Crick, Francis, 1916-2004
The outstanding advance in the life sciences in this century has been the creation of a new branch of science: molecular biology. In this, Crick has been a central figure and its key concept, that the self-replicating genetic material DNA has the form of a double helix with complementary strands, is due to him and J D Watson.
Crick graduated in physics in London but his first research was interrupted by war service, working on naval mines. After the war he was attracted to Cambridge and to biology and by 1949 was with the Cambridge Medical Research Council Unit, then housed in the Cavendish physics laboratory. His field of expertise was the use of X-ray crystal diffraction methods (originally devised by the Braggs) to examine the structure of biopolymers. The overall head of the Cavendish Laboratory was then Sir Lawrence Bragg. In the 1950s and under his patronage, the team led by Perutz and including J C Kendrew (1917 - 1997), Watson, H E Huxley, Crick and later Brenner were to have as dramatic an effect on molecular biology as Rutherford’s team had on particle physics in the 1930s, and in the same building.
In 1951 Watson joined the group. He was 23, a zoologist with experience of bacterial viruses and an enthusiasm for genetics. He and Crick quickly became friends; they shared an optimistic enthusiasm that it should be possible to understand the nature of genes in molecular terms, and in under 2 years they were to succeed. Important background material was available for them. There was good evidence from Avery’s work that the DNA of genes formed the key genetic material. A R Todd had shown that DNA consists of chains of sugar residues (deoxyribose) linked by phosphate groups and carrying base molecules (mainly of four types) attached to the sugar rings. Chargaff had shown that the number of these bases had a curious ratio relation. Helical structures had been met with; Pauling had shown, as had Crick, that the protein keratin consists of chains of protein arranged in helical form; Pauling, like Crick, was an enthusiast for making molecular models as an aid to deducing possible structures.
Crick had devised a general theory that would show whether a given X-ray pattern was due to a helical structure; and his friendship with M H F Wilkins (1916 - ) at London gave him limited access to the X-ray pictures made there by Wilkins’s colleague, Rosalind Franklin. With all this in mind, Crick and Watson built their models and in 1953 focused on a model in the form of a double helix, with two DNA chains. It could accommodate all known features of DNA, with acceptable interatomic angles and distances, and would accord with Franklin’s observed X-ray diffraction pattern. The sequence of atoms in them give the DNA chains a direction, and the pair of chains forming the double helix run in opposed directions. Also, the helices are right-handed. The model had its sugar and phosphate chains on its outside and the bases (linked in pairs, A with T, C with G) on the inside (see diagram). The model explains how DNA replicates, by the uncoiling of its double-helical strands, with these strands then acting as templates. It also suggested how genetic information could be encoded, by the sequence of bases along the chains. Crick proposed as a ‘central dogma’ the scheme DNA→RNA→ protein, with the first arrow representing transcription and the second representing translation. (The conversion DNA→DNA, shown in the diagram, is known as replication.)
Crick and Watson had found the broad answer to the question ‘how do genes replicate and carry information?’ and in the succeeding years most work on molecular biology has been directed to confirming, refining and extending these ideas. Crick himself has done much in this area, for example in work with Brenner demonstrating that the code is read in triplets of bases (codons) each defining one specific amino acid used to make a protein, and in showing that adjacent codons do not overlap. He also studied the structure of small viruses and collagen, and the mechanism by which transcription and translation occurs; and he has offered novel ideas on the origin of life on Earth and on the nature of consciousness. He worked mainly in Cambridge until 1977 when he moved to the Salk Institute in San Diego, CA. He shared the Nobel Prize for physiology or medicine in 1962 with Watson and Wilkins.
F H C Crick, in about 1954.
DNA replication, following Watson and Crick. The two strands of the double helix separate, and a daughter strand is laid down alongside each with a constitution determined by the base sequence of its parent strand.