Fundamentals of Anatomy and Physiology

Cellular Metabolism and Reproduction: Mitosis and Meiosis

mitosis, look-alike chromosomes did not pair up with one
another. In meiosis, homologous chromosomes are brought
so close together that they line up side by side in a process
called synapsis. We now have a pair of homologous
chromosomes each with two sister chro-matids. The visible
pair of chromosomes is called a tetrad (Figure 4-14). The
chromosomes are so close together that they may actually
exchange genetic material in a process called crossing-
over. Actual segments of DNA are exchanged between the
sister chromatids of the homologous chromosomes.
Crossing-over is a com-mon but random event and it occurs
only in meiosis. Evidence of crossing-over can be seen with
a light microscope as an X-shaped structure known as a
chiasma or chiasmata (key-AZZ-mah-tah) (plural). The
spindle forms from microtubules just as in mitosis;

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paired chromosomes separate slightly and orient them-

selves on the spindle attached by their centromere.

Metaphase I

Spindle microtubules attach to the kinetochore only on the

outside of each centromere, and the centromeres of the two

homologous chromosomes are attached to microtubules

originating from opposite poles. This one-sided attachment

in meiosis is in contrast to mito-sis whose kinetochore on

both sides of a centromere is held by microtubules. This

ensures that the homologous chromosomes will be pulled to

opposite poles of the cell. The homologous chromosomes

line up on the equato-rial plate. The centromeres of each

pair lie opposite one another. The orientation on the spindle

is random; thus, either homologue might be oriented to

either pole.

Terminal chiasma

Homologues

Terminal chiasma

Meiosis Mitosis

Sister

chromatids

Kinetochore

Spindle

microtubules

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Figure 4- 14 The pairing up of homologous chromosomes in prophase I of meiosis produces figures
called tetrads. When crossing--over occurs, it is visible and it is called chiasmata.