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558 Chapter 17 NEL

17.117.1 The Cell Cycle

All the estimated 100 trillion cells that make up your body arose from a single fertilized

egg. As with the frog egg shown in Figure 1, this fertilized egg cell underwent a series

of divisions that increased the number of cells, thus increasing the size and complexity

of your body until eventually you reached your current size. Cell division also maintains

a fully grown individual. All multi-cellular eukaryotic organisms grow in size and main-

tain the cells of their body (the somatic cells) by a sequence of events called the

cell cycle.

Learning Tip

DNA, the cell’s hereditary

information, is found in the

chromosomes of a cell. In

eukaryotic cells (cells with a

nucleus), the chromosomes

are found in the nucleus.

Review this information in

Section 6.5 of this book.

Figure 1

Early stages of cell division of a fertilized frog egg

one division several divisions

The cell cycle is often described as taking place in phases (Figure 2, next page). However,

the cycle is a continuous process and does not pause after each phase. During the divi-

sion phase (mitosis,or M), the components of the cytoplasm and the components of the

nucleus of the parent cell are divided to give rise to two identical daughter cells by two

processes, mitosis and cytokinesis. Mitosis ensures the equal distribution of the nuclear

contents. This process includes the duplication of chromosomes, so that each daughter

cell ends up with the same number of chromosomes as the parent cell.Cytokinesis

divides the cytoplasm and its constituent organelles of the parent cell roughly equally

between the daughter cells.

For most cells, the nuclear division that occurs during mitosis marks only a small

part of their cycle. The stage between division phases, called interphase, is marked by

a period of rapid growth (gap 1, or G1), the duplication of chromosomes (synthesis,

or S), another period of growth (gap 2, or G2), and preparation for further divisions. Cells

carry out their particular functions during interphase.

Chromosome Structure

Before looking at the details of mitosis, you will need to know something about the

structure of chromosomes. In animals such as humans, the DNA is divided among a

number of chromosomes. Chromosomes contain both DNA and a number of proteins.

cell cyclethe sequence of stages
through which a cell passes from
one cell division to the next

mitosis (M)a type of cell division
in which a daughter cell receives
the same number of chromosomes
as the parent cell

cytokinesisthe division of
cytoplasm

interphasethe time interval
between nuclear divisions when a
cell increases in mass, roughly
doubles the cytoplasmic
components, and duplicates its
chromosomes