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.

Protein synthesis Protein synthesis is quite complex. The stages are the coding by triplets of DNA bases to produce mRNA, the linking of mRNA to tRNA at ribosomes, and the linking of amino acids to form protein. Since DNA is in the nucleus of the cell and the amino acids are joined in the cytoplasm the information on the DNA has to be copied so that it can be taken into the cytoplasm. Sections of DNA that code for one protein are copied, making molecules called mRNA (messenger RNA). The mRNA moves out of the nucleus into the cytoplasm. In the cytoplasm, mRNA joins a ribosome. Then molecules called tRNA (transfer RNA) bring amino acids. The amino acids are arranged in sequence following the code on the mRNA, and then they are joined to make protein.

Codons Geneticists identify the codons by the initial letters of the constituent bases – for example, the base sequence of codon CAG is cytosine–adenine–guanine. The meaning of each of the codons in the genetic code has been worked out by molecular geneticists. There are four different bases, which means that there must be 4 × 4 × 4 = 64 different codons. Proteins are usually made up of only 20 different amino acids, so many amino acids have more than one codon (for example, GGT, GGC, GGA, and GGG all code for the same amino acid, glycine). The first chromosome to have its DNA sequenced by geneticists was chromosome 22 (one of the smallest human chromosomes, linked to schizophrenia), which had its estimated 800 genes and 33.5 million bases sequenced in 1999, leaving only a few gaps.

Blueprint for the organism The information encoded by the codons is transcribed (see transcription) by messenger RNA and is then translated into amino acids in the ribosomes and cytoplasm. The sequence of codons determines the precise order in which amino acids are linked up during manufacture and, therefore, the kind of protein that is to be produced. Because proteins are the chief structural molecules of living matter and, as enzymes, regulate all aspects of metabolism, it may be seen that the genetic code is effectively responsible for building and controlling the whole organism.

Laboratory techniques The sequence of bases along the length of DNA can be determined by cutting the molecule into small portions, using restriction enzymes. This technique can also be used for transferring specific sequences of DNA from one organism to another.

Ancient DNA DNA is surprisingly stable – the DNA of a nemonychid weevil trapped in amber from the Cretaceous period, and estimated as being 120–135 million years old has been extracted and analysed. US researchers extracted DNA from human hair 10,000 years old in 1994. Dinosaur DNA was also extracted in 1994 from unfossilized dinosaur bones found in a coal mine in Utah.

Dimensions of DNA If the DNA of one human cell was unwound it would be almost 2 m/6.6 ft in length. The cell nucleus that contains the DNA is only 10 micrometres in diameter.

DNA

Microsoft's marketing name for a software architecture announced in 1997 and designed to extend the use of Microsoft's PC-based client/server products and standards – including COM and ActiveX – onto and across the Internet. One of the aims is to enable programmers used to working on personal computers in local area networks to use the same tools on the Internet instead of having to learn the Internet's way of doing things. One example is Active Server Pages (ASP).