Question

6. (8pts) A. The following three boxes show one pair of sister chromatids during spindle formation. What are the identities of Structures A & B? A- B- B. Please draw spindle microtubules to connect appropriate combinations of Structures A & B to show: -2 different examples of unstable attachments 1 example of a stable attachment ready to pass through the metaphase-anaphase transition Unstable -Example 1 Unstable Example 2 Stable C. How is an unstable vs. stable attachment sensed by the cell during the spindle-assembly checkpoint? D. Suppose a cell loses the ability to sense whether sister chromatids are correctly attached to the spindle. How can this loss lead to outcomes like aneuploidy?

Answer 1

a) A- spindle pole or centriole, B- Centromere or kinetochore

b) 2 examples of unstable attachments- 1. Syntelic attachment- both sister kinetochores are attached to the microtubules from the single spindle pole. 2. Merotelic attachment. Microtubules from both the spindle poles are attached to only one sister kinetochore.

Example of stable attachment: Bi-oriented attachment: two sister kinetochores are attached to microtubules arising from opposite spindle poles. Refer the diagram.

Synkelic aHachrnonk Mesotelic- attach-nenト Br-onientel attachment

c) The stability of the attachment is sensed via the kinetochore binding proteins commonly known as KMN network. These proteins can recognise the low tension that is created because of the improper attachment of microtubules to the kinetochore. These protein signals themcore control network which inturn stabilizes the attachment. The KMN network is composed of 3 kinetochore binding proteins namely- kinetochore null protein 1 (KNL1), the mis-segregation 12 (MIS12) complex ,and the nuclear division cycle 80 (NDC80) complex.

D) Aneuploidy is the presence of abnormal number of chromosomes. When a cell loses the ability to sense incorrectly attached sister chromatids, failure in proper segregation of sister chromatids occurs. This inturn leads to  either an increase or decrease in normal chromosome count. One daughter cell might get an extra copy of a chromosome whereas other cell might lack a copy.