Glossary

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DNA

Term used to describe the chemical make-up of the genetic information

The building blocks of DNA (deoxyribonucleic acid) are called “nucleotides”, each comprising a sugar (deoxyribose), a phosphate, and a base.

These building blocks combine to form a giant molecule comprising two strands of nucleotides, the famous “double helix”.

DNA is always present in the cell in conjunction with special protein molecules (histones), which act as “spools” around which the DNA strands wind.

The sequence of nucleotides in each of the two strands carries the entire genetic code, which is therefore present in full in every cell of an organism.

The two strands are held together by the pairing of the bases adenine (A), cytosine (C), guanine (G) and thymine (T). A always pairs up with T and C with G. This complementary structure enables DNA to replicate itself during cell division. It divides like a zip into two single strands, each of which then replicates to form a double strand again. DNA also splits open to reveal sites which are “read” by RNA during the transfer of genetic information. A gene is a specific sequence of DNA.

In 1953 James Watson and Francis Crick unlocked the molecular structure of DNA.

It was previously thought that each gene provided the blueprint for a specific protein (one gene, one protein hypothesis), but we now know that a specific gene can code for different proteins with the aid of complex regulatory mechanisms. Protein-coding genes, however, account for only 1 to 2% of DNA. In humans this amounts to around 30,000 to 40,000 genes, which code for an estimated 300,000 proteins. Around 95% of DNA is comprised partly of regulatory units, but mainly of sequence segments whose functions have yet to be discovered.