The chromosome field

Knobs are well defined cytological markers consisting of large chromomeres. They are observed at pachytene and at prophase of mitosis. A total of 264 knobs has been identified in 15 species and varieties belonging to four unrelated families. The distribution of knobs is so regular that four regions can be distinguished within the arm frame: (1) Forbidden region (near the kinetochore) where no knobs have been found. (2) Permitted region where they occur with a frequency of 27.7%. (3) Favoured region where they increase in number along the arm length. (4) Optimal region (telomere zone) where 72.3% of the knobs are found. In some species, the knobs appear in every chromosome and in every arm of the complement. This means that their location is not directly related to the particular genetic constitution of each chromosome or arm. The location of knobs is also independent of the species or the family. It is postulated that the occurrence of knobs on chromosomes is dependent on three factors: (i) a DNA segment with knob-formation capacity, (ii) a molecular message from the telomere which favours their location in its close vicinity and (iii) a kinetochore molecular message with suppressing effect that spreads along the arm. There is a tendency for knobs not to appear in very short arms. A distance between telomere and kinetochore of less than 15 μm does not favour their appearance.