DNA

The molecular building blocks of life are DNA, RNA and proteins. DNA stores the information of the protein structure, RNA participates in the assembly of the proteins, and the proteins are the final devices that perform the tasks. Two other prominent classes of molecule are lipids (they form membranes and, thus, compartments), and sugars (they are the product of the photosynthesis; ultimately life depends on the energy from the Sun). Sub-units from these building blocks show up in different contexts. For example ATP is the ubiquitous energy storing molecule, but adenine (the A in ATP) is also one of the DNA bases; similarly, the sugar ribose is also a component of the DNA backbone.

Before entering into the details of the basic building units, let us list their respective sizes. We do this with respect to their information content. DNA is used as the main information storage molecule. The information resides in the sequence of four bases: A, T, C and G. Three subsequent bases form a codon, which codes for one of the 20 amino acids. The genetic code is shown in Table 3.1. The weight of a codon storage unit is larger for DNA (double stranded and 330 Da per nucleotide, 1 Da = 1 g/mol) than for RNA (single stranded) and proteins.

1. DNA: 2000 Da per codon,

2. RNA: 1000 Da per codon,

3. proteins: 110 Da per amino acid.

Thus whereas a 300 amino acid protein weighs 33 kg/mol, the stored information at the DNA level has a mass of 600 kg/mol.