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 next, and repeating this builds a long strand with a backbone of alternating sugar and phosphate groups and the bases pointing sideways.

The double helix

A DNA molecule is made of two such strands lying side by side and held together by hydrogen bonds between their bases. Each hydrogen bond is weak, but a molecule has millions of them, so the two strands stay together unless a cell deliberately separates them. The pairing follows a fixed rule: adenine pairs with thymine, and cytosine pairs with guanine.

Because of this rule the two strands are complementary: if one strand reads ATGCCA, the other must read TACGGT opposite it. The paired strands twist around each other into a spiral, the double helix, which is the natural shape of a DNA molecule. A drawing of it is usually simplified to a stick diagram, in which a ladder shows the two sugar-phosphate backbones as the uprights, the bases as the rungs, and the letters of the pairing written in.

A stick diagram is drawn like this. Draw two parallel vertical lines for the backbones and divide them into units, each unit being one nucleotide. Label one sugar and one phosphate on each side, draw a rung between each facing pair of bases, and write the pairing on the rungs (A-T, C-G), with the hydrogen bonds shown as dotted lines.

Functions of DNA

DNA does two jobs. It makes up genes, which carry hereditary information from one generation to the next, because a copy of each gene is passed to the offspring in the gametes. It also contains the coded information that the cell uses to make proteins: the order of bases along a gene determines the order of amino acids in a protein, and the proteins (enzymes, structural proteins, hormones) largely determine how an organism looks and works.

Not all DNA codes for a protein. Some stretches are non-coding DNA, which is not translated into protein. Some of it has a regulating role, switching genes on and off or controlling how much of a protein is made, and some has no known function.

Discovery of the structure

The shape of DNA was worked out in the early 1950s by combining evidence from different people. Rosalind Franklin and Maurice Wilkins, working at King's College London, used X-ray diffraction to produce images of DNA, and Franklin's images showed that the molecule was a helix. James Watson and Francis Crick, at Cambridge, used this evidence together with chemical data to build a model of the double helix, and published it in 1953. Watson, Crick and Wilkins shared the 1962 Nobel Prize; Franklin had died in 1958.

How RNA differs

RNA is also made of nucleotides, but it differs from DNA in three ways. The sugar is ribose instead of deoxyribose, the base uracil (U) takes the place of thymine, and the molecule is single-stranded. These differences are covered in full with protein synthesis.