DNA

DNA (deoxyribonucleic acid) Molecule found in all cells, and in some viruses, which is responsible for forming the genetic code. It consists of two long chains of alternating deoxyribose sugar molecules and phosphate groups linked by nitrogenous bases. A base and its associated sugar are known as a nucleotide; the whole chain is a polynucleotide chain. The genetic code is formed in terms of the sequence of nucleotides: three nucleotides code for one specific amino acid and a series of them constitute a gene. A single human cell contains 4m (13ft) of DNA made up of all the information needed to make a human being. Each chromosome is believed to involve more than 100,000 different genes each representing one of the instructions needed to make and maintain the organism from which it originated. DNA directs development and maintains life of an organism by instructing cells to make proteins - the versatile molecules on which all life depends. DNA is permanently locked into the nucleus. But the machinery for protein synthesis is situated in the cytoplasm - outside the cell membrane. DNA communicates with this machinery through a messenger molecule known as rna. In eukaryote cells, DNA is stored in chromosomes inside the nucleus. Loops of DNA also occur inside chloroplasts and mitochondria. In 2003, the international Human Genome Project completed the sequence of the 3 billion DNA bases in the human genome that carries all the genetic information of an individual. See also recombinant DNA research