BIOLOGY

CHAPTER-1

CELL CYCLE

Are you aware that all organisms, even the largest, start their life from a
single cell? You may wonder how a single cell then goes on to form such
large organisms. Growth and reproduction are characteristics of cells,
indeed of all living organisms. All cells reproduce by dividing into two,
with each parental cell giving rise to two daughter cells each time they
divide. These newly formed daughter cells can themselves grow and divide,
giving rise to a new cell population that is formed by the growth and
division of a single parental cell and its progeny. In other words, such
cycles of growth and division allow a single cell to form a structure
consisting of millions of cells.

10.1 CELL CYCLE

Cell division is a very important process in all living organisms. During
the division of a cell, DNA replication and cell growth also take place. All
these processes, i.e., cell division, DNA replication, and cell growth, hence,
have to take place in a coordinated way to ensure correct division and
formation of progeny cells containing intact genomes. The sequence of
events by which a cell duplicates its genome, synthesises the other
constituents of the cell and eventually divides into two daughter cells is
termed cell cycle. Although cell growth (in terms of cytoplasmic increase)
is a continuous process, DNA synthesis occurs only during one specific
stage in the cell cycle. The replicated chromosomes (DNA) are then
distributed to daughter nuclei by a complex series of events during cell
division. These events are themselves under genetic control.

10.1.1 Phases of Cell Cycle

A typical eukaryotic cell cycle is illustrated by
human cells in culture. These cells divide once
in approximately every 24 hours (Figure 10.1).
However, this duration of cell cycle can vary
from organism to organism and also from cell
type to cell type. Yeast for example, can progress
through the cell cycle in only about 90 minutes.
The cell cycle is divided into two basic
phases:
􀁺 Interphase
􀁺 M Phase (Mitosis phase)
The M Phase represents the phase when the
actual cell division or mitosis occurs and the
interphase represents the phase between two
successive M phases. It is significant to note
that in the 24 hour average duration of cell
cycle of a human cell, cell division proper lasts
for only about an hour. The interphase lasts
more than 95% of the duration of cell cycle.
The M Phase starts with the nuclear division, corresponding to the
separation of daughter chromosomes (karyokinesis) and usually ends
with division of cytoplasm (cytokinesis). The interphase, though called
the resting phase, is the time during which the cell is preparing for division
by undergoing both cell growth and DNA replication in an orderly manner.
The interphase is divided into three further phases:
􀁺 G1 phase (Gap 1)
􀁺 S phase (Synthesis)
􀁺 G2 phase (Gap 2)
G1 phase corresponds to the interval between mitosis and initiation
of DNA replication. During G1 phase the cell is metabolically active and
continuously grows but does not replicate its DNA. S or synthesis phase
marks the period during which DNA synthesis or replication takes place.
During this time the amount of DNA per cell doubles. If the initial amount
of DNA is denoted as 2C then it increases to 4C. However, there is no
increase in the chromosome number; if the cell had diploid or 2n number
of chromosomes at G1, even after S phase the number of chromosomes
remains the same, i.e., 2n.
In animal cells, during the S phase, DNA replication begins in the
nucleus, and the centriole duplicates in the cytoplasm. During the G2
phase, proteins are synthesised in preparation for mitosis while cell growth continues.