DNA, short for deoxyribonucleic acid, is the molecule that contains the genetic code of organisms. This includes animals, plants, protists, archaea and bacteria. <\/p>\n
DNA is in each cell in the organism and tells cells what proteins to make. A cell's proteins determine its function. DNA is inherited by children from their parents. This is why children share traits with their parents, such as skin, hair and eye color. The DNA in a person is a combination of the DNA from each of their parents. <\/p>\n
Viruses use either DNA or RNA to infect organisms.[1] The genome replication of most DNA viruses takes place in the cell's nucleus, whereas RNA viruses usually replicate in the cytoplasm. <\/p>\n
DNA has a double helix shape, which is like a ladder twisted into a spiral. Each step of the ladder is a pair of nucleotides. <\/p>\n
A nucleotide is a molecule made up of: <\/p>\n
DNA is made of four types of nucleotide: <\/p>\n
The 'rungs' of the DNA ladder are each made of two bases, one base coming from each leg. The bases connect in the middle: 'A' only pairs with 'T', and 'C' only pairs with 'G'. The bases are held together by hydrogen bonds. <\/p>\n
Adenine (A) and thymine (T) can pair up because they make two hydrogen bonds, and cytosine (C) and guanine (G) pair up to make three hydrogen bonds. Although the bases are always in fixed pairs, the pairs can come in any order (A-T or T-A; similarly, C-G or G-C). This way, DNA can write 'codes' out of the 'letters' that are the bases. These codes contain the message that tells the cell what to do. <\/p>\n
On chromosomes, the DNA is bound up with proteins called histones to form chromatin. This association takes part in epigenetics and gene regulation. Genes are switched on and off during development and cell activity, and this regulation is the basis of most of the activity which takes place in cells. <\/p>\n
When DNA is copied this is called DNA replication. Briefly, the hydrogen bonds holding together paired bases are broken and the molecule is split in half: the legs of the ladder are separated. This gives two single strands. New strands are formed by matching the bases (A with T and G with C) to make the missing strands. <\/p>\n
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