501. Which sequence correctly represents the usual order of key pre-fertilisation events in humans?
ⓐ. Acrosomal reaction → insemination → capacitation → sperm entry
ⓑ. Insemination → capacitation → acrosomal reaction → sperm entry
ⓒ. Capacitation → implantation → sperm entry → ovulation
ⓓ. Ovulation → implantation → capacitation → zygote formation
Correct Answer: Insemination → capacitation → acrosomal reaction → sperm entry
Explanation: Insemination places sperm in the female reproductive tract. After that, sperm undergo capacitation, which functionally prepares them for fertilisation. When a capacitated sperm interacts with the zona pellucida, the acrosomal reaction occurs and helps sperm entry into the oocyte. This sequence correctly places the preparatory and entry events in order.
502. Which event normally occurs before the acrosomal reaction during human fertilisation?
ⓐ. Capacitation of sperm in the female tract
ⓑ. Implantation of the blastocyst
ⓒ. Completion of cleavage to morula
ⓓ. Formation of the placenta
Correct Answer: Capacitation of sperm in the female tract
Explanation: Capacitation is a preparatory change that sperm undergo after entering the female reproductive tract. Only after this functional activation can the sperm effectively participate in interaction with the zona pellucida and undergo the acrosomal reaction. Implantation and placentation occur much later. So capacitation is the earlier event in the fertilisation sequence.
503. A sperm has reached the secondary oocyte and has begun releasing enzymes after contact with the zona pellucida. This event is called
ⓐ. spermiation
ⓑ. ovulation
ⓒ. acrosomal reaction
ⓓ. implantation
Correct Answer: acrosomal reaction
Explanation: The acrosomal reaction occurs when the sperm contacts the zona pellucida and releases enzymes from the acrosome. These enzymes help the sperm penetrate the coverings of the oocyte. The event is therefore directly linked with sperm entry during fertilisation. It should not be confused with ovulation or spermiation.
504. Which of the following is not a part of sperm preparation or entry during fertilisation in humans?
ⓐ. Capacitation
ⓑ. Acrosomal reaction
ⓒ. Sperm interaction with zona pellucida
ⓓ. Implantation in the endometrium
Correct Answer: Implantation in the endometrium
Explanation: Capacitation, acrosomal reaction, and interaction with the zona pellucida all belong to events around fertilisation. Implantation occurs later, after the zygote has undergone cleavage and the blastocyst reaches the uterus. It is therefore not part of sperm preparation or sperm entry. This question helps separate fertilisation events from post-fertilisation development.
505. Blockage at the ampullary-isthmic region of the oviduct would most directly interfere with which event in normal human reproduction?
ⓐ. Implantation of the blastocyst in the uterus
ⓑ. Shedding of the endometrium during menstruation
ⓒ. Formation of the corpus luteum in the ovary
ⓓ. Fertilisation of the secondary oocyte
Correct Answer: Fertilisation of the secondary oocyte
Explanation: In humans, fertilisation usually occurs at the ampullary-isthmic junction of the oviduct, where the transported sperm commonly meets the secondary oocyte. A blockage in this region would therefore most directly disrupt fertilisation, not implantation, ovulation-related ovarian changes, or menstrual shedding.
506. Which event follows sperm entry into the secondary oocyte during human fertilisation?
ⓐ. Formation of the corpus luteum
ⓑ. Completion of meiosis II in the oocyte
ⓒ. Shedding of the endometrium
ⓓ. Regression of the Graafian follicle
Correct Answer: Completion of meiosis II in the oocyte
Explanation: The secondary oocyte is arrested in metaphase II at the time of ovulation. When a sperm enters it, meiosis II is completed. This leads to formation of the ovum and an additional polar body. So sperm entry is the trigger for finishing the second meiotic division.
507. Diploidy is restored in human reproduction when
ⓐ. the Graafian follicle ruptures
ⓑ. the endometrium becomes secretory
ⓒ. the secondary oocyte completes meiosis II
ⓓ. the male and female pronuclei fuse
Correct Answer: the male and female pronuclei fuse
Explanation: Human sperm and ovum are both haploid. Diploidy is restored only when the male and female pronuclei unite after fertilisation. This fusion creates the diploid zygote. Completion of meiosis II is necessary, but diploidy is restored specifically at pronuclear fusion.
