Unit 9 - Cell Reproduction
• In this unit, we look at the mechanics of cells
multiplying for both growth and reproductive
processes.
Cell Division
• Cells divide into two for three main purposes:
• Enlargement of the organism – cells will divide in
order for the total number of cells, an thus the total
size of the whole organism, to increase. In children,
cells such as bone cells continuously divide in order
to create larger bones.
• Maintenance – as cells die off, they need to be
replaced. In order to maintain a constant state,
dead cells must be continuously replaced. An
example of this occurs in the skin.
Cell Division
• The third purpose of cell division is to create new
organisms. This may mean one cell splitting into two
identical clones, or may mean the create of
sperm/egg cells to create a new organism.
• All of these processes where new cells are made
are called reproduction.
• Growth specifically refers to the process of an
individual cell increasing in size.
Cell Division
• Cells must divide because once they reach a
certain size, they hit the limit of their surface-tovolume ratio and can’t grow any more. They
cannot grow because they don’t have enough
surface to bring in food/oxygen and to pump out
wastes. Therefore, organisms decide instead to split
into two entire cells, solving this problem.
Reproduction
• There are two types of cellular reproduction.
• Sexual reproduction creates new, geneticallyunique organisms from specialized cells from two
organisms.
• Asexual reproduction creates identical organisms
(clones) to the original. This requires only one
organism and does not create generic variety.
Asexual Reproduction
• Asexual reproduction
comes in four types:
• Binary Fission – used by
prokaryotes only. It is
simply the duplication
of the DNA, and then
the cell splitting into
two, each one with a
copy of the DNA.
Asexual Reproduction
• Fragmentation – when
an organism is broken
apart and each piece
will regrow into a full
organism. Most plants
and sea stars can do
this.
• This requires the
organism to be simple
enough that parts can
live on without the
whole organ system.
Asexual Reproduction
• Budding – this is a
process where a
portion of an organism
is purposely released to
act independently.
• The hydra creates a
second entire organism
on its stalk which
eventually buds off as
a whole.
• This is NOT the same as
releasing spores,
however.
Asexual Reproduction
• Mitosis – this is the
eukaryotic process of
making two identical
cells from one original.
In some ways it
resembles binary fission.
• However, the multiple
chromosomes of a
eukaryote make it
more complex than
diving the single DNA
piece in prokaryotes.
Mitosis
• Mitosis is broken down into four steps.
o
o
o
o
Prophase
Metaphase
Anaphase
Telophase
Mitosis - Interphase
• Between cell divisions,
the cell exists in
interphase. During this
time, the DNA is loose
in the nucleus so that it
can be accessed.
• This is the “normal”
state of a cell, growing,
eating, etc.
• When it comes time to
divide, the cells leaves
interphase and goes
into the mitotic phase.
Mitosis - Prophase
• The first phase of mitosis
is prophase. In this step,
the DNA condenses
into visible
chromosomes.
• A chromosome has an
X-shape because it is
made of two pieces of
DNA (each called a
chromatid) attached
together at the centre
at the centromere.
Mitosis - Prophase
• The nuclear membrane
begins to break down.
This will allow the DNA to
be carried into each of
the two cells to be
created.
• A special organelle
called the centrosome
starts to activate. This
organelle has two “cores”
which control a number
of microtubules called
the mitotic spindle.
Mitosis - Metaphase
• By metaphase, the
nuclear membrane has
completely dissolved
and the chromosomes
have all aligned at the
“equator” of the cell.
Mitosis - Metaphase
• The centrosomes have
fully set up the spindle
network. Each
centromere has tubules
attached to it. These
help align the
chromosomes and
make sure neither of
the new cells gets too
many or too few
chromosome.
Mitosis - Anaphase
• Once anaphase begins,
the spindle fibers pull the
chromosomes apart,
dragging one chromatid
of each chromosome
towards the ends of the
cell.
• This way, each of the new
cells will have half of
every chromosome and
will be able to make the
other by copying the first.
Mitosis - Telophase
• Telophase sees the
DNA fully reach the
poles of the cell.
• In each end, the
nuclear membrane
begins to reform.
• The spindle fibers begin
to break down and the
centromeres
deactivate.
• Telophase is similar to
prophase in reverse.
Mitosis - Telophase
• The final process is to
separate into two cells.
This process is called
cytokinesis and
involves the center of
the cell’s membrane
pinching in to create a
cleavage furrow that
ultimately completely
separates the
membranes.
Mitosis - Interphase
• Once complete, the
two cells will each
enter interphase.
