Mike Terrell
Debi Hollander
Dan Hand
Cris Salazar
Cell Division Lab: Mitosis and Meiosis
Introduction:
Cell division in eukaryotes occurs in two distinct ways through mitosis, in which cell
divide into two “identical” diploid cells and meiosis, in which cells are divided into diploids and
then divided again into haploids. In mitosis this is accomplished through six stages; interphase,
prophase, metaphase, anaphase, telophase, and finally cytokinesis. In meiosis these steps are
basically repeated further dividing the cells. The majority of cells spend their time in Interphase
which is split into three stage Gap 1, S phase, and Gap 2. During S phase of interphase, the DNA
and centrosomes are replicated and the cell prepares for mitosis.
Mitosis actually begins with prophase, in which the chromatin condenses into visible
structures called chromosomes. However, since the DNA was replicated in S phase the
chromosomes form sister chromatids connected at the centromere. Next the nuclear
membrane dissolves allowing microtubules called spindle fibers, which form at the
centrosomes, to begin attaching themselves to the centromeres of the chromatids. Metaphase
then takes place. During metaphase, spindle fibers push and pull the chromosomes to the
equatorial plate which marks the spot of division. “This organization helps to ensure that in the
next phase, when the chromosomes are separated, each new nucleus will receive one copy of
each chromosome.”(UA) Anaphase occurs when the chromosomes are pulled apart at the
centromere and the chromatids begin moving toward opposite ends, or poles of the cell. And
during telophase the chromatids reach the poles and new membranes form around the two
new nuclei. It is also during telophase that the chromosomes disperse and cytokinesis begins.
Cytokinesis occurs differently in animals then in plants. In plants the rigid cell wall must be
divided with a cell plate, while animal cells create a cleavage furrow which simply pinches the
cell in half, each with a nucleus. Thus, after cytokinesis two identical diploid cells have been
produced.
Meiosis I begins with prophase I, in which homolgous chromosomes, that are pairs of
chromosomes with identical gene characteristics, come together on the nuclear membrane to
form tetrads. These tetrads are bound by the synaptonemal complex and the scene is then set
for crossing over. During crossing-over, portions of the homologous chromosomes break off
and bind to the other chromosomes, forming a chiasma, or “x” where the chromosomes have
exchanged information. Crossing-over is just one of the processes that create the biological
diversity of sexually reproducing organisms. During Metaphase I, homologous pairs are forced
to the equatorial plate, which divided the “two” undeveloped cells. There, the homologous
pairs randomly arrange themselves on either side of the equatorial plate, which is known as
independent assortment and it another process that drives evolution in sexually reproducing
organisms. Next is anaphase I, in which homologous pairs are pulled apart by spindle fibers and
head to the poles of the cell. During telophase I, the sister chromatids arrive at the poles of the
cell and a new nuclear membrane forms around the two diploid cells.
Meiosis II is much like meiosis I, but the DNA is not synthesized again. Because of this
the processes of prophase II, metaphase II, anaphase II, telophase II and ultimately cytokinesis
creates not a “2n” diploid like meiosis I but a single “n” or haploid cell. It is because of the
haploid cell, which contains only half of the genetic information needed for a cell to develop
and mature, that genetic diversity is so abundant in sexually reproducing organisms. Since two
different haploids must be fused together, or fertilized a truly unique offspring is created,
different form its mother, father, and even siblings.
Observation/Discussion
When we saw the slide of prophase we all noticed that the chromosomes were all
visible and as we see, they have starting to condense, since they are able to be seen under a
microscope. At the later stage of prophase the nuclear envelope breaks down and the
kinetohore microtubules appear and connect the kinetochores to the poles. The chromosomes
are held together by a protein called cohesin.
We saw the cells being aligned by the microtubules into a plate. When we looked we
noticed that the plate could show up as a line or a disk, and that both were metaphase. In this
step the microtubules push and pull as a sort of lever to line up the chromosomes into a line.
The spindle apparatus is amazing, although about as accurate as a shotgun, every centromere
gets a microtubule. During metaphase the sister chromatids are still held by cohesin although
most has been lost.
During anaphase the paired sister chromatids separate and the daughter chromosomes
begin to move toward the poles. The microtubules pull the chromosomes by the centromeres
into the poles. At anaphase the enzyme separase hydrolyzes the remaining cohesin and the
chromosomes separate.
When we looked at the slide we saw telophase as the cells began to reform. The
daughter chromosomes reach the poles, and the nuclear envelope and nucleoli re-form and the
cell is ready for cytokinesis. This leads directly into the next generation of cells as the cell is
now ready to start interphase and begin again.
Our group saw all of this on the lab. We all shared the cooler looking cells that we saw
and discussed the findings. Using the microscopes allowed us to get a in depth look at all four
visible steps, and let us see for ourselves what each step looked like. The microscopes allowed
all of us our first look at the inner workings of cells, and it was amazing.
The slides all had examples of the separate cells in each phase of mitsosis. By allowing
us to see this for ourselves it helped our understanding of the entire process to be expanded.
This lab was amazing because it allowed us to look at the book picture and relate it to what we
were seeing on our slides.
Citations:
"Index of /courses/bio141/lecguide/unit6/genetics/DNA/DNArep/images. N.p., n.d. Web.
8 Feb. 2010.
<http://student.ccbcmd.edu/courses/bio141/lecguide/unit6/genetics/DNA/DNArep/image
s>.
Bailey, Regina. "Stages of Mitosis: Prophase." Biology. N.p., n.d. Web. 8 Feb. 2010.
<http://biology.about.com/od/mitosis/ss/mitosisstep_2.htm>.
"Mitosis: Metaphase." Prentice Hall Bridge page. N.p., n.d. Web. 8 Feb. 2010.
<http://www.phschool.com/science/biology_place/biocoach/mitosisisg/meta.html>.
"The Cell Cycle & Mitosis Tutorial." The Biology Project. N.p., n.d. Web. 8 Feb. 2010.
<http://www.biology.arizona.edu/Cell_bio/tutorials/cell_cycle/cells3.html>.