508. Completion of meiosis II in the secondary oocyte produces
ⓐ. a primary oocyte and a first polar body
ⓑ. two equal ova
ⓒ. an ovum and a second polar body
ⓓ. a zygote and a blastomere
Correct Answer: an ovum and a second polar body
Explanation: The secondary oocyte completes meiosis II only after sperm entry. Because cytokinesis remains unequal, one large ovum and a small second polar body are formed. The zygote is produced later, after fusion of the male and female pronuclei. Thus, the immediate products of meiosis II are the ovum and second polar body.
509. Which option correctly matches an event with its product in human fertilisation?
ⓐ. Fusion of male and female pronuclei — zygote
ⓑ. Capacitation of sperm — blastocyst
ⓒ. Acrosomal reaction — corpus luteum
ⓓ. Ovulation — morula
Correct Answer: Fusion of male and female pronuclei — zygote
Explanation: The zygote is formed when the haploid male and female pronuclei fuse after fertilisation. Capacitation and the acrosomal reaction are earlier preparatory events, and ovulation releases the secondary oocyte rather than forming an embryo. This makes pronuclear fusion the correct event-product match. It is the defining moment of zygote formation.
510. Assertion: Fertilisation in humans usually occurs in the ampullary-isthmic junction of the oviduct. Reason: Sperm entry into the secondary oocyte leads to completion of meiosis II.
ⓐ. Both Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.
ⓑ. Both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
ⓒ. Assertion is true, but the Reason is false.
ⓓ. Assertion is false, but the Reason is true.
Correct Answer: Both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
Explanation: The assertion is correct because the ampullary-isthmic junction is the usual site of fertilisation in humans. The reason is also correct since sperm entry does trigger completion of meiosis II in the secondary oocyte. However, that fact does not explain why fertilisation occurs specifically at that site. So both statements are true, but the reason is not the explanation of the assertion.
511. After one sperm enters the secondary oocyte in humans, what immediate change helps prevent entry of additional sperms?
ⓐ. Formation of the corpus luteum
ⓑ. Shedding of the endometrium
ⓒ. Changes in the oocyte membrane and its coverings that block polyspermy
ⓓ. Regression of the zona pellucida
Correct Answer: Changes in the oocyte membrane and its coverings that block polyspermy
Explanation: Human fertilisation normally involves entry of only one sperm into the secondary oocyte. After that entry, changes occur in the oocyte membrane and surrounding coverings that prevent additional sperms from entering. This protective response is important because fusion with multiple sperms would disturb the normal chromosome balance. The block to polyspermy is therefore an essential immediate post-fertilisation event.
512. Which statement best explains why polyspermy is normally prevented in human fertilisation?
ⓐ. Once one sperm enters, the oocyte undergoes changes that stop other sperms from entering
ⓑ. The uterus destroys all remaining sperms before they reach the oocyte
ⓒ. The secondary oocyte leaves the oviduct immediately after the first contact
ⓓ. Only one sperm is produced in each ejaculation
Correct Answer: Once one sperm enters, the oocyte undergoes changes that stop other sperms from entering
Explanation: Polyspermy means entry of more than one sperm into the female gamete, and this must be avoided for normal development. After one sperm enters, the oocyte shows membrane and covering changes that block further sperm entry. This ensures that the resulting zygote receives the correct chromosome contribution. It is therefore a safeguard built into normal fertilisation.
513. Which sequence correctly represents the key events immediately after sperm entry into the secondary oocyte?
ⓐ. Pronuclear fusion → completion of meiosis II → second polar body formation → zygote
ⓑ. Completion of meiosis II → formation of ovum and second polar body → fusion of pronuclei → zygote
ⓒ. Ovulation → cleavage → pronuclear fusion → zygote
ⓓ. Implantation → ovum formation → meiosis II → blastocyst
Correct Answer: Completion of meiosis II → formation of ovum and second polar body → fusion of pronuclei → zygote
Explanation: Sperm entry first triggers completion of meiosis II in the secondary oocyte. This produces the ovum and the second polar body. After that, the male and female pronuclei fuse, and the diploid zygote is formed. This is the correct post-entry sequence in human fertilisation.