• During this time they will
grow and duplicate
their DNA (remember
that they only have 1
chromatid of each
chromosome at first).
• Later on, the cell may
undergo mitosis again
and again.
Mitosis
Ploidy
• Normally in a cell, most organisms carry two copies
of every chromosome. This is called an diploid (2n)
organism. One of each chromosome was inherited
from each parent.
• Cells that are intended to create a new organism
(such as sperm/egg cells) are called gametes.
• Gametes only carry one of each chromosome and
are therefore haploid (n) cells. One gamete will fuse
with another to give the new organism a full set.
Ploidy
• Normally in a cell, most organisms carry two copies
of every chromosome. This is called an diploid (2n)
organism. One of each chromosome was inherited
from each parent.
• Cells that are intended to create a new organism
(such as sperm/egg cells) are called gametes.
• Gametes only carry one of each chromosome and
are therefore haploid (n) cells. One gamete will fuse
with another to give the new organism a full set.
Ploidy
• In diploid organisms, we have two of each
chromosomes. As they code for the same genes,
they are called homologous chromosomes or
homologs. Each homolog is made of two
chromatids, for a total of four.
• In mitosis, the new cells get one chromatid from
every chromosome, in sexual reproduction, each of
the gametes created will only get one chromatid
from each pair of homologs, giving them only half
of the DNA of a normal cell, making them haploid.
Meiosis
• To create gametes, which are needed for sexual
reproduction, a diploid cell undergoes two cell
divisions in a row to create four haploid cells.
• Essentially, mitosis is happening twice in a row. So
instead of leaving a cell with one chromatid of
every chromosome, each cell at the end of two
divisions will only have one chromatid from each
pair of chromosomes.
• This process is called meiosis.
Meiosis
• Meiosis works mechanically very similarly to mitosis
done twice. There are, however a few key
differences.
• For each division, there are prophase, metaphase,
anaphase, telophase, and cytokinesis.
• One key difference is that instead of entering
interphase after the cell division, in meiosis, the cell
will divide again immediately.
Meiosis I
• For the first cell division of
the cell, meiosis I,
prophase I occurs virtually
the same as prophase in
mitosis except for one
major difference.
• Instead of each
chromosome lining up
individually at the
equator of the cell, each
pair will now line up in a
structure called a tetrad
(tetra=four, as there are
four chromatids in a pair
of chromosomes).
Meiosis I
• A special process
called crossing over
occurs in prophase I.
• The two sister
chromosomes will trade
pieces DNA with each
other. This creates a
new combination of
traits not seen in the
parent, leading to
more diversity.
Meiosis I
• The crossing-over site is
called a chiasmata. On
any one pair of
homologs, multiple
chiasmata may be
present.
• The final DNA is
considered to be
recombinant as it has
been broken apart and
recombined.
Meiosis I
• At metaphase I, all the
tetrads will be aligned at
the equator, and one
chromosome will go to
each of the cells to be
made.
• In mitosis, one chromatid
of every chromosome
went to both cells, here,
both chromatids of only a
single of each
chromosome go to each
of the new cells.
Meiosis I
• Anaphase I will
separate the DNA just
like in mitosis.
• Telophase will reform
the cells as two
separate cells, just like
in mitosis.
• Cytokinesis will occur
and we are left with 2
cells.
Meiosis II
• Instead of entering
interphase, meiosis II will
occur right away. This is
important because this
means the cells had no
time to duplicate their
DNA, and thus the final
product will be cells
with only half the
needed DNA.
Meiosis II
• Meiosis II has all the
usual steps of mitosis.
• Prophase and
metaphase happen
normally, and there is
NOT crossing over in
meiosis II.
• At metaphase II, each
chromosome lines up
on the equator, and a
lone chromatid is
pulled to each of the
new cells to be made.
Meiosis II
• Telophase and
cytokinesis leave us
with a total of four
haploid cells from our
original single diploid
cell.
• Each of these gametes
only has one chromatid
for each pair of
chromosomes in the
original.
Diversity
• Due to crossing over, the diversity of traits passed on
to offspring is increased dramatically. This is how
multiple siblings can all look very different from one
another and from their parents.
• Another factor that increases diversity is that no
particular homolog of one chromosome has to be
paired with a particular homolog of another
chromosome. This means that every
chromosome/chromatid inherited is completely
independent of each other, meaning certain traits
on different chromosomes are not linked.
Diversity
• In this example, note
that a light green
chromosome may end
up with either a light
blue chromatid or a
dark green chromatid
at the final step.
• The traits on one
chromosome are not
linked to ones on other
chromosomes.