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Figure

(a) Light micrograph of seminiferous tubules (200x).

(b) Spermatogonia give rise to primary spermatocytes by mitosis; the spermatocytes, in turn, give rise to sperm cells by meiosis.

Sperm cells

Spermatogonia

Seminiferous tubule

Interstitial cells

Sustentacular cell

Sustentacular cell

Vasos Sangu Neos

Sperm cells (23 chromosomes)

Spermatid (23 chromosomes)

Secondary spermatocyte (23 chromosomes)

Primary spermatocyte (46 chromosomes)

Sperm cells

Spermatogonia

Seminiferous tubule

Interstitial cells

Sperm cells (23 chromosomes)

Spermatid (23 chromosomes)

Secondary spermatocyte (23 chromosomes)

Primary spermatocyte (46 chromosomes)

% —1—Spermatogonium (46 chromosomes)

, Chromatids

Homologous chromosomes pairing

, Chromatids

Homologous chromosomes pairing

Centrosome

Prophase I

Spindle fiber

Metaphase I

Anaphase I

Centrosome

Prophase I

Spindle fiber

Metaphase I

Nuclear envelope

Nuclear envelope

Telophase I

Figure 22.6

Stages in the first meiotic division.

division separates the chromatids, producing cells that are still haploid, but whose chromosomes are no longer in the replicated form. After meiosis II, each of the chro-matids has become an independent chromosome.

The steps of meiosis are clearer when considered in a time sequence. However, keep in mind that, like mitosis, meiosis is a continuous process. Considering it in steps simply makes it easier to follow. Refer to figure 22.6 throughout the following discussion.

1. Prophase I. Individual chromosomes appear as thin threads within the nucleus, then shorten and thicken. Nucleoli disappear, the nuclear membrane temporarily disassembles, and microtubules begin to build the spindle that will separate the chromosomes. The chromosomes have already been replicated.

As prophase I continues, homologous chromosomes pair up side by side and tightly intertwine. During this pairing, called synapsis, the chromatids of the homologous chromosomes contact one another at various points along their lengths. Often, the chromatids break in one or more places and exchange parts, forming chromatids with new combinations of genetic information (fig. 22.7). Since one chromosome of a homologous pair is from a person's mother and the other is from the father, an exchange, or crossover, between homologous chromosomes produces chromatids that contain genetic information from both parents.

Anaphase I

Telophase I completed
Essentials of Human Physiology

Essentials of Human Physiology

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