Volume 1

Gene expression / Benjamin Lewin.

Benjamin Lewin

Date:: 1974-1977

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Gene expression / Benjamin Lewin. Source: Wellcome Collection.

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$evidence that the genetic material is nucleic acid derives from studies with microorganisms; data from higher organisms, although strongly suggestive, do not constitute a formal proof. Transformation between bacteria was discovered when Griffith found in 1928 that when mice are injected with dead virulent Pneumococcus together with a live but harmless mutant, they die as the result of production of live bacteria of the virulent form. The inference is that although dead, the virulent strain contains genetic information which can be transferred in some way to use the live organization of the other bacterial cell type. Studies of cell free preparations by Avery, Macleod and McCarty in 1944 showed that the transforming factor is DNA. Confirmation that DNA is the genetic material was provided by the classic experiment in which Hershey and Chase (1951a, b) radioactively labelled either the protein or the nucleic acid components of T2 bacteriophages. Only the nucleic acid of the virus enters the cells of the host bacterium, Escherichia coli, to provide the genetic information which must be required for the sub sequent reproduction of the phages. Most viruses, bacterial cells and the cells of higher organisms utilise DNA as their genetic material; RNA constitutes the genetic information of a few small viruses. It is a common view that molecular biology began in 1953 with the elucidation of the structure of DNA. Diffraction patterns obtained by Wilkins et al. from X-ray crystallography proved to be consistent with a structure in which two very long polynucleotide chains are wound around each other to form a double helix. The model building of Watson and Crick (1953a, 1953b) suggested that this might be achieved if the two complementary strands are oriented in opposite directions, one running 5' to 3' and the other 3' to 5', held together only by hydrogen bonding between complementary base pairs. This suggests the structure for DNA represented by the illustration of figure 1.2. A covalently linked chain of deoxyribose-phosphate provides the backbone of the molecule, with the nucleotide bases located inside. DNA usually contains four types of base: the two purines adenine and guanine; and the two pyrimidines thymine and cytosine. Watson and Crick suggested that the double helix can remain the constant width demanded by its diffraction pattern only if a purine in one strand always partners a pyrimidine in the other; purines never complement purines and pyrimidines never complement mutations lie in different genes, a cell containing one copy of each is there fore wild type no matter whether their arrangement is eis or trans. But when two mutations are located in the same gene, the cell can make active protein only when their arrangement is eis ; in the trans arrangement two mutant proteins are made. When two mutations show wild type behaviour in eis array but are inactive in trans they must therefore be located in the same gene$

Gene expression / Benjamin Lewin.

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