BSCI 2201L Lecture Notes - Lecture 10: G2 Phase, G1 Phase, Sister Chromatids
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Lab Report 10
The cell Cycle refers to the series of events that cells got through in order to divide and
proliferate. In this lab session, we studies the processes of mitosis and meiosis. We examined
mitosis in dividing cells in onion root tips as well as meiosis in Ascaris oogenesis. Finally we
dissected drosophila males and examined testes under the microscope.
The Cell Cycle refers to the series of stages that living cells go through. The length and
order of the stages varies among cell lines according to the rate of proliferation and the
environment that surrounds the cells. During the S phase, the chromosomes of the cell are
duplicated in preparation for division, and in the M phase the duplicated chromosome are
segregated into individual nuclei followed by cytokinesis. The M and S phases are separated by
G1 and G2. In the G1 phase, cellular contents are duplicated (except for the chromosomes), and
in G2 phase the cell checks the duplicated chromosomes for errors. Many types of somatic cells
exit the cycle at G1 into G0 and are called terminally differentiated.
Cells utilize different mechanisms to ensure that replication happens correctly through
cell cycle checkpoints. These are commonly Cyclins and Cyclin-dependent kinases that check for
proper transition from G1 to S, G2 to M and the metaphase/anaphase transition during mitosis.
These checkpoints are crucial for proper mitosis to occur.
Mitosis is the process of duplicating one cell into two identical cells with the same
genetic information. After the chromatin of the cell is duplicated and the cell passes the G2-M
checkpoint, the cell enters prophase where chromatin condenses into dense chromosomes,
each comprised of two sister chromatids. In pro-metaphase, the nuclear membrane is
fragmented and the mitotic microtubules are formed. In Metaphase the chromosomes are
aligned on the metaphase plate in the middle of the cell while connected to the centrosomes on
each end of the cell (mitotic spindle). In anaphase the sister chromatids are separated and each
begins to migrate to opposite ends of the cell directed by microtubules. Finally, in telophase and
cytokinesis the cytoplasm is divided among the two daughter cells, the nuclear membrane is
reformed and chromosomes de-condense into chromatin.
The process of meiosis occurs in gamete formation (spermatocyte and oocyte). It
includes two meiosis events that produce four haploid cells. Therefore, meiosis is a reductional
division where each daughter cell contains half the number of chromosomes as the parent cell.
In this lab session we observed the process of mitosis in onion root tips. These cells are
continuously dividing. We also studied the structure of D.Melanogaster testes and the process
of spermatogenesis. Finally, we studied meiosis in Ascaris oocytes.
Materials and Methods:
In order to see the various stages of mitosis in onion root cap cells, we stained the cells
by first using HCL to break down the cellulose cell wall. We then stained the cells with
acetocarmine/orceine. The sample was then studies under the microscope. For the second part
of the lab, Drosophila males were dissected and the testes were removed from the abdomen
and fixed on a glass slide for viewing under the microscope.
Results and Conclusion
Fig.1 Onion root tip cells under phase contrast 40X. The cell labeled as A is in anaphase. The two
sets of chromatids are migrating to opposite ends of the cell.
Fig.2 Oogenesis in Ascaris ovaries. The cell labeled with the arrow is in Prophase II of meiosis.
The chromosomes are condensed and have begun the movement towards the metaplate which
demonstrates pro-metaphase. Because the polar body is present, the cell is in the second