RNA – Structure and Types

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  • Ribonucleic acid (RNA) is a complex compound with high molecular weight. It participates in protein synthesis within cells and also has the ability to carry genetic codes instead of DNA in some viruses.
  • When examining the structure of RNA, it is possible to notice that it’s made up of ribose nucleotides which are nitrogenous bases attached to a ribose sugar. These are joined by phosphodiester bonds that generate strands with different lengths. Adenine, cytosine, guanine and uracil are the nitrogenous bases in RNA. Uracil is the substitute of thymine in DNA.
  • The RNA’s ribose sugar has a cylindrical structure containing one oxygen and five carbons. RNA is likely to undergo hydrolysis due to the attachment of a reactive hydroxyl group (-OH) to the ribose sugar molecule’s second carbon group. DNA lacks a reactive hydroxyl group within its sugar (deoxyribose) at the same place as in RNA and so it is believed to be among the reasons why DNA has evolved to act as the genetic information carrier within a greater number of organisms.
  • In 1965, R.W. Holley described the molecular structure of the RNA. Ideally RNA is a biopolymer with single strands. The self-complementary sequences within the strands of RNA result in the intrachain base pairing as well as the coiling of ribonucleotide chain to form structurally complex forms containing helices and bulges.
  • The RNA’s 3D structure is important to its function and stability as it facilitates the nitrogenous bases and the ribose sugar to be altered in several different ways by the help of cellular enzymes that affix chemical groups like methyl groups to the chain. Modifications such as these will make the chemical bond formation between distant areas in the RNA strand possible resulting in complex twists in the chain of RNA which stabilizes the RNA structure even further.
  • Molecules with weak stabilization and structural modifications maybe destroyed quickly. For example, alteration at position 58 of an initiator transfer RNA (tRNA) molecule which does not have a methyl group could make the molecule unstable and nonfunctional. The quality control mechanisms of cellular tRNA will then destroy the nonfunctional chain.
  • RNAs have also the ability to form complexes with certain molecules called ribonucleoproteins (RNPs). At least one cellular RNP’s RNA portion is shown to behave as a biological catalyst, which is a role previously linked to proteins only.
  • The three most widely known and the commonest types of RNA that are being studied are ribosomal RNA (rRNA), transfer RNA (tRNA) and messenger RNA (mRNA). They are found in all organisms.
  • Other types of RNAs including the above mentioned three engage in biochemical reactions just like enzymes. However, in addition to that, some also carry out complex regulatory functions within cells. Due to the RNA’s engagement in several regulatory processes, versatile functions and abundance, they could be considered to play crucial roles in both diseases and general cellular processes.
  • During protein synthesis, mRNA carries genetic codes to ribosomes from the DNA in the nucleus. Protein translation in the cytoplasm occurs at the site of ribosomes. Ribosomes are made up of protein and rRNA. The protein subunits of ribosomes are synthesized in the nucleolus though encoded by rRNA. Once ready, these vital regulators of translation move to the cytoplasm to ‘read’ the code that is carried by mRNA. A specific amino acid which makes up a protein is incorporated within the sequence of three nitrogenous bases in mRNA.
  • The tRNA molecules, sometimes called the activator or soluble RNA, bring the specific amino acids to the ribosomes to form proteins upon linking. Molecules of tRNA have less than hundred nucleotides.
  • In addition to rRNA, tRNA and mRNA, RNAs could be categorized broadly into noncoding (ncRNA) and coding RNA (cRNA). The ncRNAs are of two types called the regulatory ncRNAs and housekeeping ncRNAs (rRNA and tRNA) which are divided further according to their size. Small ncRNAs contain less than two hundred nucleotides while long ncRNAs (IncRNA) contain a minimum of two hundred nucleotides.

Small ncRNAs can be divided further as follows.

  1. PIWI- interacting RNA (piRNA)
  2. Small- interfering RNA (siRNA)
  3. Small nuclear RNA (snRNA)
  4. Small nucleolar RNA (snoRNA)
  5. Micro RNA (miRNA)