Describe the events that must occur in order for sperm to penetrate an egg. What prevents more than one sperm from entering? In the case of twins, how many sperm entered the egg? Why must a man have millions of sperm in a semen sample to be considered fertile? If only one sperm were present in the semen and that sperm managed to find its way to the egg, could it fertilize the egg? Explain.
What will be an ideal response?
The sperm must penetrate the corona radiata and the zona pellucida, which surround the egg, in order to get to its
membrane. The acrosome of the sperm contains enzymes that disrupt these barriers. The first sperm to reach the
membrane binds to a receptor, the sperm and egg membranes fuse, and the sperm nucleus enters the egg. The
cortical reaction ensues, in which granules in the egg spill their contents out of the membrane, which alters it in a
way preventing additional sperm from binding. Twins result from normal fertilization, but either two eggs are
present or the early embryo splits. Only about 100 sperm manage to find the egg, with the other millions swimming
off in the wrong direction. One sperm probably does not contain enough enzymes to allow penetration of the egg
barriers.
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What will be an ideal response?