Cell Cycle and Division
Division of a Cancer Cell. From Biology, McDougal Littell, 2008.
Cell Division
• In order to reproduce, most cells will divide at some
point in their lifespan. The actual process varies,
depending on the cell type.
• Somatic, or non-reproductive cells, divide through
the process of mitosis.
• Gametic, or reproductive cells, undergo mitosis
then utilize a second process called meiosis.
Cell Cycle and Interphase
• For most cells, the majority of
their lifespan is actually spent in
interphase, the time in between
cell divisions when they
perform normal functions.
• During interphase,
the DNA takes the
form of chromatin.
– Chromatin
consists of long
strands of DNA
wrapped around a
cluster of protein
called a histone.
• During mitosis, the
chromatin
condenses into
chromosomes.
• Human cells have 46 chromosomes. This includes
23 from the mother, and 23 from the father.
• Cells in interphase can be identified under a
microscope by looking for nuclei that contain
chromatin, as opposed to the more condensed
worm-like chromosomes.
– The nucleolus may also be visible as a dark area.
• Interphase consists of
three phases.
– During the G1 phase,
the cell replicates its
organelles and cytosol
so there is enough for
two functional cells.
– During the S phase, the
DNA of the cell is
completely duplicated.
– During the G2 phase,
any additional growth
needed by the cell
before division takes
place.
Mitosis
• Mitosis is the part of the cell cycle where the
nucleus physically divides into two, each
containing an exact copy of the original DNA.
• During prophase, the
DNA begins to condense,
forming chromosomes.
• The nucleolus disappears
and spindle fibers formed
by centrioles attach to the
centromeres.
• There are two joined
copies of each
chromosome before
mitosis, called
chromatids.
–
The chromatids are
connected by a single
point, called the
centromere.
• Metaphase begins as
the chromosomes
move and align
themselves in a narrow
central zone in the cell.
• Chromosomes cannot move independently. Their migration
occurs as a result of a network of protein microtubules
called the spindle apparatus produced by the centrioles.
• Anaphase begins when
the centromere of each
chromatid pair splits.
– Now called daughter
chromosomes, they
are pulled towards
opposite ends of the
cell by the spindle
apparatus.
• With the chromosomes
now fully divided, the
nuclear membranes
form during telophase.
– The DNA begins
returning to its uncoiled
chromatin form.
– The nucleolus becomes
visible again.
Cytokinesis
• The process of mitosis only
separated the nucleus. The
cytosol, organelles, and cell
membrane are physically
divided during cytokinesis.
– The cells return to
interphase at this point.
• Cytokinesis proceeds much differently in animal
and plant cells.
• In animal cells, cytokinesis causes the formation of
a shallow indentation across the cell membrane
called a cleavage furrow.
– The furrow tightens, eventually pinching shut as the
cells separate.
• In plant cells,
vesicles containing
cell wall material
will collect at the
middle of the
dividing cell,
forming a cell plate.
Cell Division
• This is a time-lapse microscopy of an epithelial cell
with fluorescent markers undergoing cell division.
The entire process usually takes about 20 minutes.
Cell Cycle
• Once cell division is complete, the daughter cells
will re-enter interphase until receiving a signal to
divide once again.
Cancer
• Most cells of multicellular organisms do not divide
constantly, rather, they have signals or conditions
needed to initiate the process of mitosis.
• Growth factors are proteins secreted by body cells
that stimulate other cells to divide.
• Cells may also exhibit density-dependent
inhibition, where they stop dividing when
conditions become too crowded.
• Cells also exhibit anchorage dependence, meaning
they must be in contact with a solid surface before
they will divide.
• Cancer cells disregard these normal controls and
grow and divide uncontrollably, forming masses of
cells called tumors.
– If the tumor is encapsulated by other tissue and does
not spread, it is considered benign.
– If part of the tumor is able to break free and spread
through the circulatory system of the body to other
places to divide, it is considered malignant.
Cell Division and
Reproduction
• In multicellular organisms, mitosis is primarily
used for tissue growth, regeneration, and repair.
• In many unicellular organisms, mitosis is the
primary form of reproduction.
Asexual Reproduction
• Asexual reproduction is reproduction that involves
a single parent producing an offspring.
– The offspring produced are, in most cases, genetically
identical to the single cell that produced them.
– Asexual reproduction is a simple, efficient, and
effective way for a single organism to produce a large
number of offspring by using only mitosis.
• Prokaryotic organisms reproduce
asexually.
• Only a few eukaryotic organisms,
like sponges, are able to reproduce
asexually.
Sexual Reproduction
• In sexual reproduction, offspring are produced by
the fusion of two sex cells – one from each of two
parents. These fuse into a single cell before the
offspring can grow.
– The offspring produced inherit some genetic
information from both parents.
– Most animals and plants, and some single-celled
organisms, reproduce sexually.
• Organisms that reproduce sexually do not produce
clones – their offspring have more genetic diversity.
– This requires the use of both mitosis and meiosis, a
separate cell division process.
Spermatogenesis
• The male gametes, called sperm, begin as stem cells
in the testes.
• During spermatogenesis, the formation of sperm,
the stem cells will undergo mitosis produce two
exact copies, called daughter cells.
– One copy remains behind
in the testes to generate
future sperm.
– One copy begins to migrate
towards the epididymis.
• At this point, the gametes are given the diploid (2n)
notation, meaning they contain a full set of 46
chromosomes .
• The daughter cell that migrates away will undergo a
second type of cell division called meiosis, which
will result in the production of four genetically
different cells with only half the normal number of
chromosomes.
– The end products of
meiosis are given the
haploid (n) notation,
since they only have
half the normal
number of
chromosomes.
Meiosis
• There are two separate sets of cell divisions that
occur in meiosis. These are labeled as Meiosis I
and Meiosis II.
• During prophase I, the homologous chromosomes
from each of the father’s parents come together to
form a tetrad instead of remaining separate.
• The tetrads then line up in the middle during
metaphase I.
Crossing Over
• A unique phenomenon
occurs in prophase I of
meiosis, called crossing
over.
• The homologous
chromosomes exchange
segments, producing
combinations of genes
unique to those of the
original stem cell.
In this diagram, blue indicates
chromosomes from the father,
and red from the mother of the
individual producing the games.
• Following anaphase I and telophase I, the diploid (2n)
stem cell has divided into two haploid (n) daughter
cells.
• The stages of meiosis II mirror what happens
normally during mitosis; the two haploid (n)
daughter cells duplicate.
– The end result is four haploid (n) genetically different
daughter cells.
• Meiosis also occurs during oogenesis as the ovaries
produce eggs, but cytoplasm is all concentrated in
only one of the four daughter cells.
– The rest are called polar bodies and are re-absorbed by
the body.