Looking at chromosomes / by John McLeish and Brian Snoad.

  • McLeish, John.
Date:
[1958], ©1958
Catalogue details

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

Credit: Looking at chromosomes / by John McLeish and Brian Snoad. Source: Wellcome Collection.

    31/116 (page 17)
    MITOSIS AND DEVELOPMENT 17 capable of regenerating a new plant with characters identical to those of the plant from which they were originally taken. The results of a variety of biochemical and genetical experiments are beginning to demonstrate how the nucleus controls organised growth through two remarkable properties of its DNA. DNA makes copies of itself by replication but, in addition, it can act as a template in making some forms of RNA which pass from the nucleus to the cytoplasm. These RNA molecules, carrying copies of the genetic 'message', then help to regulate metabolic activity by directing the syntheses of specific proteins at appropriate sites in the cytoplasm. The ability of cells to metabolise, to undergo mitosis and to become integrated with other cells in the formation of a particular tissue, is the result of control by specific regions of the DNA molecules which are activated or inactivated at appropriate times during the life of the organism. In any one cell, at any given point in time, only a small proportion of the genetic DNA is likely to be active, the rest being 'masked' by proteins, particularly the histones. At the same time, different regions may be 'un¬ masked' and active in other cells which are performing other functions. The way in which this differential activation is brought about is still unknown but there is no doubt that it depends on the exchange of substances between neighbouring cells as well as on the chemical interaction between nucleus and cytoplasm.