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576 Chapter 17 NEL

parent cell. In contrast, meiosis results in four daughter cells that are different from each

other and from the parent cell. The daughter cells have different genetic information

from each other and from the parent cell and carry half the number of chromosomes as

the parent cell.

Figure 7and Figure 8(next page) summarize the similarities and differences between

mitosis and meiosis. As you examine Figures 7and 8 , make note of the chromosome

number of the cell or cells, whether the chromosome number is haploid or diploid, and

during which stage the chromosome number changes.

Meiosis, combined with fertilization, explains the variation in traits that is observed

in species that reproduce sexually. The variation occurs through three mechanisms.

First, crossing over during prophase I exchanges genes on the chromosomes. Second,

during metaphase I, the paternal and maternal chromosomes are randomly assorted.

Although homologues always go to opposite poles, a pole could receive all the maternal

chromosomes, all the paternal ones, or some combination. Lastly, during fertilization,

different combinations of chromosomes and genes occur when two gametes unite.

prophase I metaphase I anaphase I telophase I

The replicated chromosomes
condense. Homologous
chromosomes come together in
synapsis and crossing over
occurs. Chromosomes attach to
the spindle.

Homologous 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.

Figure 7

Stages of meiosis I. During meiosis I, crossing over occurs and homologous pairs separate.

These events do not occur during mitosis.

Purpose Design Analysis

Problem Materials Evaluation

Hypothesis Procedure Synthesis

Prediction Evidence

To perform this investigation, turn to page 590.

Comparing Mitosis and Meiosis

Scientists often use models to help them to understand complex
processes. To understand the consequences of mitosis and
meiosis, you must have a clear view of the similarities and
differences between these two modes of cell division. In this
investigation, you construct and use models to investigate these
essential processes.

INVESTIGATION 17.3Introduction Report Checklist