Genetica

, Volume 41, Issue 1, pp 231–256 | Cite as

Meiosis VII: “Detorsive bending” as a basis for geometric shapes of late prophase bivalents

  • L. T. Douglas
Article

Abstract

A new model depending on mechanical properties of chromosomes is adduced as a basis for diplotene opening-out and for curvature occurring in grasshopper bivalents, during and subsequent to diplotene. Conditions underlying the model are: (1) rigid physical binding exists between sister chromatids, (2) each chromatid remains free of torsional strain if its pairing face is straight, i.e. the chromatid is bilaterally symmetrical, (3) reciprocal exchange together with stiff binding between sisters produces twist in each chromated before diplotene begins, (4) stiffening of the bivalent during late meiotic prophase removes the twist resulting from reciprocal exchange, (5) since sister binding prevents untwisting of chromatids about their long axes, untwisting would be achieved only in conjunction withbending of each chromated. It is shown that this bending, called “detorsive bending”, automatically produces opening out, not only in bivalents with one chiasma but also in those with more than one, especially if the chiasmata are interstitial.

In bivalonts with two chiasmata, classes of curvature resulting when both chiasmata are interstitial (II), when one is interstitial and one terminal (IT) and when both are terminal (TT) are attributed to differences in strength of opening out at interstitial and at terminal chiasmata respectively. It is postulated that mechanisms responsible for opening out at terminal chiasmata are basically different from those at interstitial chiasmata.

A theoretical basis of a method for cytological detection of chromatid interference is outlined and arguments are presented against the electrostatic hypothesis.

Keywords

Mechanical Property Chromated Theoretical Basis Geometric Shape Sister Chromatid 

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Copyright information

© Martinus Nijhoff 1970

Authors and Affiliations

  • L. T. Douglas
    • 1
  1. 1.Department of GeneticsUniversity of NijmegenThe Netherlands

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