Dna & Rna

DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are nucleic acids that are involved in the storage and transmission of genetic information which must be decoded by the cell to make new proteins. Both consist of individual building blocks that are joined to form strands however, DNA consists of two strands that are bound by hydrogen bonds and run in opposite directions (antiparallel) whereas RNA consist of one. The building blocks consist of a pentose ring sugar (deoxyribose in DNA, ribose in RNA) that has one nitrogenous base (either adenine, thymine or uracil in RNA, guanine or cytosine) attached to its 1 carbon. The 2 carbon has either an -OH group (RNA) or -H group (DNA) attached and the 5 carbon is attached to a phosphate group. Thus, both molecules have a sugar-phosphate backbone with differences in code arising due to different sequences of bases. The stability, and hence its use as a permanent source code, of DNA is due to the hydrogen bonding that occurs between the complementary bases present on the antiparallel strands. The bonding pattern follows Chargaff s rules, i.e Adenine only bonds with thymine and guanine only bonds with cytosine. Adenine and guanine are purine bases whereas thymine and guanine are pyrimidine bases. The hydrogen bonding between guanine and cytosine is stronger due to the formation of three bonds compared to two between adenine and thymine (Alberts et al., 2008).

Due to the total length of DNA inside a cell it must be packaged efficiently. To facilitate this, DNA is packaged as chromatin which consists of DNA and supporting histone proteins. When the DNA is wound around histones and package tightly it forms nucleosomes. These nucleosomes coil into a chromatin fibre which loops and anchors to supporting proteins. Further folding results in the formation of a chromosome. In contrast, three classes of RNA have been proposed (Brosius Rabbe, 2016). Messenger RNA and transfer RNA (tRNA) are best characterised due to their essential role in protein production. Although tRNA is a single stranded molecule it is stabilised by intramolecular bonding between bases, and it forms transient bonds with mRNA during primary protein assembly. There is an increasing appreciation of the role of other regulatory, non-coding, RNAs such as micro RNAs that negatively regulate protein production by binding mRNA and triggering degradation (O Brien et al., 2018).

Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., and Walter, P. (2008). Molecular biology of the cell (8th ed.). Garland Science.

Brosius, J., Raabe, C. A. (2016). What is an RNA? A top layer for RNA classification. RNA Biology, 13(2), 140-144.

O`Brien, J., Hayder, H., Zayed, Y., Peng, C. (2018). Overview of MicroRNA biogenesis, mechanisms of actions, and circulation. Frontiers in Endocrinology, 9, 402.