Explain the process of oogenesis in meiosis.

Oogenesis in meiosis is the process by which female gametes, or eggs, are produced in the ovaries.

Oogenesis begins in the female fetus before birth. The process starts with the formation of oogonia, which are diploid cells containing 46 chromosomes. These oogonia undergo mitosis to increase their number. Each oogonium then enters the first stage of meiosis, known as meiosis I, to become a primary oocyte. However, the primary oocytes do not complete meiosis I immediately. Instead, they are arrested in the prophase stage of meiosis I and remain in this state until puberty.

At puberty, hormonal changes trigger the completion of meiosis I in one primary oocyte each menstrual cycle. The primary oocyte divides unequally, producing a large secondary oocyte and a small polar body. The secondary oocyte contains most of the cytoplasm and organelles, while the polar body contains a small amount of cytoplasm and eventually degenerates. This unequal division ensures that the egg, if fertilised, has enough resources to support the early stages of embryonic development.

The secondary oocyte then begins meiosis II but is again arrested, this time in the metaphase stage. Meiosis II is only completed if the secondary oocyte is fertilised by a sperm cell. Upon fertilisation, the secondary oocyte divides again unequally to produce a large ovum and a second polar body. The ovum is the mature egg cell ready for implantation in the uterus, while the second polar body, like the first, degenerates.

In summary, oogenesis is a complex process that begins before birth and continues until menopause. It involves two rounds of cell division, meiosis I and II, both of which are paused at different stages until specific triggers prompt their completion. The result is the production of a single mature egg from each primary oocyte, ready for potential fertilisation and the creation of a new life.