Three models for contractile-ring positioning in animal cells are presented in the schematic diagrams below. Answer the following question(s) according to these models.

-In classic experiments carried out in the 1960s and schematized below, an egg of the sand dollar Echinarachnius parma was made into a torus shape using a glass bead on the end of a needle, to study the influence of microtubules on cleavage-furrow positioning. The cells were allowed to undergo two rounds of mitosis. After the second mitosis, an extra furrow (indicated by a red arrowhead) was observed between the two asters that did not have a spindle between them. Which of the models (1 to 3) is more consistent with this observation? Write down 1, 2, or 3 as your answer.

Question

Three models for contractile-ring positioning in animal cells are presented in the schematic diagrams below. Answer the following question(s) according to these models.
 
-In classic experiments carried out in the 1960s and schematized below, an egg of the sand dollar Echinarachnius parma was made into a torus shape using a glass bead on the end of a needle, to study the influence of microtubules on cleavage-furrow positioning. The cells were allowed to undergo two rounds of mitosis. After the second mitosis, an extra furrow (indicated by a red arrowhead) was observed between the two asters that did not have a spindle between them. Which of the models (1 to 3) is more consistent with this observation? Write down 1, 2, or 3 as your answer.

Quizplus · Accepted Answer

The astral stimulation model (1) predicts this outcome. The central spindle stimulation model (2) predicts the formation of two furrows when there are two spindles. However, it is also likely that a midzone-like structure formed between the two spindles in these experiments, and so model 2 could also be correct here. The astral relaxation model (3) predicts the formation of two furrows in the first division.

Three Models for Contractile-Ring Positioning in Animal Cells Are Presented