The Cell - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 25

Their presence allows for selective transport between the cells of material having
molecular weight less than 1000 Da. Molecularly a channel-like structure made up
of proteins, they permit movement of inorganic ions and are crucial in regulating the
electrical signals at neural and neuromuscular junctions [] .
1.8 Cell Cycle
In general, cell cycle is defined as a sequence of events where the cellular and nuclear
contents are duplicated and divided. So we can say that cell cycle ultimately brings
about cell reproduction. Cell cycle differs in different organisms, in various cells of
multicellular organisms, and even at different stages of life. But certain fundamental
processes remain the same: the genetic material is replicated and passed onto prog-
eny following division. This ensures passing the entire genome to each daughter cell.
The time duration of cell cycle also exhibits considerable variance. The eukaryotic
cell cycle is divided into four major phases: the M phase and the other three phases,
collectively termed the interphase.
The M phase is the most spectacular phase—a complex phase of cell cycle in
which the replicated genome splits during mitosis, in other words, the nuclear con-
tents are segregated into the daughter cells. Mitosis is further subdivided into vari-
ous phases discussed later in this chapter. This is quickly followed by cytokinesis,
the division of cytoplasmic contents into the progeny. The cytoplasmic contents are
already replicated during biosynthesis in interphase, in preparation of the impending
cell division. The transcription and protein synthesis is already in continuance during
interphase, and hence the cytoplasmic contents are easily divided into the daughter
cells. A contractile ring, made of nonmuscle myosin II and actin filaments, assembles
equatorially (in the middle of the cell) at the cell cortex, constricting the cell mem-
brane, to form a cleavage furrow eventually dividing the cell into two.
The time phase between the two M phases is the interphase, which can be further
segregated into three phases: G1, S, and G2 ( Fig. 1.11 ). The cell division must go
hand in hand with cell growth so as to maintain cell size. The G1 phase is the gap
phase between the M phase and the S phase. During the G1 phase, the cell responds to
its external and internal stimuli and decides to commit to the S phase, or extends the
gap phase to prepare for the upcoming tumultuous S phase. Sometimes the cells may
switch to a G0 phase of prolonged nondividing state. The cell responds to the external
stimuli to enter this nonproliferating phase.
The S phase is called the synthetic phase and involves the replication of the cell's
nuclear contents. The replication machinery comes into action and, as described ear-
lier, the DNA replicates to double the entire genome.
The G2 phase is the gap phase between the S phase and the next M phase. It pro-
vides additional time to grow and duplicate cytoplasmic contents, proteins, and so
on. The gap phases are also important regulatory checkpoints before the cell commits
itself to upcoming phases in the cell cycle. This is crucial because once committed, it
is difficult for the cell to revert back or halt the further processes [-0] .