Krebs Lecture

Date:
1977
Reference:
PP/CRI/H/6/7
Catalogue details

Licence: In copyright

Credit: Krebs Lecture. Source: Wellcome Collection.

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    Eur. J. Biochem. 83, 1-3 (1978) Chromosome Structure and Function. Future Prospects Francis H. C. CRICK Salk Institute, San Diego, California It is clear from the two symposia and the work shop on The Structure and Function of Chromatin that there has been a big advance in our under standing of the three dimensional organization of chromosomes at the first level of coiling of the DNA, due mainly to the concept of the core nucleosome particle (or 'platysome'), although the exact location of histone HI and the arrangement of the linker regions between platysomes are still in doubt. Even less is known about the precise location of the non histone proteins and the details of the higher levels of coiling. Some 10 to 20% of the nucleosomes (the amount depends on the tissue under study) have a looser configuration which makes them more sensitive to the nuclease DNase I. These 'active' nucleosomes appear to include much of those stretches of DNA which are being transcribed in any particular tissue. In addition several lines of work suggest that (as claimed for Escherichia coli) the DNA in eukaryotes is arranged in 'domains'. The average size of these domains is estimated, in very round terms, to be about 50000 base-pairs but the distribution of sizes about the average is as yet unknown. Nor is the exact nature and functional significance of these domains at all clear though there is no lack of informed guesses on this point. In particular one would like to know whether the nucleosomes in a single domain are, at any one time, all in the 'active' state or all in the more compact inactive state or whether, on the other hand, nucleosomes of both types occur at the same time in a single domain. During this recent period there has also been a very big improvement in our ability to study the 1D structure of DNA; that is, the nucleotide sequence. This has come from the well-known advances in genetic engineering which enable longish stretches This is an expanded version of the last part of the 8th Sir Hans Krebs lecture, entitled 'The structure of chromatin' delivered at the closing session of the 11th FEBS Meeting, Copenhagen, 19 August 1977. Abbreviations. 3D, 2D and 1 D, three dimensional, two dimen sional, one dimensional; mRNA, tRNA and hnRNA, messenger, transfer and heterologous nuclear ribonucleic acids; DNase, de- oxyribonuclease. of DNA (of the order of 10 4 base-pairs) to be inserted into microorganisms and thus multiplied up so that biochemically useful amounts of the DNA of pure eukaryotic 'genes' can be obtained for further study. The use of restriction enzymes and hybridization techniques has allowed the rapid mapping, on a rela tively coarse scale, of these DNA segments, while two extremely fast methods are available for obtaining exact nucleotide sequences. We can expect that in the next few years this detailed information about DNA sequences will grow from its present modest flow into an almost unmanageable flood. Special arrangements, probably involving some central com puting facility, will almost certainly have to be made to collect and store these sequences and to distribute them to all interested workers in the field. What will such sequence information tell us? Already it has yielded unexpected results. Sequences coding for one particular mRNA are apparently located in some 40 different places in the Drosophila genome. The genes for the five major histones have been found, in several species, to be in tandemly repeated arrays. Moreover there are large non-coding regions between the different coding sequences, not all of which are read off the same chain. Clearly we shall learn much, in the next few years, about the distribution of the various kinds of sequences in eukaryotic genomes, especially in Drosophila, not only because of the concentration of work on this organism but also because its genetics will be needed to obtain decisive answers to our questions. The location of coding sequences and of single-copy non- coding sequences, the distribution of intermediate- repetitive sequences and in particular of the finely interspersed intermediate-repetitive sequences (strange ly absent in Drosophila, though present in some of the larger diptera and in most higher organisms) should, hopefully, reveal some significant patterns. It is one of our misfortunes that while we can with ease decipher coding sequences, we still have no reliable methods to spot promoters, terminators and operators, nor those sequences which may be needed for DNA domain formation and for RNA processing, let alone other instructions for functions as yet
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