5 Steps to a 5 AP Biology, 2014-2015 Edition

Human Reproduction ❮ 203

Oogenesis

In women, the process of gamete formation is called oogenesis(Figure 16.3), which begins

quite early—as the embryo develops. Mitotic division turns fetal cells into cells known as

primary oocytes,which begin the process of meiosis and progress until prophase I; then

the wait begins. The primary oocyte sits halted in prophase I until the host female enters

puberty a number of years later. This is where the menstrual cycle begins. Each month, one

Figure 16.2 The human female reproductive system. (FromBiology,8th ed., by Sylvia S. Mader,

© 1985, 1987, 1990, 1993, 1996, 1998, 2001, 2004 by the McGraw Hill Companies, Inc.

Reproduced with permission of The McGraw-Hill Companies.)

Figure 16.3 Oogenesis.

Excess genetic info of polar body is cast away.

Polar body

Meiosis I* Excess info again cast away.

Fetal cell Primary oocyte Polar body

Can remain frozen in Meiosis II** Each primary oocyte

prophase I for many years results in a single ovum.

as a primary oocyte. Secondary oocyte

*After puberty begins, one primary oocyte re-enters
meiosis every 28 days.
**Meiosis II begins after a secondary oocyte is fertilized by a sperm. Egg

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