Fertilization

Gametes are haploid: one set of 23 chromosomes. Two haploid gametes together restore the diploid number, 46.

• Correct: Both haploid; the new cell will be diploid:
• Both diploid; the new cell will be haploid:
• Both diploid; the new cell will have 92 chromosomes:

The acrosome is a sac of digestive enzymes, made from the Golgi apparatus, that sits over the front of the sperm's head.

• The mitochondria-packed midpiece that powers the tail:
• Correct: A cap-like sac of digestive enzymes over the front of the nucleus:
• The protein coat that holds the sperm's DNA tightly:

Just before ovulation the primary oocyte finishes meiosis I. The secondary oocyte then starts meiosis II and pauses at metaphase II; it finishes only if a sperm enters.

• Prophase I, as a primary oocyte:
• Correct: Metaphase II, as a secondary oocyte:
• Finished meiosis, as a mature ovum:

Albumin pulls cholesterol from the sperm membrane, and bicarbonate and calcium enter the sperm. These are the changes of capacitation, which makes the sperm able to undergo the acrosomal reaction and fertilize. Swimming alone is not enough.

• Correct: Capacitation: Correct. The medium copies the female tract fluids that capacitate sperm.
• The cortical reaction: The cortical reaction happens in the oocyte after one sperm has fused with it. It blocks further sperm; it does not help any sperm get in.
• Completion of meiosis II in the sperm: Sperm finish both meiotic divisions in the testis, long before ejaculation. It is the oocyte that finishes meiosis II at fertilization.
• Formation of the acrosome: The acrosome forms during spermiogenesis in the testis. These sperm already had one; what they lacked was the ability to use it.

Sperm must capacitate before they can fertilize. They then cross the outer layer, the corona radiata, and release their acrosomal enzymes to cross the inner layer, the zona pellucida. Fusion of one sperm starts calcium waves in the oocyte, which trigger the cortical reaction and restart meiosis II. Finally the two haploid pronuclei combine their chromosomes into the diploid zygote.

• Correct order: 1. Sperm capacitate over several hours 2. Capacitated sperm push through the corona radiata 3. The acrosomal reaction releases enzymes that digest a path through the zona pellucida 4. One sperm fuses with the oocyte's plasma membrane 5. Calcium waves trigger the cortical reaction 6. The oocyte completes meiosis II, and the chromosomes of the two pronuclei combine into the zygote

Calcium waves drive two separate outcomes: cortical granule exocytosis and the restart of meiosis II. Blocking only the granules removes the zona block to polyspermy, so more sperm can bind and polyspermy becomes more likely, while meiosis II still finishes.

• Calcium waves in the oocyte after fusion: no change. The calcium waves are set off by the enzyme the sperm delivers. The drug acts on granule fusion, which comes after the calcium rise.
• Cutting of ZP2 in the zona pellucida: down. The enzyme that cuts ZP2 is released from the cortical granules. If they cannot fuse, it stays inside them.
• Number of sperm able to bind the zona after the first sperm enters: up. With ZP2 uncut, the zona still offers binding sites, so more sperm keep attaching.
• Chance of polyspermy: up. The zona block is lost, and only the slower membrane block remains, so a second sperm is more likely to get in.
• Completion of meiosis II: no change. Meiosis II is restarted by the calcium waves, not by the cortical granules, so it still finishes.

Sperm stored in the cervical crypts and the tube's isthmus can survive up to about five days and are released a few at a time. Some were still available when the oocyte arrived after ovulation on day 14.

• The oocyte waited in the uterine tube from day 11 until the sperm reached it there: There was no oocyte on day 11; ovulation happened on day 14. Once released, an oocyte lives only 12 to 24 hours.
• Correct: Some sperm survived in her tract for three days and fertilized the oocyte after ovulation: Correct. Sperm survival of up to about five days is what makes the fertile window about six days long.
• Fertilization happened on day 11, and the zygote then paused in the tube until day 14: Fertilization needs an ovulated secondary oocyte. None existed until day 14.
• Sperm entered the ovary and fertilized the oocyte inside its follicle: Sperm do not enter the ovary. Fertilization happens in the ampulla of the uterine tube after the oocyte is released.

The ovulated oocyte already finished meiosis I just before ovulation and is paused at metaphase II. Sperm entry releases that pause, so it completes meiosis II.

• The sperm delivers an enzyme that starts calcium waves in the oocyte: This step is right. The sperm's enzyme triggers repeated calcium release from the oocyte's endoplasmic reticulum.
• Cortical granules release their contents under the zona pellucida: This step is right. That is the cortical reaction, the zona block to polyspermy.
• Correct: The oocyte, released from its pause, completes meiosis I: This is the error. Meiosis I finished before ovulation; the oocyte completes meiosis II after sperm entry.
• A second polar body is pinched off: This step is right. Meiosis II is lopsided, and the discarded set of chromosomes leaves in the second polar body.
• Two haploid pronuclei form and copy their DNA: This step is right. Each pronucleus holds one set of chromosomes and copies its DNA before the first mitotic division.

Without an acrosome, a sperm has no enzymes to digest a path through the zona pellucida, however many try or however well they are capacitated. Injecting a sperm into the cytoplasm skips the zona entirely. Round-headed sperm often carry too little of the enzyme that activates the oocyte, so clinics may also add a treatment that raises calcium in the oocyte after the injection.

• Giving him testosterone to make more sperm: Sperm numbers are already normal. More sperm without acrosomes would fail in the same way.
• Placing many more of his sperm around each oocyte: Every one of his sperm lacks an acrosome, so more of them adds no enzymes. Numbers are not the problem.
• Correct: Injecting one sperm directly into each oocyte: Correct. Intracytoplasmic injection skips the zona, the step these sperm cannot pass.
• Adding extra albumin and calcium to capacitate his sperm: Capacitation readies a sperm to release its acrosome's enzymes. A sperm with no acrosome has nothing to release.

Two whole paternal sets plus one maternal set means two sperm delivered their nuclei into one oocyte. That is polyspermy, which the cortical reaction and the membrane block normally prevent.

• Nondisjunction of one chromosome pair during the father's meiosis: Nondisjunction of one pair adds or removes a single chromosome, giving 45 or 47, not a whole extra set of 23.
• The oocyte skipped meiosis I: If the oocyte skipped meiosis I, the extra set would be maternal, not paternal.
• The zygote copied all of its DNA twice before it first divided: Copying DNA twice would double every set equally, maternal and paternal alike, giving 92 chromosomes.
• Correct: Two sperm entered the oocyte: the block to polyspermy failed: Correct. Two sperm, each with 23 chromosomes, plus the oocyte's 23 gives 69.