HUMAN BIOLOGY

chromosome A DNA

molecule together with

the proteins attached to it.

chromosome number The

sum of the chromosomes in

a species’ body cells.

diploid Having two of each

type of chromosome occur-

ing in a species; body cells

are diploid.

life cycle The recurring

series of events in which

individuals grow, develop,

maintain themselves, and

produce a new generation.

reproduction The process

in which a parent cell or

organism produces a new

individual cell or whole

organism.

n    The continuity of life depends on the ability of cells to

faithfully reproduce themselves.

n Links to Life’s characteristics 1.1, Cell nucleus 3.6, Formation

of eggs and sperm 16.2, 16.4

In biology, reproduction is when a

“parent” cell produces a new gen-

eration of cells, or when parents pro-

duce a new individual. Reproduction

is part of a life cycle, a recurring

series of events in which individuals

grow, develop, maintain themselves,

and reproduce a new generation. The

instructions for the human life cycle

are encoded in our DNA, which we

inherit from our parents.

Remember from Section 3.6 that

DNA is organized into structures

called chromosomes in the nucleus

of a cell. When a cell is not divid-

ing, its threadlike chromosomes are

dispersed in the nucleus. As a cell

prepares to divide, however, each

chromosome is copied and each

copy is coiled and packed tightly. Each copy is condensed

as it coils back on itself again and again (Figure 18.1). Notice

that the DNA loops around some proteins (called histones),

forming beadlike structures. The “beads” then coil up into

a long fiber.

As a cell nucleus starts to divide, the DNA coils tighten up

even more to form the condensed chromosome shown at the

upper right in Figure 18.1. This tight coiling may help keep

chromosomes from getting tangled when they are moved

and sorted into parcels for daughter cells. When the coiling is

complete, a chromosome has its typical size and shape.

Body cells have two sets of 23 chromosomes

Every species of organism has a characteristic number of

chromosomes. In humans, DNA is carried on 23 differ-

ent types of chromosomes (Figure 18.2A). In all cells but

gametes, however—that is, in all somatic (“of the body”)

cells—each type of chromosome comes as a paired set, in

which one member of the pair comes from each parent.

The sum of the chromosomes in a species’ cells is called

the chromosome number. In humans the chromosome

number is 46.

A cell that has two of each type of chromosome is called

a diploid cell. The shorthand 2n indicates that a cell is

diploid. The n stands for the number of chromosomes in

one full set. All the body’s somatic cells are diploid. When

a soon-to-divide diploid cell copies its chromosomes, it

has four sets of them. The division process called mitosis,

which we consider shortly, puts half of this doubled genetic

reproduction: Continuing the Life Cycle

At its most condensed, a duplicated chromo-

some is packed tightly into an X shape.

A chromosome unravels as a hollow cylinder

formed by coiled coils.

The coiled coils consist of a long molecule of

DNA (blue) and the proteins that are associated

with it (purple).

At regular intervals, the DNA molecule is

wrapped twice around a core of proteins. In this

“beads-on-a-string” structure, the “string” is the

DNA, and each “bead” is called a nucleosome.

The DNA molecule itself has two strands that

are twisted into a double helix. Chapter 21

discusses DNA’s structure.

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1

2

2

3

3

4

4

(^55)
6
6
The DNA inside the nucleus of a cell is
typically divided up into a number of chromo-
somes. Inset: a duplicated human chromosome.
Figure 18.1 Animated! DNA in a chromosome is looped and coiled. (© Cengage Learning; Andrew Syred/Science Source)
18.1
354 Chapter 18
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