Nucleic acids

Substances found in all living matter that have a fundamental role in the propagation of life. Nucleic acids provide the inherited coded instructions (or “blueprint”) for an organism’s development. There are 2 types of nucleic acid: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). In all plant and animal cells, including human cells, DNA permanently holds the coded instructions, which are translated and implemented by RNA. DNA is the main constituent of chromosomes, which are carried in the nucleus (central unit) of the cell. DNA and RNA are similar in structure, both comprising long, chain-like molecules. However, DNA usually consists of 2 intertwined chains, whereas RNA is generally single-stranded. The basic structure of DNA has been likened to a rope ladder, the chains forming the 2 sides, with interlinking structures in between forming the rungs. The ladder is twisted into a spiral shape called a double helix. Each DNA chain has a “backbone” consisting of a string of sugar and phosphate chemical groups. Attached to each sugar is a chemical called a base, which can be any of 4 types (adenine, thymine, guanine, and cytosine) and forms half a rung of the DNA ladder. The 4 bases can occur in any sequence along the chain. The sequence, which may be many millions of individual bases long, provides the code for the activities of the cell (see genetic code). RNA is like a single strand of DNA; the main difference is that the base thymine is replaced by another base, uracil. DNA controls a cell’s activities by specifying and regulating the synthesis of enzymes and other proteins in the cell. Different genes (sections of DNA that code for information) regulate the production of different proteins. For a particular protein to be made, an appropriate section of DNA acts as a template for an RNA chain. This “messenger” RNA then passes out of the nucleus into the cell cytoplasm, where it is decoded to form proteins (see genetic code; protein synthesis). When a cell undergoes mitotic (see mitosis) division, identical copies of its DNA must go to each of the 2 daughter cells. The 2 DNA chains separate, and 2 more chains are formed, side by side with the original chains. Because only certain base pairings are possible, the new double chains are identical to the original DNA molecule. Each of a person’s cells carries the same DNA replica that was present in the fertilized ovum, so the DNA message passes from one generation of cells to the next.