Download Prokaryotic and Eukaryotic Cell Division

Critical Reading 1
Prokaryotic and Eukaryotic Cell
Cycle
In order to reproduce, a cell must be able to duplicate
its DNA and pass along identical copies to each new
daughter cell. This is true for both prokaryotic and
eukaryotic cells. However, the two types of cells do not
go about DNA replication in the same way. Examination
of the differences between prokaryotic and eukaryotic
cells cycles gives us an insight into the evolution of
more complicated cellular life. Prokaryotes are
considered to be simpler than eukaryotic cells because
they have only a single, circular molecule of DNA.
Prokaryotic cells do not have nuclei, and the DNA
molecule is unconfined within the cell membrane. Most
prokaryotic cells grow rapidly, and the process of DNA
replication occurs throughout most of the cell cycle.
There is not enough room in the cell for two complete
molecules of DNA. Therefore, when the DNA copy is
nearly complete, the replication process stops just long
enough for the cell to divide. No visible apparatus, such
as the mitotic spindle seen in eukaryotic cells,
Name:______________________________
participates in the division. The two daughters DNA
strands are linked to different locations on the plasma
membrane to ensure that, during separation, each
daughter cell receives an entire copy of the DNA. Unlike
the mitosis in eukaryotic, there is no condensation or
de-condensation of DNA. Eukaryotic cells grow and
divide at quite different rates. A yeast cell can divide
and double in number in 2 hours; most plant and animal
cells take from 10 to 20 hours. The rate at which a cell
divides is determined by many factors. However, the
chemicals that control the phases of the cell cycle,
called growth factors, play an important role. The
resting phase of the cell cycle is controlled by the G0
factor. When the G1 factor is released, the cell becomes
committed to replication. From this point on, a long
series of chemical switches turn specific processes on
and off. The building of the mitotic spindle and
condensation are both under the guidance and timing
of theses chemical switches. This situation is far more
complex than replication in prokaryotic cells, in which
replication only stops for the brief moment of cell
division. The failure of the chemical switches in
eukaryotic cells can cause certain types of cancer. In the
final phases of eukaryotic cell life, the cells are
programmed for death. The chromatin in the
chromosomes begins to degrade and then the DNA
breaks into fragments. The cell eventually digests itself.
Programmed cell death is different from cell death
resulting from injury or disease-it is important to the
health of an organism. Without it, dead cells would be
left to decompose in the body, and other body systems
would be required to eliminate the decomposing cell.
Answer the following questions on a separate piece of paper in COMPLETE SENTENCES.
1. Why are prokaryotes considered to be simpler than eukaryotes?
2. What prompts the prokaryote to stop replication and undergo cell division?
3. Do eukaryotic cells all divide at the same rate? If not, what controls the rate of doubling?
4. What controls the process of DNA replication in eukaryotes?
5. How is natural cell death different from cell death as a result of injury or disease?
Critical Reading 2
Name:_______________________
Control of the Eukaryotic Cell Cycle
Control of the Cell Cycle
How does the cell know when to divide? How does the cell know when to replicate the DNA? The answers to these
questions have to do with the control of the cell cycle. But how is the cell cycle controlled? The cell cycle is controlled by
a number of protein-controlled feedback processes. Two types of proteins involved in the control of the cell cycle are
kinases and cyclins. Cyclins activate kinases. Cyclins are a group of proteins that is rapidly produced at key stages in the
cell cycle. Kinases activate other target molecules. It is this precise regulation of proteins that triggers advancement
through the cell cycle.
The cell cycle has key checkpoints. When the cell receives key signals or information (feedback regulation), the cell can
begin the next phase of the cell cycle. The cell can also receive signals that delay passage to the next phase of the cell
cycle. These signals allow the cell to complete the previous phase before moving forward. Three key checkpoints are the
cell growth (G1) checkpoint, the DNA synthesis (G2) checkpoint, and the mitosis checkpoint. The cell growth (G1)
checkpoint allows the cell to proceed into the S phase of the cell cycle and continue on to divide. The cell spends most of
the cycle in the G1 phase. G1 is where the cell carries out its main functions. If the cell has performed its functions and
has grown to significant size to be divided in half, key proteins will stimulate DNA replication to begin. If the cells are not
to divide, such as some muscle and nerve cells, the cell will stop at this checkpoint and move into a resting phase. Some
cells may stay in this resting period permanently, never dividing. The DNA synthesis (G2) checkpoint determines if the
cell is ready for mitosis. DNA repair enzymes check the replicated DNA at this point. If the checkpoint is passed, the
many molecular mechanisms and processes needed for mitosis will begin. The mitosis checkpoint determines the end of
one cycle and the beginning of the next. This checkpoint signals the end of mitosis, allowing the cell to prepare for the
beginning of G1 of the next cell cycle.
Please answer these questions thoroughly, in COMPLETE SENTENCES on the same lined paper.
1. Does the cell have any control over its progression through the cell cycle? If so, how is it regulated?
2. What happens at a cell cycle checkpoint?
3. What are the five main phases of the cell cycle? What are the main events in each?
4. List and describe (thoroughly) the three main cell cycle checkpoints.