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the DNA in a cell is really just a pattern made up of four different parts called nucleotides.

Imagine a set of blocks that has only four shapes, or an alphabet that has only four letters. DNA is a long string of these blocks or letters. Each nucleotide consists of a sugar (deoxyribose) bound on one side to a phosphate group and bound on the other side to a nitrogenous base.

Watson and Crick discovered that DNA had two sides, or strands, and that these strands were twisted together like a twisted ladder -- the double helix.

A subunit of the DNA polymerase proofreads the new DNA

Before a cell can reproduce, it must first replicate, or make a copy of, its DNA.

DNA replication occurs in the cytoplasm of prokaryotes and in the nucleus of eukaryotes. Regardless of where DNA replication occurs, the basic process is the same.

Dolly the sheep, as the first mammal to be cloned from an adult cell, is by far the world's most famous clone. However, cloning has existed in nature since the dawn of life. From asexual bacteria to ‘virgin births’ in aphids, clones are all around us and are fundamentally no different to other organisms. A clone has the same DNA sequence as its parent and so they are genetically identical.

To produce Dolly, scientists used an udder cell from a six-year-old Finn Dorset white sheep. They had to find a way to 'reprogram' the udder cells - to keep them alive but stop them growing – which they achieved by altering the growth medium (the ‘soup’ in which the cells were kept alive). Then they injected the cell into an unfertilised egg cell which had had its nucleus removed, and made the cells fuse by using electrical pulses. The unfertilised egg cell came from a Scottish Blackface ewe. When the research team had managed to fuse the nucleus from the adult white sheep cell with the egg cell from the black-faced sheep, they needed to make sure that the resulting cell would develop into an embryo. They cultured it for six or seven days to see if it divided and developed normally, before implanting it into a surrogate mother, another Scottish Blackface ewe. Dolly had a white face.

Since 1996, when Dolly was born, other sheep have been cloned from adult cells, as have cats, rabbits, horses and donkeys, pigs, goats and cattle. In 2004 a mouse was cloned using a nucleus from an olfactory neuron, showing that the donor nucleus can come from a tissue of the body that does not normally divide.

Genes affect our chances of having several common illnesses, like heart disease, asthma and diabetes but so do many other factors, such as diet and lifestyle.

Many genetic and non-genetic factors affect our health, but scientists don't yet know what they all are, or how they interact with each other.