The first step is to review the different stages of the cell cycle
which are essential to the acquisition of chromosomes suitable
The cell cycle is a process of successive cell divisions (mitosis)
interrupted by so-called "resting" periods (interphase). Actu-
ally, the resting cell is very active metabolically with continuous
molecular interactions between DNA, ribonucleic acid (RNA),
The interphase is the period wherein the cell is in a nondividing
state and can be at different stages: the first gap (G1) is between
the last mitosis and the S-phase (phase of DNA synthesis) and
the second gap (G2) is between the completion of the S-phase
and the next mitosis (M). The mitotic division occupies only a
short time in the cell cycle. If the cell reaches its ultimate stage of
differentiation and will not divide anymore, the cell is said to be
in phase GO of the cycle. GO applies also for those cells that have
temporarily stopped dividing (Fig. 2.1).
During the G1 phase, the cell is metabolically active and
requires many organelles for protein synthesis while acquiring
the potential for the DNA-doubling process. The duration of the
entire cycle depends on the time of the G1 phase, which varies
according to different conditions and tissue types. G1 phase
may last from only a few hours to weeks or months, depend-
ing on the mitotic rate of the tissue. The phase of DNA synthe-
sis (chromosome replication) has a duration of approximately
8 hours. The replication is not homogeneous throughout the
genome, and asynchronism of replication occurs, particularly in
the synthesis of the heterochromatin composing the inactivated
DNA replication is achieved when all the chromosomes are
duplicated in two identical sister chromatids with the conse-
quence that the total amount of DNA is now doubled compared
to the normal
value of the interphase nucleus. The following
phase, G2, takes about 4 hours and accumulates the cytoplasmic
organelles necessary to complete the mitosis.
This step-by-step progression is controlled by a series of
checkpoints which stop the process if the previous phase is not
achieved. Different proteins act sequentially on the cell cycle:
the cyclin-dependent kinases (CDKs), the cyclins, and the CDK
Activation of kinases by cyclins positively regulates the cycle
by allowing the cell to enter the successive phases. If the quality
of DNA synthesis is impaired, CKIs would automatically stop
the process and drive the cell to apoptosis.
Although the cell cycle is a continuous process, mitosis has four
distinct phases (Fig. 2.2).
Condensation and fragmentation of the chromatin into chro-
mosomes becomes evident. The nucleolus vanishes and the cen-
trioles, replicated in G2, migrate to opposite poles of the cell.
Each chromosome is still attached to the nuclear membrane and
composed of a double strand of sister chromatids. A constricted
Schematic representation of the
cell cycle with the four
(see text). The cell cycle checkpoints are located at
the G1/S and G2/M transitions.
area called centromere becomes apparent on the chromosomes
and the nuclear membrane disintegrates.
The chromosomes are aligned at the equatorial plate of the
mitotic spindle and attached by their centromere to the network
of microtubules. Metaphase chromosomes are composed of two
sister chromatids joined together by the centromere.
The centromeres are split into two parts and both strands of the
sister chromatids are attracted to opposite pole by shortening of
the spindle fibers. The chromosomes, pulled apart, are clustered
at each pole of the cell.
Telophase results in the formation of a nuclear membrane.
The constriction of the cellular membrane starts the division
of the cytoplasm (cytokinesis). The chromosomes progres-
sively melt back into a chromatin network. At the end, both
daughter cells have the same number of chromosomes as the
The meiosis is a more complex process by which the gonad cell
undergoes two cellular divisions.
The meiosis I follows stages similar to the mitotic division.
During prophase I, each chromosome is duplicated. Chroma-
tid exchanges occur between paired homologue chromosomes
which are linked together by their sites of junction: the chiasmas.
This process, called "crossing over," results in genetic recombi-
nation, with the consequence that genomes between maternal
and daughter cells will not be strictly identical. Anaphase I start
with the migration of homologue chromosomes to the opposite
poles of the cell without splitting of their centromere. Meiosis II
arises without previous DNA synthesis and produces the longi-
tudinal separation of the two chromatids, thereby reducing the
cell to a haploid n number of 23 single-stranded chromosomes.
The fecundation of the ovule by the spermatozoid will restitute