Let's talk about cell division. Why do cells divide? Cells can divide and make exact copies
with the same number and kind of chromosomes, or they can divide and make unique cells,
with half the number of chromosomes. These are called gametes. When would a cell make exact copies?
When it grows, when an organism is injured, and when cells need to be replaced. The exact
copies of cells are made by mitosis. Consider the human karyotype shown here. A karyotype
is a photograph of chromosomes taken during mitosis. If you count the chromosomes, you
will find that there are 46. This is the number of chromosomes found in all body cells. So
every time a body cell divides, the daughter cells must each have 46 chromosomes. Although
there are 46 chromosomes in every body cell, these chromosomes come in pairs, called homologous
pairs or homologues. One of this pair came from the female parent,
shown here in red, and one is from the male parent, shown here in yellow. The homologues
carry genes for the same traits, but not necessarily the same version of that trait. For example
the chromosome from the female parent might carry blood type A and the male parent blood
type O. To produce more cells that have the same number of chromosomes, each one must
be replicated. The replicated arms of a chromosome are called sister chromatids. The centromere
is the part of the chromosome where the sister chromatids are joined. You can see the sister
chromatids in the karyotype, where each chromosome has two visible arms. So how and when do the
chromosomes become replicated? Here is a diagram of the cell cycle. The gray part of the circle
is interphase, and it consists of 3 subphases:
G1, S, and G2. "G" stands for "growth" so in the G1 phase, the cell grows in size and
makes important molecules. S stands for synthesis, so in this phase, the DNA is replicated to
make sister chromatids. In the G2 phase the cell grows in size and makes proteins.
In the mitotic phase shown here in dark blue, the sister chromatids separate
and become 2 complete chromosome sets, within 2 cells. The division of the cytoplasm is
called cytokinesis. In this schematic you can see that cells are always diploid, or
2n, throughout the cell cycle leading to mitosis. Chromosome replication occurs during interphase,
before mitosis. The times 2 designation refers to the replicated chromosomes, which have
2 sister chromatids. Here are some photos of a plant cell undergoing mitosis. These
plant cells are larger and have bigger chromosomes, so cell division is easier to see. The first
cell is in interphase, and although the chromosomes are replicating, they appear as a ball of
spaghetti. The chromosomes are more easily seen because they have become denser as you
see in the this photo of prophase. In this photo we see that the chromosomes are in the center
of the cell, as spindle fibers have attached to the centromeres. The diagram above the
photo more clearly shows this attachment. Up till this point, each chromosome is replicated,
that is, it has sister chromatids. In this photo, the chromatids have now separated and
have become single chromosomes, destined for 2 different cells. Finally in the last photo,
we see the beginning of cytokinesis occur with the formation of the cell plate.
Here we can see the entire process from interphase through cytokinesis.
When does cell produce cells with only one set of chromosomes? These cells are gametes, egg and sperm in
animals, and are used for reproduction. This process is called meiosis. Cells preparing
for meiosis undergo interphase (G1, S , and G2), but the process is not a cycle, since
the gametes cannot repeat the process and go through the cycle again, as the daughter
cells of mitosis can. There are 2 divisions in meiosis. The replication of the chromatids
has already occurred in the S phase of the cell cycle. The x2 represents this replication.
After meiosis I, the cells are now haploid, or n, but the chromosomes are still replicated,
denoted by the x2. Haploid means that there is only one of each homologous chromosome
in the cell. After meiosis II the sister chromatids separate, and each cell is not only haploid,
or n, but chromosomes are single. So in meiosis I, homologous chromosomes are separated, and
in the second division. and in meiosis II, chromatids are separated.
Next we will simulate meiosis and mitosis using bean models.
Be sure to review these terms before you watch the beads activity video.