South African NSC Life Sciences

Nucleic acids are the molecules that store and use the instructions for building an organism. There are two kinds, **DNA** (deoxyribonucleic acid) and **RNA** (ribonucleic acid), and both are polymers made of repeating units called **nucleotides**. ## Where DNA is found Most of a eukaryotic cell's DNA is in the nucleus, where it is called nuclear DNA. It is wound together with protein into **chromosomes**, and a stretch of DNA that carries the code for one characteristic (in practice, for one protein or one functional RNA) is a **gene**. Chromatin is the loosely coiled form of this DNA and protein when the cell is not dividing; it condenses into visible chromosomes when the cell is about to divide. A small amount of DNA is found outside the nucleus. Mitochondria contain their own circular DNA, in animals as well as plants, and chloroplasts contain DNA too. This extranuclear DNA codes for some of the proteins these organelles need, and in humans mitochondrial DNA is passed on only through the egg cell, which matters later when it is used to trace female ancestry. ## The nucleotide Every DNA nucleotide is built from three parts joined together: - a phosphate group; - a five-carbon sugar called deoxyribose; - a nitrogenous base, which is one of four: adenine (A), thymine (T), cytosine (C) or guanine (G). The phosphate and sugar are the same in every nucleotide, so the four kinds of nucleotide differ only in their base. The sugar of one nucleotide bonds to the phosphate of the…