Crack the Question: Week 22 (Cell Biology)

Q1. Source: CSIR NET JUNE 2015

The key determinant of the plane of cytokinesis is the position of:

a. Chromosomes

b. Central spindle

c. Centrioles

d. Pre-prophase band

Explanation

The central spindle (also known as the spindle midzone) is the primary determinant of the plane of cytokinesis in animal cells. During anaphase, overlapping microtubules of the central spindle recruit proteins that signal the formation of the contractile ring, ensuring that the cleavage furrow forms at the correct position.

Let’s examine the other options:

A. Chromosomes : Incorrect. Chromosomes carry genetic information but do not determine where the cleavage furrow forms.

C. Centrioles : Incorrect. Centrioles organize the spindle apparatus but do not directly specify the plane of cytokinesis.

D. Pre-prophase band : Incorrect. The pre-prophase band is unique to plant cells and predicts the future division plane before mitosis. It is not the key determinant in animal cell cytokinesis.

Therefore, the correct answer is:

b. Central spindle

Q2. Source: CSIR NET JUNE 2015

Glycolipids and sphingomyelin are produced by the addition of sugars or phosphorylcholine to ceramide on cytosolic and luminal surfaces, respectively, of the Golgi apparatus. Finally, after such modification, these molecules are located on the outer half of the plasma membrane. What key events are responsible for such localization?

a. Membrane fusion only

b. Action of flippase and membrane fusion

c. Flip-flop of these molecules in the Golgi membrane catalyzed by proton pump

d. Action of only flippase

Explanation

Membrane lipid asymmetry is established and maintained through a combination of flippase activity and vesicle-mediated membrane fusion.

• Flippases are membrane proteins that selectively move specific lipids from one leaflet of the membrane bilayer to the other, helping establish membrane asymmetry.
• During vesicle transport and membrane fusion, the orientation of membrane leaflets is preserved. As a result, lipids present on the luminal side of the Golgi become exposed on the outer leaflet of the plasma membrane after exocytosis.

Let’s examine the remaining options:

A. Membrane fusion only : Incorrect. Membrane fusion preserves membrane orientation but does not establish lipid asymmetry on its own.

C. Flip-flop catalyzed by proton pump : Incorrect. Proton pumps do not catalyze lipid flip-flop.

D. Action of only flippase : Incorrect. Flippases establish asymmetry, but vesicular transport and membrane fusion are also required to position these lipids on the extracellular leaflet.

Therefore, the correct answer is:

b. Action of flippase and membrane fusion

Your Score?

2/2 – Cell Biology Expert!

1/2 – Great effort! Keep revising.

0/2 – Time to revisit the fundamentals.

See you next week with another edition of Crack the Question!