Biology (Holt)

Section 1 Chromosomes

118 CHAPTER 6Chromosomes and Cell Reproduction

Formation of New Cells

by Cell Division

About 2 trillion cells are produced by an adult human body every

day. This is about 25 million new cells per second! These new cells

are formed when older cells divide. Cell division, also called cell

reproduction, occurs in humans and other organisms at different

times in their life. In Figure 1,the cells of the fawn that is growing

and developing and the cells in the wound that is healing are under-

going cell division. The type of cell division differs depending on the

organism and why the cell is dividing. For example, bacterial cells

undergoing reproduction divide by one type of cell division. Eukary-

otic organisms undergoing growth, development, repair, or asexual

reproduction divide by a different type of cell division. And the for-

mation of gametes involves yet a third type of cell division.

are an organism’s reproductive cells, such as sperm or egg cells.

Regardless of the type of cell division that occurs, all of the infor-

mation stored in the molecule DNA (deoxyribonucleic acid) must

be present in each of the resulting cells. Recall from Chapter 3 that

DNA stores the information that tells cells which proteins to make

and when to make them. This information directs a cell’s activities

and determines its characteristics. Thus, when a cell divides, the

DNA is first copied and then distributed. Each cell ends up with a

complete set (copy) of the DNA.

Gametes

Objectives

Identifyfour examples of
cell division in eukaryotes
and one example in
prokaryotes.

Differentiatebetween
a gene, a DNA molecule,
a chromosome, and a
chromatid.

Differentiatebetween
homologous chromosomes,
autosomes, and sex
chromosomes.

Comparehaploid and
diploid cells.

Predicthow changes in
chromosome number
or structure can affect
development.

Key Terms

gamete

binary fission

gene

chromosome

chromatid

centromere

homologous

chromosome

diploid

haploid

zygote

autosome

sex chromosome

karyotype

The cells of these organisms are undergoing some type of cell division.

Figure 1Cell division

Repair Growth and development

4A 4B 6E

6A 6E

6A 6E

6E

6C 6F