DNA

Molecular basis of heredity. It is a complex giant molecule that contains, in chemically coded form, the information needed for a cell to make proteins. In other words it determines the order in which amino acids are joined to make a specific protein in a cell. DNA is a ladder-like double-stranded nucleic acid, which forms the basis of genetic inheritance in all organisms, except for a few viruses that have only RNA. DNA is organized into chromosomes and, in organisms other than bacteria, it is found only in the cell nucleus.

DNA is a ladder-like molecule, which means that it is made up of two halves (the ladder sides), formed of chains of nucleotide subunits. Each nucleotide contains a deoxyribose sugar, a phosphate, and a base. A set of three bases – known as a codon – acts as a blueprint for the incorporation of a particular amino acid, the subunit of a protein molecule. The two halves are joined together by the bases – a purine (adenine or guanine) or pyrimidine (cytosine or thymine) – forming pairs (the rungs). The bases form into two specific base pairs: adenine with thymine and guanine with cytosine. The sequence of base pairs along the DNA acts as a code carrying information about the sequence of amino acids in proteins. Three base pairs in sequence (triplet) name an amino acid and the next three name the next amino acid that needs to be joined and so on, to make a specific protein. The specific way in which the pairs form means that the base sequence is preserved from generation to generation. Hereditary information is stored as a specific sequence of bases.

It is important that inherited information is passed on correctly. In the process of DNA replication, which takes place before any cell divides, the two halves of DNA separate and new halves are made. Because of specific base pairing, the inherited information is copied exactly. Despite this, a mistake sometimes occurs and the sequences of bases is altered. This may change the sequence of amino acids in a protein. This is mutation. Ionizing radiation increases the risk of mutation. In plants and animals DNA is organized into linear chromosomes and is found in the nucleus of cells. DNA in bacteria is organized differently. Bacteria have one large circular DNA molecule carrying most of their inherited information. Some bacteria also have small circular molecules of DNA, known as plasmids. These may be used in genetic engineering to transfer genes from one organism to another.