WORLD OF MICROBIOLOGY AND IMMUNOLOGY Cell cycle and cell division103•
has also been awarded the Passano Foundation Young
Scientist Award and the Harrison Howe Award in 1984; the
Pfizer Award in Enzyme Chemistry in 1985; the U. S. Steel
Award in Molecular Biology; and the V. D. Mattia Award in- In 1988, he won the Newcombe-Cleveland Award, the
Heineken Prize, the Gairdner Foundation International Award,
the Louisa Gross Horwitz Prize, and the Albert Lasker Basic
Medical Research Award; he was presented with the Bonfils-
Stanton Award for Science in 1990.
Cech was made full professor in the department of
chemistry at the University of Colorado in 1983. Cech and his
wife have two daughters. In the midst of his busy research
career, Cech finds time to enjoy skiing and backpacking.
See alsoViral geneticsCELL-MEDIATED IMMUNE RESPONSE•see
IMMUNITY, CELL MEDIATEDCCell cycle and cell divisionELL CYCLE AND CELL DIVISION
The series of stages that a cell undergoes while progressing to
division is known as cell cycle. In order for an organism to
grow and develop, the organism’s cells must be able to dupli-
cate themselves. Three basic events must take place to achieve
this duplication: the deoxyribonucleic acid DNA, which makes
up the individual chromosomeswithin the cell’s nucleusmust
be duplicated; the two sets of DNA must be packaged up into
two separate nuclei; and the cell’s cytoplasmmust divide itself
to create two separate cells, each complete with its own
nucleus. The two new cells, products of the single original
cell, are known as daughter cells.
Although prokaryotes (e.g. bacteria, non-nucleated uni-
cellular organisms) divide through binary fission, eukaryotes
(including, of course, human cells) undergo a more complex
process of cell division because DNA is packed in several
chromosomes located inside a cell nucleus. In eukaryotes, cell
division may take two different paths, in accordance with the
cell type involved. Mitosis is a cellular division resulting in
two identical nuclei that takes place in somatic cells. Sex cells
or gametes (ovum and spermatozoids) divide by meiosis. The
process of meiosis results in four nuclei, each containing half
of the original number of chromosomes. Both prokaryotes and
eukaryotes undergo a final process, known as cytoplasmatic
division, which divides the parental cell in new daughter cells.
Mitosis is the process during which two complete,
identical sets of chromosomes are produced from one origi-
nal set. This allows a cell to divide during another process
called cytokinesis, thus creating two completely identical
daughter cells.
During much of a cell’s life, the DNA within the nucleus
is not actually organized into the discrete units known as chro-
mosomes. Instead, the DNA exists loosely within the nucleus,
in a form called chromatin. Prior to the major events of mito-sis, the DNA must replicate itself, so that each cell has twice
as much DNA as previously.
Cells undergoing division are also termed competent
cells. When a cell is not progressing to mitosis, it remains in
phase G0 (“G” zero). Therefore, the cell cycle is divided into
two major phases: interphase and mitosis. Interphase includes
the phases (or stages) G1, S and G2 whereas mitosis is subdi-
vided into prophase, metaphase, anaphase and telophase.
Interphase is a phase of cell growth and metabolic activ-
ity, without cell nuclear division, comprised of several stages
or phases. During Gap 1 or G1 the cell resumes protein and
RNAsynthesis, which was interrupted during previous mitosis,
thus allowing the growth and maturation of young cells to
accomplish their physiologic function. Immediately following
is a variable length pause for DNA checking and repair before
cell cycle transition to phase S during which there is synthesis
or semi-conservative replication or synthesis of DNA. During
Gap 2 or G2, there is increased RNA and protein synthesis,
followed by a second pause for proofreading and eventual
repairs in the newly synthesized DNA sequences before tran-
sition to mitosis.
The cell cycle starts in G1, with the active synthesis of
RNA and proteins, which are necessary for young cells to grow
and mature. The time G1 lasts, varies greatly among eukaryotic
cells of different species and from one tissue to another in the
same organism. Tissues that require fast cellular renovation,
such as mucosa and endometrial epithelia, have shorter G1
periods than those tissues that do not require frequent renova-
tion or repair, such as muscles or connective tissues.
The first stage of mitosis is called prophase. During
prophase, the DNA organizes or condenses itself into the spe-
cific units known as chromosomes. Chromosomes appear as
double-stranded structures. Each strand is a replica of the
other and is called a chromatid. The two chromatids of a chro-
mosome are joined at a special region, the centromere.
Structures called centrioles position themselves across from
each other, at either end of the cell. The nuclear membrane
then disappears.
During the stage of mitosis called metaphase, the chro-
mosomes line themselves up along the midline of the cell.
Fibers called spindles attach themselves to the centromere of
each chromosome.
During the third stage of mitosis, called anaphase, spin-
dle fibers will pull the chromosomes apart at their centromere
(chromosomes have two complementary halves, similar to the
two nonidentical but complementary halves of a zipper). One
arm of each chromosome will migrate toward each centriole,
pulled by the spindle fibers.
During the final stage of mitosis, telophase, the chro-
mosomes decondense, becoming unorganized chromatin
again. A nuclear membrane forms around each daughter set of
chromosomes, and the spindle fibers disappear. Sometime
during telophase, the cytoplasm and cytoplasmic membrane of
the cell split into two (cytokinesis), each containing one set of
chromosomes residing within its nucleus.
Cells are mainly induced into proliferation by growth
factors or hormones that occupy specific receptors on the sur-
face of the cell membrane, being also known as extra-cellularwomi_C 5/6/03 2:04 PM Page 103