| 释义 |
chiasma Anat.|kaɪˈæzmə|
Also chiasm.
[mod.L., a. Gr. χίασµα arrangement of two lines (sticks, etc.) crossed like the letter Χ (χῖ), decussation; χιάζ-ειν to mark with or like a chī (Χ, χ).]
1. Intercrossing or decussation. optic chiasma: the crossing or decussation of the fibres of the optic nerves at the base of the brain; the optic commissure.
1839–47Todd Cycl. Anat. III. 769/1 The existence of a chiasma is not general throughout the animal series. 1872Mivart Elem. Anat. 387 This decussation of fibres is called the optic commissure or chiasma. 1880Gunther Fishes 104 In Palæichthyes the two nerves are fused together, immediately after their origin, into a chiasma.
2. Cytology. Pl. chiasmata. [Fr. (F. A. Janssens Cellule (1909) xxv. 391).] The exchange of segments of chromatids during nuclear division.
1911T. H. Morgan in Science XXXIV. 637/1 The chiasma type of cross union between the homologous chromosomes is grasped. Ibid., If, then, the genes do not themselves split when the chiasma is formed there is no opportunity offered for a quantitative division. 1929Darlington in Nature 20 July 99/2 In the longer chromosomes these chromatids change partners, forming what are called ‘chiasmata’... In the short chromosomes..there is rarely more than one chiasma. 1949New Biol. VII. 83 Each chiasma represents a point where two of the four daughter chromatids, one from each of the pairing partners, have broken and rejoined with one another, thus giving rise to two new continuous strands which cross over from one partner to the other. |