Biology (Holt)

BIO

graphic

Stages of Meiosis

Four cells are produced, each with half as much genetic material as the original cell.

1 Prophase I 2 Metaphase I 3 Anaphase I 4 Telophase Iand cytokinesis

Crossing-over

Spindle

Homologous

chromosomes

Chromosomes become

visible. The nuclear

envelope breaks down.

Crossing-over occurs.

Pairs of homologous

chromosomes move to

the equator of the cell.

Homologous chromosomes

move to opposite poles of

the cell.

Chromosomes gather

at the poles of the cell.

The cytoplasm divides.

Figure 1

Section 1 Meiosis

144 CHAPTER 7Meiosis and Sexual Reproduction

Formation of Haploid Cells

Some organisms reproduce by joining gametes to form the first cell of

a new individual. The gametes are haploid—they contain one set of

chromosomes. Imagine how the chromosome number would increase

with each generation if chromosome reduction did not occur!

(meye OH sihs)is a form of cell division that halves the

number of chromosomes when forming specialized reproductive

cells, such as gametes or spores. Meiosis involves two divisions of

the nucleus—meiosis I and meiosis II.

Before meiosis begins, the DNA in the original cell is replicated.

Thus, meiosis starts with homologous chromosomes. Recall that

homologous chromosomes are similar in size, shape, and genetic

content. The stages of meiosis are summarized in Figure 1.

Step Prophase IThe chromosomes condense, and the nuclear

envelope breaks down. Homologous chromosomes pair

along their length. occurs when portions of a

chromatid on one homologous chromosome are broken

and exchanged with the corresponding chromatid portions

of the other homologous chromosome.

Crossing-over

Meiosis

Objectives

Summarizethe events that
occur during meiosis.

Relatecrossing-over, inde-
pendent assortment, and
random fertilization to genetic
variation.

Comparespermatogenesis
and oogenesis.

Key Terms

meiosis

crossing-over

independent

assortment

spermatogenesis

sperm

oogenesis

ovum

4B

4B

4B