1.1 Genome description

The common point to all organisms is to own a genome containing the biological information necessary to their construction, maintenance and survival. Most genomes are made of DNA (deoxyribonucleic acid), with the exception of viruses that have an RNA (ribonucleic acid) genome. DNA and RNA are both polymeric molecules composed of chains of monomeric subunits called nucleotides.

The human genome, which is representative of the genomes of all multicellular animals, consists of two parts:

  • The nuclear genome including about \(3.2 \times 10^9\) nucleotides of DNA divided into 24 linear molecules, the chromosomes. These 24 chromosomes consist of 22 autosomes and two sex chromosomes, X and Y.

  • The mitochondrial genome is a circular DNA molecule present in multiple copies in the organelles called mitochondria. The human mitochondrial genome contains 37 genes.

In the animalia taxon, the vast majority of cells are diploid which means that each autosome are present in two copies plus two sex chromosomes, XX for females and XY for males. These cells are known as somatic cells in contrast to sex cells, or gametes, which are haploid and possess only one copy of each chromosome. The use of the biological information contained in the DNA requires the coordinated action of several proteins participating in a series of complex biochemical reactions referred to as genome expression. The direct product of genome expression is the transcriptome, a collection of RNA molecules derived from the protein-coding genes. The transcriptome is maintained by the process of transcription, in which individual genes are copied into RNA molecules. The indirect product of genome expression is the proteome, the cell’s collection of proteins. The proteins constituting the proteome are synthesized by translation of the individual RNA molecules present in the transcriptome.

DNA is a polymer, a polynucleotide, in which the monomeric subunits are four chemically distinct nucleotides linked together in chains that can reach length of thousands, even millions of units in length (Figure 1.1). Each nucleotide in a DNA polymer is made up of three components: a deoxyribose, which is a pentose, a nitrogenous base (cytosine, thymine, adenine or guanine) and a phosphate group. A molecule made up of just the pentose and base is called a nucleoside and adding a phosphate group converts it into a nucleotide.

Double-Helix structure of DNA molecule. University of Leicester / Licence Creative commons

Figure 1.1: Double-Helix structure of DNA molecule. University of Leicester / Licence Creative commons

What makes DNA such a unique molecule is its famous double-helix structure discovered by (Crick and Watson 1954). The key feature of the double-helix structure that convinced biologists that genes are made of DNA is the constrained base pairing between the nucleotides. Indeed, the limitation that adenosine can only be paired with thymine, and guanine with cytosine, means that DNA replication can result in perfect copies of a parent molecule simply by using the sequences of the pre-existing strands to build the sequences of the new strands.

References

Crick, F.H.C., and J.D. Watson. 1954. “The Complementary Structure of Deoxyribonucleic Acid.” Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences 223 (1152): 80–96.