Question

Describe the steps of oogenesis when fertilization occurs and when it does not occur. Why are polar bodies produced?

Short answer

Oogenesis is the process of egg formation in females, which begins in the fetus as oogonia form primary oocytes that pause in Prophase I of meiosis. During a menstrual cycle, a primary oocyte may complete meiosis I, form a secondary oocyte and a first polar body, and then enter meiosis II while being arrested at metaphase II. If fertilization occurs, meiosis II resumes, resulting in a mature ovum and a second polar body. If fertilization does not occur, the secondary oocyte degenerates and is expelled during menstruation. Polar bodies are small cells produced during the uneven divisions of meiosis, their purpose is to eliminate extra sets of chromosomes while conserving the cytoplasm for the developing egg and ensuring the proper genetic material is present in the zygote.

Step-by-step solution

Phase 1: Understanding Oogenesis

Oogenesis is the process in the female body that leads to the formation of an ovum or egg. This process starts before a female is born and continues until menopause.

Formation of Primary Oocytes

The process of oogenesis begins in the female fetus. Cells in the ovaries called oogonia multiply by mitotic division to form primary oocytes. These primary oocytes enter the first phase of meiosis (a type of cell division that results in four daughter cells with half the chromosomes of the parent cell) but they pause in this process in Prophase I and will only continue under certain circumstances.

Formation of Secondary Oocytes

The primary oocyte only continues through meiosis I if it is selected for ovulation during a menstrual cycle. If this occurs, the primary oocyte finishes meiosis I to form a secondary oocyte and a smaller cell known as a first polar body. The secondary oocyte then enters meiosis II but is arrested in the metaphase II unless fertilization occurs.

Fertilization and Completion of Oogenesis

Fertilization by a sperm triggers the completion of meiosis II, producing a mature ovum and a second polar body. If fertilization does not occur, the secondary oocyte degenerates and is expelled from the body during menstruation.

Phase 2: The Role and Purpose of Polar Bodies

Polar bodies are small cells that are byproducts of oogenesis, produced during the two rounds of meiosis. They contain extra sets of chromosomes not needed by the mature ovum.

Genesis of Polar Bodies

The polar bodies form during the uneven division of the oocyte during meiosis. In each round of meiosis, the primary and then the secondary oocyte divide unevenly, producing one large cell (which continues to the next stage of oogenesis) and one smaller cell or polar body.

Purpose of Polar Bodies

The key role of polar bodies is to eliminate the extra sets of chromosomes while preserving most of the cytoplasm for the developing egg. This is essential to ensure the egg, and hence the resulting zygote if fertilization occurs, has the right amount of genetic material. If the polar bodies did not remove the extra chromosomes, the egg would have too many chromosomes which can result in severe genetic disorders.