the 2' carbon of ribose is also an important donor and acceptor of hydrogen bonds.

Types of RNA

Messenger RNA (mRNA) carries the genetic information from DNA to the ribosomes, where protein synthesis takes place. The nucleotide sequence of mRNA determines the amino acid sequence of the protein. For this reason, it is called coding RNA.

However, many RNAs are noncoding RNAs, which do not produce proteins. Some of these originate from their own genes or from introns removed during splicing. Coding RNAs include messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA), which play fundamental roles in translation, as well as regulatory RNAs of various types.

Some noncoding RNAs, called ribozymes, are capable of catalyzing chemical reactions, such as cutting other RNA molecules or forming peptide bonds in the ribosome during protein synthesis. These RNAs are involved in protein synthesis.

Messenger RNA

Messenger RNA (mRNA) carries information about the amino acid sequence of a protein from DNA, where it is inscribed, to the ribosome, where proteins are synthesized in the cell. Therefore, the mRNA molecule is an intermediate between DNA and protein, which is why it is called "messenger". In eukaryotic cells, mRNA is synthesized in the nucleus and then transported to the cytosol through the nuclear envelope, where ribosomes are located.

Transfer RNA

Transfer RNA (tRNA) is a short polymer of approximately 80 nucleotides that transfers a specific amino acid to the growing polypeptide chain. It has a specific site for attaching the amino acid (3' end) and an anticodon formed by a triplet of nucleotides that is complementary to the mRNA codon through hydrogen bonds. tRNA binds to the ribosome during translation.

Ribosomal RNA

Ribosomal RNA (rRNA) is combined with proteins to form ribosomes, where it represents about two-thirds of their mass. In prokaryotes, the large ribosomal subunit contains two rRNA molecules and one small molecule, while the small subunit contains one rRNA molecule. In eukaryotes, the large subunit contains three rRNA molecules and the small subunit contains one. In both cases, the ribosomal structure is constituted by rRNA associated with specific proteins.

rRNA is a very abundant RNA and makes up about 80% of the RNA in the cytoplasm of eukaryotic cells. Ribosomal RNA is the main catalytic component of ribosomes: it catalyzes the formation of peptide bonds between amino acids during protein synthesis, acting as a ribozyme.