2.3 – The Importance of Cell Division
Cells undergo cell division for reproduction, growth, and repair
Cell Division for Reproduction
Reproduction involves the transfer of genetic information from the
parent(s) to the offspring.
There are two types of reproduction
Sexual reproduction
the process of producing
offspring by the fusion of two
gametes; the production of
offspring that have genetic
information from each parent
Asexual reproduction
the process of producing
offspring from only one parent;
the production of offspring that
are genetically identical to the
parent
Cell Division for Growth
As multicellular organisms grow, their cells duplicate their genetic
information and divide.
Plant and animal cells need a source of energy, nutrients, water
and gases. They also need to excrete carbon dioxide and other
waste materials.
Chemicals diffuse into, throughout, and out of cells. This process
must happen quickly enough for the cell to function properly. This
is why the cell divides to form more cells instead of getting larger
when it grows
Cell Division for Repair
When part of an organism is damaged, the remaining cells divide
to repair the injury
The Cell Cycle
Cells follow a cell cycle that includes growth and preparation for
division (interphase) followed by cell division (mitosis and
cytokinesis).
Cell Division
Cell division occurs in two stages: mitosis (the division of the
contents of the nucleus) and cytokinesis (the division of the rest of
the cell, such as cytoplasm, organelles, and cell membrane).
Interphase
Cells appear inactive but this is the longest period of the cell
cycle. The cell continues to carry out growth and cellular
respiration.
In the Interphase stage:
 DNA replicates
 The centrioles divide
 Proteins are actively produced
At the end of interphase, the spindle fibres form
Microtubules of the cytoskeleton are responsible for cell shape,
motility and attachment to other cells
(The building blocks of these microtubules are used to grow the
mitotic spindle from the region of the centrosomes.)
Prophase
 The first stage of mitosis
 The nucleolus fades
 Chromatin (replicated DNA and associated proteins) condenses
into chromosomes.
 Each replicated chromosome comprises two chromatids, both
with the same genetic information.
 Microtubules disassemble.
Prophase
 The chromosomes (containing DNA)
become visible.
 Each chromosome consists of two
identical strands called sister chromatids.
 The sister chromatids are held together
by a centromere.
 The spindle fibres attach to the
centromeres of each chromosome.
 The nuclear membrane dissolves.
Metaphase
Tension applied by the spindle fibers aligns all
chromosomes in one plane at the center of
the cell
Metaphase
 The chromosomes line up in the middle of the cell.
 The spindle fibres apply tension to the chromosomes aligning
them into the centre of the cell
Anaphase
Spindle fibers shorten, the kinetochores
separate, and the chromatids (daughter
chromosomes) are pulled apart and begin
moving to the cell poles.
Anaphase
 The centromere splits.
 The sister chromatids separate into daughter chromosomes.
 The spindles fibres pull each daughter chromosome towards
opposite ends of the cell
Telophase
The daughter chromosomes arrive at the
poles and the spindle fibers that have pulled
them apart disappear
Telophase
 Is the final phase of mitosis
 The daughter chromosomes (chromatids) stretch out and
become invisible.
 A new nuclear membrane forms around each group of daughter
chromosomes.
 The cell appears to have two nuclei
Cytokinesis
The spindle fibers not attached to
chromosomes begin breaking down until only
that portion of overlap is left. It is in this
region that a contractile ring cleaves the cell
into two daughter cells. Microtubules then
reorganize into a new cytoskeleton for the
return to interphase
Cytokinesis
 Mitosis is followed by cytokinesis, which results in the entire cell
dividing into two new daughter cells.
 Animal cell cytokinesis features the cell pinching off to form two
daughter cells.
 Plant cell cytokinesis features a new cell wall forming to
separate the two daughter cells.
Checkpoints in the Cell Cycle
specialized proteins monitor cell activities and the cell’s
surroundings
these proteins send messages to the nucleus which then instructs
the cell whether to divide or not
A cell should remain in interphase and not divide if
signals from surrounding cells tell the cell not to divide
there are not enough nutrients to provide for cell growth
the DNA within the nucleus has not been replicated
the DNA is damaged
Mitosis
Mitosis is the division of the nucleus into two identical nuclei.
Mitosis has four phases: prophase, metaphase, anaphase, and
telophase (PMAT).
Stages of Mitosis
http://www.cellsalive.com/mitosis.htm