All
living organisms are dependent on three essential
biopolymers for their biological functions:
DNA,
RNA and
proteins. Each of these molecules is required for life since each plays a distinct, indispensable role in the
cell. The simple summary is that
DNA makes RNA, and then RNA makes proteins.
DNA,
RNA, and proteins all consist of a repeating structure of related building blocks (
nucleotides in the case of DNA and RNA,
amino acids in the case of proteins). In general, they are all unbranched polymers, and so can be represented in the form of a string. Indeed, they can be viewed as a string of beads, with each bead representing a single nucleotide or amino acid monomer linked together through
covalent chemical bonds into a very long chain. In most cases, the monomers within the chain have a strong propensity to interact with other amino acids or nucleotides. In DNA and RNA, this can take the form of
Watson–Crick base pairs (
G–
C and
A–
T or A–
U), although many more complicated interactions can and do occur.
Structural features Because of the double-stranded nature of DNA, essentially all of the nucleotides take the form of
Watson–Crick base pairs between nucleotides on the two complementary strands of the
double helix. In contrast, both RNA and proteins are normally single-stranded. Therefore, they are not constrained by the regular geometry of the DNA double helix, and so fold into complex
three-dimensional shapes dependent on their sequence. These different shapes are responsible for many of the common properties of RNA and proteins, including the formation of specific
binding pockets, and the ability to catalyse biochemical reactions.
DNA is optimised for encoding information DNA is an information storage macromolecule that encodes the complete set of
instructions (the
genome) that are required to assemble, maintain, and reproduce every living organism. DNA and RNA are both capable of encoding genetic information, because there are biochemical mechanisms which read the information coded within a DNA or RNA sequence and use it to generate a specified protein. On the other hand, the sequence information of a protein molecule is not used by cells to functionally encode genetic information. The single-stranded nature of protein molecules, together with their composition of 20 or more different amino acid building blocks, allows them to fold in to a vast number of different three-dimensional shapes, while providing binding pockets through which they can specifically interact with all manner of molecules. In addition, the chemical diversity of the different amino acids, together with different chemical environments afforded by local 3D structure, enables many proteins to act as
enzymes, catalyzing a wide range of specific biochemical transformations within cells. In addition, proteins have evolved the ability to bind a wide range of
cofactors and
coenzymes, smaller molecules that can endow the protein with specific activities beyond those associated with the polypeptide chain alone.
RNA is multifunctional RNA is multifunctional, its primary function is to
encode proteins, according to the instructions within a cell's DNA. They control and regulate many aspects of protein synthesis in
eukaryotes. RNA encodes genetic information that can be
translated into the amino acid sequence of proteins, as evidenced by the messenger RNA molecules present within every cell, and the RNA genomes of a large number of viruses. The single-stranded nature of RNA, together with tendency for rapid breakdown and a lack of repair systems means that RNA is not so well suited for the long-term storage of genetic information as is DNA. In addition, RNA is a single-stranded polymer that can, like proteins, fold into a very large number of three-dimensional structures. Some of these structures provide binding sites for other molecules and chemically active centers that can catalyze specific chemical reactions on those bound molecules. The limited number of different building blocks of RNA (4 nucleotides vs >20 amino acids in proteins), together with their lack of chemical diversity, results in catalytic RNA (
ribozymes) being generally less-effective catalysts than proteins for most biological reactions. == Branched biopolymers ==