The Spiral Structure of Chromosomes

Summary . i. Sufficient examples of spiral structure of chromosomes are now known from dicotyledons, monocotyledons, pteridophytes and perhaps gymnosperms, together with some animals, to show that the phenomenon is a fundamental one in the plant and animal kingdoms, at least in the higher groups. 2. Various methods found to be effective for revealing the spiral are listed, though the spiral itself cannot be an artifact. 3. Since 1939, new observations have been made on the following principal topics: (a) chromonema length; (b) direction of coiling, especially in a somatic chromosome; (c) prophase development leading to generalizations about ‘gyre elimination’ as a normal part of the spiralization cycle; (d) geometrical form of the spiral. 4. Unification of terminology is recommended by general adoption of the terms plectonemic and par anemic to describe the geometrical form of the spiral. The term ‘standard coil’ is recommended to replace ‘minor spiral’ for contrasting the major spiral with those of other types of division. 5. The need is stressed for further evidence on the minor spiral in the strict sense. 6. The importance of the interpretation of the ‘telophase split’ and of the origin of relational coiling for an understanding of the functional importance of the whole phenomenon of spiral structure is stressed. The suggestion made in earlier publications is repeated that the functional significance of the spiral might be that of co‐ordinating the component strands of a chromosome so that these may act in a unified manner when outside the resting nucleus.