CL: Development 1 1. (G) Imagine that a mutation causes a mother to produce eggs
ID: 273588 • Letter: C
Question
CL: Development 1 1. (G) Imagine that a mutation causes a mother to produce eggs that have defective Na channels which do not open after sperm and egg fuse. a. Describe what will happen to the membrane potential of the egg when it fuses with a sperm, and explain why b. Describe the effect of this mutation on the development of the slow block to polyspermy, and explain your answer c. If you could directly observe the first sperm fuse with the plasma membrane of this egg describe a simple manipulation that might allow you to rescue/recreate the normal developmental sequence. Explain why it should work. Imagine that you find some eggs (belonging to a non-Mammalian Amniote) with lots of yolk that is asymmetrically distributed. You take them back to the lab to observe their development a. Explain why you are surprised to see that they display holoblastic cleavage. 2. Explain one mechanical and one phylogenetic reason why you are surprised to see that gastrulation begins at a blastopore. b.Explanation / Answer
Ans1 (a):- the sodium potassium channels on the egg membrane helps in the blocking the polyspermy which is achieved by changing the electric potential of the egg plasma membrane.This membrane acts as the selective barrier between the egg cytoplasm and the outside environment, and the ionic concentration of the egg differs greatly from that of its surroundings. This concentration difference is especially significant for sodium and potassium ions. Seawater has a particularly high sodium ion concentration, whereas the egg cytoplasm contains relatively little sodium. The reverse is the case with potassium ions. This condition is maintained by the plasma membrane, which steadfastly inhibits the entry of sodium ions into the oocyte and prevents potassium ions from leaking out into the environment. Within 1–3 seconds after the binding of the first sperm, the membrane potential shifts to a positive level, about +20 mV. This change is caused by a small influx of sodium ions into the egg. Although sperm can fuse with membranes having a resting potential of –70 mV, they cannot fuse with membranes having a positive resting potential, so no more sperm can fuse to the egg. But in case of mutation which leads to the defective sodium channel hence the barrier mechanism disscussed above is disrupted and which causes polyspermy.