During reproduction, the chromosome number (2n) reduces to half ( $n$ ) in the gametes and again the original number ( $2 n$ ) is restored in the offspring. What are the processes through which these events take place?
The meiotic cell division reducers the chromosome numbers to half during gametogenesis and diploid (2n) number of chromosome is restored by the union of male and female gamete through process of fertilisation.
What is the difference between a primary oocyte and a secondary oocyte?
Primary oocyte is a diploid cell formed in foetal ovary when the gamete mother cell, oogonia is arrested at prophase-l of meiosis. Secondary oocyte is the haploid cell formed from primary oocyte that completes its first meiotic division, during puberty and produces the female gamete ova(n).
What is the significance of ampullary-isthmic junction in the female reproductive tract?
The act of fertilisation takes place in the female genital track that is at the junction of the isthmus and ampulla (ampullary - isthmic junction) of the Fallopian tube.
How does zona pellucida of ovum help in preventing polyspermy?
When a sperm penetrates ovum, it induces changes in the membrane that make the zona pellucida layer impenetrable to additional sperms. Thus, it ensures that only one sperm can fertilise an ovum and stops polyspermy.
Mention the importance of LH surge during menstrual cycle.
Rapid secretion of LH leading to its maximum level during the mid menstrual cycle (14th day) called LH surge induces rupture of Graafian follicle and thereby the release of ovum (ovulation).
The ovulation (ovulatory phase) is followed by the luteal phase during which the remaining parts of the Graafian follicle transform as the corpus luteum. The corpus luteum secretes large amounts of progesterone which is essential for maintenance of the endometrium. Such an endometrium is necessary of implantation of the fertilised ovum and other events of pregnancy.