Download 03 - The Cell Cycle

The Cell Cycle
If we are the product of one fertilized egg, a single cell, how is it
we have become living, functioning organisms of over a trillion
cells?
Without cell division, living organisms cannot grow and develop.
Each new cell must have a complete nucleus with a complete copy
of all genetic instructions (DNA chromosomes)
The process which ensures this is called MITOSIS.
Replication - Before mitosis can begin, the nucleus must make a
copy of its chromatin so that there are two complete sets of
DNA.
We have 6 feet of
DNA in every
nucleus!
The phases of Mitosis:
End result: replication and then separation of DNA into two
complete identical sets, one for each new nucleus.
There are four major phases of mitosis:
Prophase
Metaphase
Anaphase
Telophase
PROPHASE:
 Double stranded chromosomes are now large and dense enough
to be visible with light microscope.
 Nucleus and nuclear membrane disappear.
 In animal cells, spindle fibres begin to form and stretch across
the cell from centrioles.
 Centrioles move to opposite ends of the cell (no centrioles in
plant cells)
 Spindle fibres attach to one side of each centromere
METAPHASE
 Tugging action of spindle fibres pulls double-stranded
chromosomes into a line across the middle of the cell
ANAPHASE
 spindle fibres contract and shorten, pulling centromere apart,
allowing one of each replicated strands to move to opposite
ends of the cell.
TELOPHASE
 final phase of mitosis
 one complete set of chromosomes at each
pole of cell
 spindle fibres disappear
 nuclear membrane forms around each set
of chromosomes
 chromosomes uncoil back into chromatin
 Two nuclei on one cell
Mitosis is over, there is still only one cell but with two sets of
genetic instructions.
Cell Division:
In animal cells, cell membrane pinches in near the middle of the
cell, dividing cytoplasm into two new cells. This is call cytokinesis.
In plant cells, a cell plate develops across the centre of the cell,
forming a new cell wall between the two new cells.
Interphase:
 Makes up most of the cells life
 Cell grows, replicates its DNA, and becomes prepared for the
first phase of mitosis.