When you were born, your body

consisted of about a trillion cells. The growth that transforms

a zygote—the first cell of a new individual—into an infant, and

eventually into an adult, depends on one of the two types of cell

division that occur in the body. The other type of cell division

takes place only in cells of the testes or ovaries that produce

gametes—a male’s sperm or a female’s eggs. In this chapter, we

look at both these mechanisms by which cells reproduce. As you’ll

see, understanding cell division provides answers to some basic

biological questions. For example, how do cells receive the genetic

information that determines their structure and function? And how

are traits passed from one generation to the next?

This chapter builds on the discussion in Chapter 3 of the

cell nucleus and chromosomes and explains how microtu-

bules assist in cell division (3.6, 3.9).

You will also learn more about how eggs and sperm form

during the processes of oogenesis and spermatogenesis

(16.2 and 16.4).

You will gain a fuller understanding of how the union of

sperm and egg at fertilization (16.6) provides a zygote with

the full set of parental chromosomes required for normal

development.

Key ConCepts

LinKs to earLier ConCepts

Basic Principles of Cell

Division

Cells reproduce by duplicating their

chromosomes and then dividing the

chromosomes and cell cytoplasm

among the daughter cells.

Sections 18.1–18.2

Mitosis: Body Growth

and Repair

The body grows and tissues are

repaired when cells divide by the

type of cell division called mitosis.

This mechanism divides the nucleus

so that each newly formed cell has

the same number of chromosomes

as the parent cell. Sections 18.3,

18.4, 18.9

Meiosis: Cells for Sexual

Reproduction

Sperm and oocytes form by the

type of cell division called meiosis.

Meiosis reduces the number of

chromosomes so that each gamete

has half the number of chromo-

somes of the parent cell. Sections

18.6 –18.9 © iStockphoto.com/ArtisticCaptures

353

Top: L. Willatt, East Anglian Regional Genetics Service/Photo

Researchers, Inc.; Middle and bottom: © Cengage Learning

CeLL reproduCtion

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