DNA stores, proteins work, RNA does a bit of both: life's giant molecules
Every living thing depends on three long chain molecules, each shaped for a different job. DNA is built for safekeeping, proteins for getting chemistry done, and RNA sits awkwardly between them, able to carry genetic instructions and even catalyse reactions, but doing neither quite as well as its specialist cousins.
A macromolecule is a very heavy molecule made by repeating small units over and over. The category spans nylon, polyethylene and synthetic rubber as well as the biopolymers of life: proteins are amino acids linked by peptide bonds, DNA and RNA are chains of nucleotides, and starch, cellulose and chitin are strings of sugars. Polyethylene is made on such a vast scale that ethylene is the chemical industry's main product. In nature, lignin makes up about a third of the mass of trees.
DNA, RNA and proteins are mostly unbranched, like beads on a string, and the order of the beads matters. DNA's advantage for storing the genome comes from three features. It is double-stranded, so each gene exists in at least two copies, with bases paired G to C and A to T across the helix. It lacks a particular hydroxyl group on each sugar, making it far more resistant to breakdown than RNA. And cells run elaborate systems to monitor and repair it.
Proteins are the workers, powering everything from photosynthesis to vision and movement. Because they are single-stranded and assembled from 20 or more different amino acids, they fold into an enormous range of shapes with pockets that grip other molecules, and that chemical variety lets many act as enzymes.
RNA can do some of each. Messenger RNA carries the instructions for building proteins, and many viruses keep their entire genome in RNA. Being single-stranded, it can fold into shapes that bind molecules and speed reactions. Yet it breaks down quickly and has no repair crew, which makes it a poor long-term archive, and with only 4 building blocks instead of more than 20, its catalysts are less versatile than proteins. In short, DNA makes RNA, and RNA makes proteins.
Source: Macromolecule
