Biology 12

Appendix 4 • MHR 555

The cells in a multicellular organism fit into one
of two categories: somatic cells and germ cells.
Somatic cellsdivide by mitosis to make up the
specialized cells of the body that the organism
relies on for life processes. Somatic cells also
include stem cells that retain the ability to
develop into required specialized forms. The
mature body has several different kinds of stem
cells, each kind capable of forming a limited
number of specialized cells. In addition to these
cells, most multicellular organisms also have
germ cells— those cells that are set aside to
produce the next generation of organisms. Germ
cells include the specialized gametes (eggs and
sperm) formed by meiosis and the unspecialized
cells that produce them.

Part A: Somatic Cells

Cells follow a cell cycle of growth and division.
Unspecialized cells, like those found in the
early embryo, can divide rapidly. As cells begin
to specialize, some will specialize to the point
that they are terminally differentiated, and
are no longer able to divide. Terminally
differentiated cells are fully specialized and
unable to undergo further change. All of these
cells follow the cell cycle as outlined in Figure
A4.1. The cell cycle can be divided into the
two main phases of mitosis (division) and
interphase (growth and metabolism).
Interphase can be further divided into three
discrete phases: G 1 (Gap 1), S (Synthesis) and
G 2 (Gap 2). Cell activity varies through these
different phases. Cells rely on a system of timed
interactions among cell structures, and proteins
that control these interactions control the
sequence of events that lead towards cell
division. Some cells leave the cell cycle for G 0
(Gap 0) as quiescent postmitotic cells that are
metabolically active but do not grow or
continue along the cell cycle towards mitosis.
During the G 1 phase, the cell grows and there
is an increased level of protein synthesis and
DNA repair. Certain protein molecules act as
inhibitory factors that stop further progress and
keep a cell in G 1. Mutationsassociated with

some of these inhibitory factors can lead to

cancerous growth. Growth factors can induce

postmitotic or quiescent cells to move into G 1

to continue in the cell cycle. This is important

to stimulate tissue cells required to repair a

wound or activate immune system cells that are

required for an immune response. During the S

phase, DNA replicates to double the genome of

the cell. The centrosome, where microtubules

are created and co-ordinated, also duplicates in

preparation for mitosis. During the G 2 phase,

DNA replication stops and various protein

factors prepare the cell for mitosis, the M phase.

Figure A4.1The cell cycle. Cells in the cell cycle go

through G 1 (Gap 1), S (Synthesis), G 2 (Gap 2), and

M (Mitosis). A specialized cell may leave the cell cycle

for G 0 (Gap 0).

During the M phase, the cell goes through

visibly dramatic activity, as illustrated in

Figure A4.2 on page 556. During prophase,

the replicated DNA in the nucleus coils and

condenses to form distinct chromosomes.

Initially the chromosomes appear as single

threads, but later they appear as double threads

showing two chromatids. The centrosomes

separate to move away from each other and

form poles that will define the daughter cells.

In

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G 1

G 0

G 2

S

growth and

repair

cell leaves the cell cycle

as a quiescent cell

replication

of DNA

synthesis of

proteins

time

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cytokinesis

telophase

anaphase

metaphase

prophase

4

APPENDIX

Cell Division