514. Which structure is formed immediately after fusion of the male and female pronuclei?
ⓐ. Morula
ⓑ. Blastocyst
ⓒ. Secondary oocyte
ⓓ. Zygote
Correct Answer: Zygote
Explanation: The male and female pronuclei are the haploid nuclear contributions from the two gametes. When they fuse, diploidy is restored and a zygote is formed. The morula and blastocyst are later developmental stages produced by cleavage of this zygote. So the immediate product of pronuclear fusion is the zygote.
515. In humans, the chromosomal sex of the offspring is determined by
ⓐ. the uterine environment during implantation
ⓑ. the chromosome carried by the fertilising sperm
ⓒ. the phase of the menstrual cycle during ovulation
ⓓ. the number of polar bodies formed in oogenesis
Correct Answer: the chromosome carried by the fertilising sperm
Explanation: The ovum always contributes an X chromosome in humans. The fertilising sperm may carry either an X chromosome or a Y chromosome. Because of this difference, the sperm determines whether the zygote becomes XX or XY. Sex determination in humans is therefore dependent on the sperm.
516. If an X-bearing sperm fertilises the secondary oocyte, the resulting zygote will have the sex chromosome combination
ⓐ. YY
ⓑ. XY
ⓒ. XX
ⓓ. XO
Correct Answer: XX
Explanation: The ovum contributes one X chromosome in every normal case. If the fertilising sperm also contributes an X chromosome, the resulting zygote becomes XX. This chromosomal combination is associated with the female sex in humans. The sperm therefore decides whether the second sex chromosome is X or Y.
517. If a Y-bearing sperm fertilises the secondary oocyte, the resulting zygote will be
ⓐ. XX
ⓑ. XY
ⓒ. YY
ⓓ. XO
Correct Answer: XY
Explanation: The female gamete always contributes an X chromosome. If the sperm contributes a Y chromosome, the resulting zygote has the combination XY. This chromosomal pattern is associated with the male sex in humans. The answer therefore depends on the chromosome carried by the sperm.
518. Which statement correctly compares the chromosome contribution of the two human gametes in sex determination?
ⓐ. Both sperm and ovum may contribute either X or Y
ⓑ. Ovum determines sex because it may carry X or Y
ⓒ. Sperm and ovum both always contribute X only
ⓓ. Ovum always contributes X, whereas sperm may contribute X or Y
Correct Answer: Ovum always contributes X, whereas sperm may contribute X or Y
Explanation: In human females, the ovum is always X-bearing. Human males produce two types of sperm, one carrying X and the other carrying Y. Because the sperm can vary while the ovum does not, the sperm determines the chromosomal sex of the offspring. This is a standard principle of sex determination in humans.
519. Assertion: Human fertilisation normally results in a diploid zygote. Reason: One haploid pronucleus from the sperm and one haploid pronucleus from the ovum fuse after fertilisation.
ⓐ. Both Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.
ⓑ. Both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
ⓒ. Assertion is true, but the Reason is false.
ⓓ. Assertion is false, but the Reason is true.
Correct Answer: Both Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.
Explanation: Human gametes are haploid, so each carries only one set of chromosomes. After fertilisation, the male and female pronuclei fuse, and their chromosome sets combine. This restores diploidy in the newly formed zygote. The reason therefore directly explains why the zygote is diploid.
520. Assertion: Entry of more than one sperm into the human oocyte is normally prevented. Reason: After one sperm enters, immediate post-fertilisation changes help block additional sperm entry.
ⓐ. Both Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.
ⓑ. Both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.
ⓒ. Assertion is true, but the Reason is false.
ⓓ. Assertion is false, but the Reason is true.
Correct Answer: Both Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.
Explanation: Normal human fertilisation requires that only one sperm contribute its nucleus to the female gamete. After entry of the first sperm, the oocyte undergoes changes that help prevent additional sperm from entering. This protective mechanism avoids abnormal chromosome combinations in the developing zygote. The reason therefore correctly explains the assertion.
