DNA, which is short for Deoxyribonucleic Acid, is the molecule that contains the genetic code of organisms. This includes animals, plants, protists, archaea and bacteria.
DNA is in each cell in the organism and tells cells what proteins to make. These proteins are mostly enzymes. 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.
DNA was first discovered by Swiss scientist Johannes Friedrich Miescher in 1869.
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. A nucleotide is a molecule made up of:
- deoxyribose, a kind of sugar with 5 carbon atoms,
- a phosphate group made of phosphorus and oxygen, and
- a nitrogenous base.
DNA is made out of four types of nucleotides:
- Adenine (A)
- Thymine (T)
- Cytosine (C)
- Guanine (G)
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 G only pairs with G. The bases (called so because they are bases rather than acids) are held together by hydrogen bonds.
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.
When DNA is copied this is called DNA replication. 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.
First, an enzyme called DNA helicase splits the DNA down the middle by breaking the hydrogen bonds. Then after the DNA molecule is broken into two separate pieces, another molecule called DNA polymerase makes a new strand that matches each of the strands of the split DNA molecule. Each copy of a DNA molecule is made of half of the original molecule and half of new bases.
When DNA is copied, mistakes are sometimes made – these are called mutations. There are three main types of mutations:
- Deletion, where one or more bases are left out,
- Substitution, where one or more bases are substituted for another base in the sequence,
- Insertion, where one or more extra base is put in, and
- Duplication, where a sequence of bases pairs are repeated.
Mutations may also be classified by their effect on the structure and function of proteins, or their effect on fitness. Mutations may be bad for the organism, or neutral, or of benefit. Sometimes mutations are fatal for the organism – the protein made by the new DNA does not work at all, and this causes the embryo to die. On the other hand, evolution is moved forward by mutations, when the new version of the protein works better for the organism.