Microsoft Word - blank.doc

NEL Cell Division 573

Section17. 3

homologous

chromosome pair

As the chromosomes

move closer together,

synapsis occurs.

Chromatids break,

and genetic information

is exchanged.

Figure 2

Crossing over occurs between

homologous pairs of chromosomes

during prophase I of meiosis.

prophase I metaphase I anaphase I telophase I

Figure 3
During meiosis I, homologous chromosomes are segregated.

The replicated chromosomes

condense. Homologous

chromosomes come together

in synapsis and crossing over

occurs. Chromosomes attach

to the spindle.

Chromosomes line up at the

equatorial plate.

Each chromosome separates

from its homologue. They move

to opposite poles of the cell.

The nucleus completes its

division. The chromosomes

are still composed of sister

chromatids. The cytoplasm

divides after telophase.

Metaphase I follows prophase I (Figure 3). The homologous chromosomes attach them-

selves to the spindle fibres and line up along the equatorial plate.

During anaphase I, the homologous chromosomes move toward opposite poles. The

process is known as segregation. At this point of meiosis, reduction division occurs. One

member of each homologous pair will be found in each of the new cells. Each chromo-

some consists of two sister chromatids.

During telophase I, a membrane begins to form around each nucleus. However,

unlike in mitosis, the chromosomes in the two nuclei are not identical because each of

the daughter nuclei contains one member of the homologous chromosome pair.

Although homologous chromosomes are similar, they are not identical. They carry

genes for the same traits (for example, eye colour), but those genes may differ (for

example, coding for brown eyes or coding for blue eyes). The cells are now ready to

begin the second stage of meiosis.

Crossing Over

This Audio Clip will discuss the

timing of crossing over and the

benefit that a species derives from

this process.

http://www.science.nelson.com GO

+ EXTENSION