culminating in the determination of its structure the deoxy ribonucleic
acid (DNA) in 1953 by James Watson(DNA) and Francis Crick. Scientists
proved that mitosis takes place in all body cells which retains same number
of chromosomes. Meiotic division takes place in sex cells where the
chromosome number is halved. Observe the fig-27.
Cell division in Human beings
We know that cell as the structural and functional units of life of any
organism. In all organisms the cell divide and form new cells. The process
of cell divisoin is same in unicellular organisms and highly evolved
multicellular organisms like human being.
Cell division is the process that transforms a human fertilized egg
into a baby in nine months and into an adult in the next 20 years. Cell
division and function in a multicellular organism is highly regulated. It
occurs only when there is a need for it.
Cells in some organs, such as heart and brain of an individual never
divide. On the other hand bone marrow cells actively divide to produce
red blood cells, which have a short life span 120 days in the body. For
example, if you cut your finger and bleed, soon a blood clot forms to stop
the bleeding. This brings in various chemicals to the site that stimulate
skin cells to divide and heal the wound. Cell division ceases as the wound
is completely healed. In contrast, cancer cells do not respond to such
growth regulating factors and continuously divide at the expense of normal
cells, thus ultimately killing the host. So it becomes important to
understand the processes involved in cell division. The cell cycle will help
us understand this better
The process of cell division is called
'Mitosis', which is completed in 40 to 60
minutes (this is the time of active
division). The period between two cell
divisions is called 'Interphase'. This is
actually the period when the genetic
material makes its copy so that it is
equally distributed to the daughter cells
during mitosis. Interphase can be divided
into three phases.