First 417 words of the document:
The Cell Cycle
This is the time between one cell division and the next.
Growth of an organism is due to cell multiplication and cell enlargement (synthesis of new
The cell cycle is of two main stages :-
o Interphase- synthesis of new organelles, membrane & DNA. It has three phases
A) G1 phase- growth and production of new organelles.
B) S phase synthesis of DNA (semi conservative)
C) G2 phase a brief period of growth in preparation for cell division.
The length of Interphase varies from one cell type to another
The S and G2 phases are of a fairly fixed duration.
G1 can vary enormously from a few hours to several months.
Some specialized cells remain locked in G1 phase for their life time EG neuron
NB Although the chromosomes have duplicated in the S phase they are not visible.
o Cell division- Sharing of the chromosomes and cytoplasmic contents. It has two stages:-
A) nuclear division mitosis or meiosis
1) There are two types of nuclear division:
Mitosis- for growth and repair
o This type of nuclear division produces two daughter nuclei which
are identical to each other and the parent nucleus.
o For our convince we divide it into four stages but in reality it is a
Prophase- chromosomes become visible and each
chromosome appears to be made of two strands
(chromatids) which are joined at the centromere. The
nuclear membrane and nucleolus disintegrate. In animals
the spindle forms centriole.
Metaphase- the chromosomes a line up at the equator
of the spindle and attach to the spindle fibers to the
Anaphase- the spindle fibers shorten and the
chromatids of each chromosome are separated and
pulled to opposite ends of the spindle
Telophase- the chromosomes unravel and are no
longer apparent. A new nuclear membrane forms
around each group of chromosomes and a nucleolus
appears in each new nucleus. See pg 116-117
Meiosis- producing gametes
B) Cytoplasmic division
In animals the cytoplasm divides around the two new nuclei and a new cell
membrane in formed.
In plant cells a cell plate forms in the center of the original cell and a new cell wall
and cell membrane is built up upon this. See figure 3.24 pg 119