A chromosome achieves syntelic attachment — both sister kinetochores connect to microtubules from the SAME pole. Why does this incorrect attachment fail to silence the spindle assembly checkpoint?
ABecause syntelic attachment leaves one kinetochore completely unattached, which the SAC directly detects
BBecause the KMN network cannot bind microtubules from the same pole simultaneously
CBecause syntelic attachment does not generate tension across the kinetochore pair — without opposing pole pulling, there is no stretch between sisters, so checkpoint kinases remain active
DBecause Mad1 and Mad2 are only released when both kinetochores are occupied by at least 10 microtubules each
The SAC monitors both attachment AND tension. Syntelic attachment means both sisters are occupied by microtubules, but they pull in the same direction — so the kinetochore pair experiences no opposing forces and generates no tension. Tension physically stretches the structure, separating kinase substrates from their phosphatases and stabilizing correct attachments. Without tension, checkpoint signaling continues even though microtubules are present. This is why 'attached' is not sufficient — only amphitelic (bi-oriented) attachment generates the mechanical force that satisfies the checkpoint.
Question 2 Multiple Choice
Why do kinetochore proteins form low-affinity, rapidly exchanging contacts with microtubule plus-ends rather than forming a rigid, stable clamp?
ABecause high-affinity binding would trigger apoptosis pathways in the cell
BBecause the NDC80 complex lacks the structural domains needed for stable microtubule binding
CBecause chromosome movement depends on riding the energy of microtubule depolymerization, which requires the kinetochore to maintain attachment to a shrinking end without irreversibly clamping it
DBecause stable binding would prevent other chromosomes from attaching to the same microtubule
Spindle microtubules undergo dynamic instability — constant cycles of polymerization and depolymerization. Chromosomes congress toward the metaphase plate and ultimately segregate by coupling to these dynamics: kinetochores can be pulled poleward by a depolymerizing microtubule, using the energy released by tubulin GTP hydrolysis. For this to work, the NDC80 complex must maintain contact with a shrinking tubulin end without locking it in place. Low-affinity contacts that exchange rapidly allow the kinetochore to 'track' the end, staying attached while the microtubule shortens. A rigid clamp would detach the instant depolymerization began.
Question 3 True / False
Kinetochores are static structures that serve as simple anchor points holding chromosomes to spindle microtubules — their primary role is mechanical stability.
TTrue
FFalse
Answer: False
Kinetochores are highly dynamic and serve at least two critical functions. Mechanically, they actively track dynamic microtubule plus-ends through rapidly exchanging NDC80-tubulin contacts, generating force for chromosome movement. Biochemically, they serve as the scaffold for the spindle assembly checkpoint — assembling Mad1, Mad2, and BubR1 when unattached or under-tensioned, and dismantling this checkpoint signal once correct amphitelic attachment generates sufficient tension. Far from static anchors, kinetochores continuously remodel as microtubules polymerize and depolymerize, and they integrate mechanical and biochemical signals to coordinate chromosome segregation.
Question 4 True / False
Correct (amphitelic) attachment of sister kinetochores to microtubules from opposite poles generates tension that physically stabilizes the attachment and contributes to silencing the spindle assembly checkpoint.
TTrue
FFalse
Answer: True
Tension is the physical readout of correct bi-orientation. When sister kinetochores connect to opposite poles, the pulling forces stretch the kinetochore structure. This stretch physically separates kinase substrates from their phosphatases at the outer kinetochore — phosphatase access stabilizes the attachment by preventing premature detachment. At the same time, the tension-induced structural change reduces the scaffolding activity for Mad1/Mad2 checkpoint complex assembly, contributing to SAC silencing. Incorrect attachments (syntelic, merotelic) don't generate proper tension, maintaining the checkpoint signal and triggering error correction by Aurora B kinase.
Question 5 Short Answer
Explain how the kinetochore distinguishes amphitelic (correct) from syntelic or merotelic (incorrect) attachment, and what molecular mechanism converts this physical distinction into a biochemical checkpoint signal.
Think about your answer, then reveal below.
Model answer: Correct amphitelic attachment pulls sister kinetochores toward opposite poles, generating tension that physically stretches the kinetochore. This stretch spatially separates kinase substrates (at the inner kinetochore, near centromeric DNA) from their phosphatases (concentrated toward the outer kinetochore), causing a net dephosphorylation state that stabilizes microtubule contacts. Incorrect attachments — syntelic (both sisters to same pole) or merotelic (one kinetochore to both poles) — generate little or incorrect tension, leaving the inner kinetochore in a phosphorylated state that destabilizes the attachment and maintains recruitment of Mad1/Mad2 checkpoint proteins. Aurora B kinase acts as the error-correction enzyme: it phosphorylates NDC80 and other outer kinetochore proteins to weaken microtubule binding, selectively destabilizing low-tension (incorrect) attachments while high-tension (correct) attachments resist its activity because their inner kinase substrates are shielded.
The elegance of this system is that tension converts a geometric property (which poles microtubules came from) into a structural change at the kinetochore (stretch), which then drives a biochemical change (kinase/phosphatase balance). The cell does not directly 'see' which pole a microtubule came from — it only senses whether the resulting attachment produces adequate tension. This mechanical read-out is what ultimately determines whether anaphase is permitted to proceed.