Question

Egg cell of a mouse has 20 chromosomes. What should be the chromosome number of epithelial cells of its ovary and by which type of cell division is the egg cell formed?

1. 20; mitosis
2. 10; meiosis
3. 40; meiosis
4. 40; mitosis

Answer 1

Answer:

several ways. Whereas the gamete formed by spermatogenesis is essentially a motile nucleus, the gamete formed by oogenesis contains all the materials needed to initiate and maintain metabolism and development. Therefore, in addition to forming a haploid nucleus, oogenesis also builds up a store of cytoplasmic enzymes, mRNAs, organelles, and metabolic substrates. While the sperm becomes differentiated for motility, the egg develops a remarkably complex cytoplasm.

The mechanisms of oogenesis vary among species more than those of spermatogenesis. This difference should not be surprising, since patterns of reproduction vary so greatly among species. In some species, such as sea urchins and frogs, the female routinely produces hundreds or thousands of eggs at a time, whereas in other species, such as humans and most mammals, only a few eggs are produced during the lifetime of an individual. In those species that produce thousands of ova, the oogonia are self-renewing stem cells that endure for the lifetime of the organism. In those species that produce fewer eggs, the oogonia divide to form a limited number of egg precursor cells. In the human embryo, the thousand or so oogonia divide rapidly from the second to the seventh month of gestation to form roughly 7 million germ cells (Figure 19.19). After the seventh month of embryonic development, however, the number of germ cells drops precipitously. Most oogonia die during this period, while the remaining oogonia enter the first meiotic division (Pinkerton et al. 1961). These latter cells, called the primary oocytes, progress through the first meiotic prophase until the diplotene stage, at which point they are maintained until puberty. With the onset of adolescence, groups of oocytes periodically resume meiosis. Thus, in the human female, the first part of meiosis begins in the embryo, and the signal to resume meiosis is not given until roughly 12 years later. In fact, some oocytes are maintained in meiotic prophase for nearly 50 years. As Figure 19.19 indicates, primary oocytes continue to die even after birth. Of the millions of primary oocytes present at birth, only about 400 mature during a woman's lifetime.

Answer 2

Correct answer is option 3. 40 ; meiosis

Explanation:

• egg cell is haploid cell (n) where the epithelial cells of the ovary are diploid cells (2n)
• so, if n= 20 then 2n=40
• so, the chromosome number of epithelial cell would be 40.
• within ovary, primordial germ cell or oogonium give rise to the primary oocyte by mitosis. primary oocyte give rise to secondary oocyte and then ovum by meiosis.
• egg is female gamete which can be either secondary oocyte or ovum.
• So, the egg cell is formed by meiosis.