Human chromosomes / E.H.R. Ford.

  • Ford, E. H. R. (Edward Hugh Rawlinson)
Date:
1973
Catalogue details

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

Credit: Human chromosomes / E.H.R. Ford. Source: Wellcome Collection.

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    5. OUTLINE OF MOLECULAR GENETICS 109 chromosomes of Diptera {q-v.) certain areas can be seen to form puffs, areas in which the chromosome seems partly to unravel itself. Such puffs have been proved to be sites of transcription. Similarly, in the lampbrush chromosomes of amphibian oocytes {q.v.) the lateral loops have also been shown to be sites of RNA formation. Autoradiographic studies using tritiated uridine show that this is incorporated into newly-formed RNA in these loops, starting at the thin end and proceeding to the thick end, over a period of some 10 days. The loops contain a strand of DNA, and if treated with actinomycin D (which inhibits RNA synthesis) they appear to disappear. Since the lampbrush chromosome structure has been seen in mammalian oocytes, and indeed may represent a basic model of chromosome structure, sites of synthesis in the chromosomes of some higher organisms can be deduced by analogy. Thus Baker and Franchi (1967) suggest that the lampbrush-like loops of the mammalian dictyotene chromosome are similarly sites of RNA synthesis. It must be admitted that studies of the fine structure of mammalian metaphase chromosomes have not shown any obvious correlation between chromosome structure and possible sites of synthesis, although some workers have claimed to find finer chromosomal fibrils in certain areas, which may represent sites of DNA replication or RNA synthesis. This topic is discussed in greater detail in the section on chromosome structure (Chapter 6). It may be mentioned again here that the secondary constrictions on certain chromosomes (in particular the satellited acrocentrics 13-15 and 21-22) probably represent sites of synthesis of rRNA, and the nucleoli which are found adjacent to these areas appear like great masses of ribosomes. B. Genetic Regulation of Development There are considerable differences in the proportions and types of RNA found in cells in different organs in the body, and this must, as has been said before, relate to differential action of the total DNA in RNA synthesis in differentiated cells. However almost nothing is known of the regulation of this. A little work has been done on Anurans (frogs and toads) on the sorts of RNA formed during the earliest stages of development. It has been found that during oogenesis the RNA formed is mainly rRNA, which is conserved against future activity in protein synthesis. After fertilization, the