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3 Information: genes and genomes

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Figure 3.1 The DNA helix. Space-filling (a) and ball-and-stick (b) models of a short section of DNA (Protein Data Bank code 1BNA). The form of DNA shown is the B form, which is most prevalent in cells. B DNA consists of two strands of DNA wrapped around each other to form the famous ‘double helix’. The strands are coloured differently here to help you distinguish them. (c) The sculpture ‘Mutual Wrap’ is situated in the grounds of Clare College, Cambridge. It was commissioned to commemorate the discovery of the double helix structure by James Watson and Francis Crick and is one of a series created by the sculptor Charles Jencks to represent the structure of DNA and show how biological information flows from it. Source : Shutterstock.

coronaviruses, one of which, SARS-CoV-2, is the cause of the Covid-19 pandemic. Viruses are not considered to be cells as they cannot replicate independently and rely on some functions of their hosts to do so, so they do not violate the rule that all cells have a DNA genome.

organization of the genome, and its role in reproduction and inherit ance are the main subjects of this chapter, while Chapter 5 explains how the information contained in the genome is ‘read’, directing the synthesis of proteins that carry out cellular functions. While DNA is the material of genomes, it is thought that at the origin of life genetic information was carried in the closely related molecule ribonucleic acid (RNA). Despite being superseded by the DNA genome as cellular life evolved, RNA retains great impor tance as an intermediate in genome-directed protein synthesis. For this reason, in this chapter we describe the basic molecular struc ture of RNA alongside that of DNA. Protein synthesis is a two-stage process. The first stage is transcription , in which the information held in the DNA is copied to make a corresponding messenger RNA molecule, and the second stage is translation , in which the information held in the messenger RNA is used to link amino acids together in a specific order to make a protein. Transcription and translation are described in detail in Chapter 5, but you will find it useful to be aware of these two stages of protein synthesis while studying the material in this chapter. It is also worth noting that certain classes of virus have an RNA genome, including human immunodeficiency virus (HIV) and

3.1 The chemical structures of DNA and RNA

DNA and RNA are termed nucleic acids because DNA was orig inally isolated from cell nuclei. Their chemical compositions are surprisingly simple, to the extent that for decades after the dis covery of nuclear DNA in 1869 scientists had difficulty believ ing that it could constitute the genetic material, instead focusing their attention on proteins. Acceptance of the DNA genome came from experiments such as those by Avery, MacLeod, and McCarty, and by Hershey and Chase in the 1940s and 1950s. We explore the work of Avery and his colleagues in Scientific Process 3.1.

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