Biology · Human Reproduction · NEET
The divisions are unequal so that one cell keeps nearly all the cytoplasm. The ovum needs a large store of cytoplasm and nutrients to feed the embryo during the early days after fertilisation, before it reaches the uterus. If the cytoplasm were split equally into 4 cells, no cell would have enough food. So the cell divides unequally: one large cell (the future ovum) and small polar bodies that carry away the extra chromosomes but almost no cytoplasm.
Three polar bodies form in total. The primary oocyte completes meiosis I to give one secondary oocyte plus the first polar body. Later, if a sperm enters, the secondary oocyte completes meiosis II to give one ovum plus the second polar body. The first polar body may also divide into two. So the final count is one functional ovum and up to three polar bodies. Only the ovum is functional; the polar bodies degenerate.
The first polar body forms when the primary oocyte completes the first meiotic division (meiosis I). This happens inside the ovary, before ovulation, and it produces the secondary oocyte plus the first polar body. Important NEET point: the first polar body is linked to the secondary oocyte, not the primary oocyte. The primary oocyte gives rise to it while dividing.
The second polar body is extruded after the sperm enters the ovum but before fertilisation (nuclear fusion) is complete. Sperm entry through the zona pellucida and plasma membrane triggers the secondary oocyte to finish meiosis II. This forms the ovum plus the second polar body. So the secondary oocyte only completes meiosis at the time of fusion of sperm with the ovum. This exact timing has been asked in NEET 2019 and 2020.
In spermatogenesis both meiotic divisions are equal, so one primary spermatocyte gives four equal, functional sperms. In oogenesis both divisions are unequal, so one primary oocyte gives only one functional ovum and three non-functional polar bodies. This is why one man's meiosis makes 4 gametes but one woman's meiosis makes only 1 gamete.
Extrusion of second polar body from egg nucleus occurs:
Meiotic division of the secondary oocyte is completed:
Consider the following: A. The reductive division for human female gametogenesis starts earlier than that of male gametogenesis. B. The gap between the first and second meiotic division is much shorter for males than females. C. The first polar body is associated with the formation of the primary oocyte. D. LH surge leads to disintegration of endometrium and onset of menstrual bleeding. Choose the correct answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Polar bodies are tiny cells with almost no cytoplasm and they degenerate soon after they form. Only the large ovum can be fertilised by a sperm. Polar bodies simply carry away the extra sets of chromosomes so the ovum ends up haploid.
Polar bodies are haploid, with 23 chromosomes each. They form during the two meiotic divisions of oogenesis, and meiosis reduces the chromosome number by half. Their job is to remove the extra chromosome sets from the future ovum.
Only one functional ovum. The other three products of meiosis become polar bodies that degenerate. This is a key difference from spermatogenesis, where one primary spermatocyte gives four functional sperms.
The early embryo has no outside food source until it implants in the uterus. The large cytoplasm of the ovum stores nutrients and cell materials that feed the dividing embryo during its first few days. Unequal division makes sure the ovum keeps this store.
