Abbreviation of deoxyribonucleic acid (a nucleic acid present in every living cell, containing the genetic information that directs the sequence in which amino acids are arranged to form a protein).(abbreviation for deoxyribonucleic acid) Complex giant molecule that contains, in chemically coded form, the information needed for a cell to make proteins. DNA is a ladderlike double-stranded nucleic acid that 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.
structure.
DNA is made up of two chains of nucleotide subunits, with each nucleotide containing either a purine (adenine or guanine) or pyrimidine (cytosine or thymine) base. The bases link up with each other (adenine linking with thymine, and cytosine with guanine) to form base pairs that connect the two strands of the DNA molecule like the rungs of a twisted ladder.
heredity.
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. A set of three bases—known as a codon—acts as a blueprint for the manufacture of a particular amino acid, the sub-unit of a protein molecule.
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 x 4 x 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.).
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.
The oldest sequenced DNA to be found is that of a nemonychid weevil trapped in amber from the Cretaceous period, and estimated as being 120–135 million years old.
US researchers extracted DNA from human hair 10,000 years old 1994. Dinosaur DNA was extracted 1994 from unfossilized dinosaur bones found in a coal mine in Utah. + prikaži više