Video Transcript
The somatic cell of a fruit fly has
eight chromosomes. How many chromosomes are found in
each of the two daughter cells produced after mitosis?
To answer this question, let’s
first review the cell cycle and then look at the different steps during cell
division, or mitosis. In order for an organism to
develop, cells need to divide. This is so the organism can grow
and can replace damaged tissue. Cell division takes place during
the cell cycle. The cell cycle can be broken up
into interphase and mitosis. Actual cell division takes place at
the end of mitosis. In order for the cell to divide, it
needs to make a copy of itself, including all of its DNA. This happens during a stage called
interphase. Before DNA is copied, it exists as
long chromosomes that are all mixed up together.
As mentioned in the question, the
somatic cells of a fruit fly contain eight chromosomes. For simplicity, let’s just pretend
there’s two chromosomes so we can see what happens more easily. During interphase, these two
chromosomes duplicate to make a copy of themselves. Each duplicated copy is now called
a chromatid. And each chromosome is now made up
of two sister chromatids. Now that the DNA is copied, this
marks the end of interphase.
Now mitosis can begin, which is
broken up into four different steps called prophase, metaphase, anaphase, and
telophase. During prophase, the duplicated
chromosomes from interphase begin to wrap up tightly and condense to form a
duplicated and condensed chromosome structure. This X-shaped chromosome shape is
what you’re probably used to seeing represented as a chromosome. Only during prophase are we finally
able to make out the individual chromosomes. During prophase, two structures
called centrosomes begin to form the mitotic spindle fibers. This will be used to separate these
duplicated chromosomes as we’ll see. Additionally, during prophase, the
nuclear membrane begins to break down.
In the next step, called metaphase,
the centrosomes are at the poles of the cell with the duplicated chromosomes along
the equator of the cell. The spindle fibers attach to each
of the sister chromatids in the duplicated chromosome.
In anaphase, these two chromatids
are separated by the retracting spindle fibers. So each chromatid goes to each pole
of the cell.
During telophase, the two
chromatids, which we can now call chromosomes, arrive at the poles of the cells as
the cell begins to divide in two. A new nuclear membrane forms around
the chromosomes. And the chromosomes begin to
decondense and are no longer visible under the microscope. Remember, we are only looking at
two chromosomes for simplicity.
After mitosis, each of the eight
chromosomes are in each daughter cell. These two cells are exact copies of
the original cell.
We now have enough information to
answer our question. In a somatic cell of a fruit fly
with eight chromosomes, the number of chromosomes in each daughter cell after
mitosis is eight.
