Page 46 - Demo 1

THE CELL CYCLE

Cells in the body are either in a state of division or non-division. The
sequence of stages from one division to the next is called the cell cycle. It is an
orderly progression of events that occur between the me a new cell has arisen
from the division of the parent cell to the point when two daughter cells are
produced. The me between divisions varies from minutes to months or even
years.

The cell cycle consists of three parts: interphase, mitosis or M phase,
and cytokinesis. Interphase is the me between divisions, while the other two
parts, mitosis and cytokinesis, define cell division.

Interphase is the first and the longest stage of the cell cycle. It is the me
when the cell performs its usual funcons. The amount of me the interphase
takes varies widely. Some cells, like those found in an embryo, complete the
cell cycle in just a few hours. An adult stem cell of a mammal typically takes
about 24 hours to complete the cell cycle where 22 hours are spent on the
interphase.

The interphase itself is composed of three phases: G1 or the “first
growth” phase, S or “synthesis” phase, and G2 or “second growth” phase.
Originally, G stood for “gap” but now that it has been revealed how
metabolically acve the cell is that using G for “growth” is more preferable.

G1 phase is the cell’s primary growth phase. During this phase, the cell
doubles its organelles (such as mitochondria and ribosomes), and accumulates
materials that will be used for DNA replicaon. For most organisms, this phase
occupies the major poron of the cell’s life span.

Aer G1, the cell enters the S phase when the DNA replicates to produce
two copies of each chromosome. At the start of this phase, each chromosome
has one chromatid consisng of a single DNA double helix. At the end of the S
phase, each chromosome is made up of two sister chromatids, each having one
double helix.

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