Fertilization and implantation
Read the following passage carefully and answer the given questions.
During copulation semen is released by the penis into the vagina. The motile sperms swim rapidly, pass through the cervix, enter into the uterus and finally reach the ampullary region of the fallopian tube. The ovum released by the ovary is also transported to the ampullary region where fertilisation takes place. During fertilization, a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms. This induces the completion of the meiotic division of the secondary oocyte. The second meiotic division is also unequal and results in the formation of a second polar body and a haploid ovum (ootid). Soon the haploid nucleus of the sperms and that of the ovum fuse together to form a diploid zygote. The zygote undergoes cleavage and forms a blastocyst, which gets implanted in the endometrium of the uterus after about 6-7 days of fertilization. Successful fertilization leads to pregnancy.
After fertilization, the zygote divides to form -
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Morula
After fertilisation the zygote starts to divide by mitosis while it moves towards the uterus. This is called cleavage. It makes 2, 4, 8 and 16 cells called blastomeres. The morula goes on dividing and becomes a blastocyst.
(1) Ovum — Incorrect. The ovum is the female gamete that is released from the ovary. The zygote is formed when a sperm fuses with it.
(2) Yolk sac — Incorrect. This is a membrane outside the embryo that forms later. It is not what the zygote forms by dividing.
(3) Morula — Correct. It is the ball of 8 to 16 blastomeres made by cleavage.
(4) Follicle — Incorrect. A follicle is the structure in the ovary in which the ovum develops.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
NCERT Extract:
Fertilisation and Implantation – “… The mitotic division starts as the zygote moves through the isthmus of the oviduct called cleavage towards the uterus (Figure 2.11) and forms 2, 4, 8, 16 daughter cells called blastomeres. The embryo with 8 to 16 blastomeres is called a morula (Figure 2.11e). The morula continues to divide and transforms into blastocyst (Figure 2.11g) as it moves further into the uterus.”
