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

204 ❯ STEP 4. Review the Knowledge You Need to Score High

of the primary oocytes frozen in the first act of meiosis returns to action and completes

meiosis I. As we saw earlier, this phase produces a polar body, which has almost no cyto-

plasm and half of the genetic information of the parent cell, and a secondary oocyte,which

has half the genetic information of the parent cell but the majority of its cytoplasm. This

asymmetrical meiosis occurs because the developing embryo will need enough food,

organelles, mitochondria, and other such structures for proper development.

As the menstrual cycle continues, ovulation frees the secondary oocyte to travel into the

fallopian tube to make its way down to the uterus. Fertilization usually occurs in the

oviduct. If a successful fertilization occurs, the secondary oocyte enters meiosis II, again

producing a polar body, as well as an egg that combines with the sperm to form a zygote.

What Is Important to Remember About This Oogenesis Business?

1.It doesn’t all happen at once for the ova. A primary oocyte could sit in the ovary for

40 years before completing the first stage of meiosis.

2.The beginning of each menstrual cycle causes a primary oocyte to resume meiosis I.

3.Oocytes undergo meiosis II only after fertilization with the sperm.

Spermatogenesis

For men, the process is less time-intensive. Let’s face it, guys... we are lazy. Less effort is

better. Less time makes sense and leaves us more time and energy to watch sports and play

video games. Males produce gametes through a process called spermatogenesis(Figure 16.4).

Unlike females, males do not begin forming gametes until puberty. Spermatogenesis occurs

in our old friends, the seminiferous tubules. Here, primary spermatocytesare produced by

mitotic division. These primary spermatocytes undergo meiosis I to produce two secondary

spermatocytes,which undergo meiosis II to produce four spermatids,which are immature

sperm. After production, they enter the epididymis, where their waiting game begins and the

maturation completes.

Each primary spermatocyte

results in four spermatids.

Meiosis II

FOUR SPERM

Meiosis I

Germ cell Primary spermatocyte

Meiosis II

Spermatids mature into

Secondary sperm in the epididymis.

spermatocytes Spermatids

Figure 16.4 Spermatogenesis.

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