Transcript Cell Reproduction-

Cell Reproduction--Mitosis
Section 8.2
Why do cells reproduce at all?
Couldn’t they grow really, really big?
• Cells are limited to small sizes, therefore
they must reproduce to make many cells
instead of one big cell.
• Reasons why big cells are bad include:
– Diffusion would be too slow.
– Too many proteins would be needed.
– Cell membrane (surface area of cell) would
not be able to increase as fast as the cell
volume and would not work properly. (volume
= l x w x h; surface area = l x w x 6)
Review: all cells come from
preexisting cells
• This means that when a cell reproduces or
divides, it creates two cells that are identical to
the original.
• This occurs not only because it is better to have
many small cells, but also because an organism
needs to grow or replace dead tissue. (This also
will cause population growth in unicellular
organisms.)
Chromosomes carry the coiled
DNA.
The Cell Cycle
• The cell cycle is the steps a cell goes
through during its life.
• First, there is a period of growth and DNA
replication called interphase. Most of a
cell’s life is spent in this stage.
• After this, the cell divides, or reproduces,
in a series of steps called mitosis. After
this 2 identical cells are formed, each with
complete set of chromosomes.
Steps of Mitosis
•
•
•
•
Prophase--prepares
Metaphase--middle
Anaphase--apart
Telophase--two
• **use the terms to provide clues for what
happens in each step**
Prophase
• First and longest step.
• Chromatin (DNA) coils into
chromosomes.
• The two halves are identical,
and are called sister
chromatids. They are held
together by the centromere.
• By the end of prophase the
nuclear membrane is
disappearing.
• In animal cells, centrioles and
spindle fiber have formed and
are lining up in the cell.
Metaphase
• Chromosomes
become attached to
the spindle fibers at
the centromere.
• Spindle fibers help to
position
chromosomes in the
middle of the cell, so
each sister chromatid
is facing a pole.
Anaphase
• Sister
chromatids are
pulled apart
towards opposite
poles.
Telophase
• Chromatids reach
opposite poles and begin
to uncoil.
• Two new nuclei are
formed at the poles.
• A new cell membrane
begins to form down the
middle of cell. This
divides the cytoplasm and
organelles and is called
cytokinesis.