In Drosophila neuroblast division, the Par complex (Par3/Par6/aPKC) localizes to the apical side and Numb localizes to the basal side. If the Par complex is mutated so that it distributes uniformly, what happens to the division?
ADivision becomes symmetric — both daughters adopt the same fate because the asymmetric localization of determinants like Numb is disrupted
BThe cell fails to divide entirely
CBoth daughters become neurons immediately
DThe cell divides normally because Par proteins are not involved in asymmetric division
The Par complex is the master regulator of cell polarity during asymmetric division. It excludes basal determinants (Numb, Miranda/Prospero) from the apical cortex, concentrating them at the basal side. When Par complex function is lost, Numb distributes uniformly, and both daughters receive equal amounts — producing symmetric division where both daughters adopt the same fate (typically both self-renew, leading to neuroblast overproliferation and tumor formation). This demonstrates that fate asymmetry requires active polarization machinery, not just the presence of fate determinants.
Question 2 True / False
Asymmetric cell division is the only mechanism by which stem cells produce one self-renewing and one differentiating daughter.
TTrue
FFalse
Answer: False
While intrinsic asymmetric division (unequal distribution of fate determinants) is one mechanism, an alternative is extrinsic (niche-mediated) asymmetry: a stem cell divides symmetrically, producing two initially equivalent daughters, but one remains in the stem cell niche (retaining stem cell identity due to niche signals) while the other is displaced from the niche and differentiates due to loss of self-renewal signals. This 'population asymmetry' mechanism is used by intestinal stem cells and some other tissue stem cells. The two mechanisms are not mutually exclusive — both can operate in the same tissue.
Question 3 Short Answer
How does spindle orientation ensure that asymmetrically localized determinants are correctly partitioned between the two daughter cells?
Think about your answer, then reveal below.
Model answer: The mitotic spindle must align with the axis of cell polarity so that the cleavage plane bisects the cell perpendicular to the polarity axis, placing apical determinants in one daughter and basal determinants in the other. Spindle orientation is controlled by cortically anchored proteins (like Pins/LGN and Mud/NuMA in Drosophila/vertebrates) that capture astral microtubules and pull the spindle poles toward specific cortical positions. If spindle orientation is randomized (by disrupting the anchoring machinery), the cleavage plane no longer correlates with the determinant distribution, and both daughters receive a mixture — abolishing fate asymmetry. Correct spindle orientation is thus as essential as determinant localization for functional asymmetric division.
In Drosophila neuroblasts, the Pins/Mud complex anchors to the apical cortex (defined by the Par complex), pulling one spindle pole apically and the other basally. This ensures the division plane separates the apical and basal cortical domains, partitioning apical (self-renewal) and basal (differentiation) determinants into different daughters.