Cell Division and Reproduction
What do you think this picture shows? If you guessed that it’s a picture of two cells, you
are right. In fact, the picture shows human cancer cells, and they are nearing the end
of the process of cell division. In this process, a single cell divides into two cells. Cell
division is just one of the stages that all cells go through during their life. This is true of
normal cells as well as cancer cells, although cancer cells divide more often and grow
out of control. In fact, this is how cancer cells cause illness.
Before Reading:
RECALL: What is the main difference between prokaryotic and eukaryotic cells?
PREDICT: Which of these two cell types do you think will grow faster? Why?
Introduction
You consist of many cells, but like all other organisms, you started life as a single cell.
How did you develop from a single cell into an organism with trillions of cells? The
answer is cell division. After cells grow to their maximum size, they separate into two
new cells. These new cells are small at first, but they grow quickly and eventually divide
and produce more new cells. This process keeps repeating in a continuous cycle.
Cell Division
Cell division is the process in which one cell, called the parent cell, divides to form two
new cells, referred to as daughter cells. How this happens depends on whether the
cell is prokaryotic or eukaryotic. Remember that a prokaryotic cell does not have a
nucleus. Cell division is simpler in prokaryotes than eukaryotes because prokaryotic
cells themselves are simpler. Prokaryotic cells have a single circular chromosome, no
nucleus, and few other organelles. Compare this to eukaryotic cells, which have
multiple chromosomes (pieces of DNA) contained within a nucleus and many other
organelles. A eukaryote makes copies of all of these cell parts and then separates
them when the cell divides.
Reading Check:
1. Which type of cells go through the process of cell division?
a) prokaryotic cells
c) prokaryotic and eukaryotic cells
b) eukaryotic cells
d) neither
2. Complete this sentence.
Cell division is a more complex process in eukaryotes because eukaryotic cells
have a _________________ (which includes multiple __________________ ) as well as
many other _________________; the eukaryotic cell must make _________________ of
all these parts, and then _________________ them when the cell divides.
3. If you were conducting a scientific experiment in a laboratory, which type of
cell would be best to study? Why?
Cell Division in Prokaryotes
Most prokaryotic cells divide by the process of binary fission. The term “binary” means
consisting of, indicating, or involving two. Binary fission is the process in which one
prokaryote splits into 2 cells.
Figure 3.1: Binary Fission in a Bacterial Cell
Cell division is relatively simple in prokaryotic
cells. The cell on the left is dividing by binary
fission. Notice how its plasma membrane is
starting to grow into the center of the cell. It will
eventually pinch apart the parent cell to form
two identical daughter cells.
Binary Fission
Although binary fission is actually a continuous process, it can
be understood as a series of three steps. The steps are
described below and also illustrated below in Figure 3.2. The
steps include DNA replication, chromosome segregation, and
cytokinesis.
Step 1 DNA Replication: Just before the cell divides, its DNA is
replicated in a process called DNA replication. This
duplication results in two identical chromosomes instead of
just one. This step is necessary so that when the cell divides,
each daughter cell will have its own chromosome.
Step 2 Chromosome Segregation: The two chromosomes
segregate, or separate, and move to opposite ends of the
cell.
Step 3 Cytokinesis: A new plasma membrane starts growing
into the center of the cell, and the cytoplasm splits apart,
forming two daughter cells. This process is called cytokinesis.
The two daughter cells that result are genetically identical to
each other and to the parent cell.
Figure 3.2: Binary Fission
Reading Check:
1. In the process of binary fission, what is the result of DNA replication?
a) one identical chromosome
b) two identical chromosomes
c) daughter cells
2. Why does each daughter cell need its own chromosome?
3. During the chromosome segregation phase, each chromosome moves to
________________ ends of the cell in preparation for ________________.
4. In cytokinesis, what two cell organelles are involved? (circle both)
a) cytoplasm
b) nucleus
c) mitochondria
d) cell membrane
5. The two daughter cells are genetically identical to ________________________
and to _____________________________.
Cell Division in Eukaryotes
Cell division is more complex in eukaryotes than prokaryotes. Before dividing, the DNA
in a eukaryotic cell’s multiple chromosomes is replicated. Its organelles are also
duplicated. Then, when the cell divides, the process occurs in two major steps: The first
step is mitosis, a multi-phase process in which the nucleus of the cell divides. During
mitosis, the nuclear membrane breaks down and later reforms. The chromosomes are
also sorted and separated to ensure that each daughter cell receives a complete set.
Mitosis is described in greater detail in the next section. The second major step is
cytokinesis. As in prokaryotic cells, during this step the cytoplasm divides and two
daughter cells form.
Reading Check:
1. What processes take place in eukaryotic cells before cell division?
(circle all correct answers)
a) mitosis
b) DNA replication
c) organelle duplication
d) cytokinesis
2. The two major steps of eukaryotic cell division are ______________ and
______________.
The Cell Cycle
Cell division is just one of several stages that a cell goes through during its lifetime. A
cycle is a number of related events that happen again and again in the same order.
The cell cycle is a repeating series of events that include growth, DNA replication,
organelle duplication, and cell division. The cell cycle in prokaryotes is quite simple:
the cell grows, its DNA replicates, and the cell divides. In eukaryotes, the cell cycle is
more complicated. As discussed previously, the eukaryotic cell cycle is a more
complex process because the cell itself is more complex.
Eukaryotic Cell Cycle
The eukaryotic cell cycle has several phases. The first phase is called interphase.
During interphase, the cell grows, performs routine life processes, and prepares to
divide by replicating the DNA and duplicating organelles. Interphase takes a much
longer time than cell division. A phase is one of the stages of a process. Cell division
consists of 2 phases and includes both mitosis, when the nucleus divides, and then
cytokinesis, when the cytoplasm divides.
Reading Check:
1. How many phases are there in the eukaryotic cell cycle?
a) 2
b) 3
c) 4
d) 5
2. What are the phases?
3. What 4 things happen during interphase?
4. Define mitosis and cytokinesis.
5. Label Figure 3.3 below using the terms interphase, mitosis, and cytokinesis.
Figure 3.3: Eukaryotic Cell Cycle
Control of the Cell Cycle
Imagine driving somewhere with no rules, no signs, and no traffic signals. You wouldn't
know when to stop or go. It would be very difficult to safely get where you need to go!
Cells need rules as well. If the cell cycle occurred without regulation, cells might go
from one phase to the next before they were ready. What controls the cell cycle?
How does the cell know when to grow, synthesize DNA, and divide? The cell cycle is
controlled mainly by regulatory proteins. These proteins control the cycle by signaling
the cell to either start or delay the next phase of the cycle. They ensure that the cell
completes the previous phase before moving on. Regulatory proteins control the cell
cycle at three key checkpoints
Cancer and the Cell Cycle
Cancer is a disease that occurs when the cell cycle is no longer regulated. This may
happen because a cell’s DNA becomes damaged. Damage can occur due to
exposure to hazards such radiation or toxic chemicals. Cancerous cells generally
divide much faster than normal cells. They may form a mass of abnormal cells called a
tumor (see Figure 3.5). The rapidly dividing cells take up nutrients and space that
normal cells need. This can damage tissues and organs and can eventually lead to
death.
Figure 3.5: These cells are cancer cells, growing out of control and forming a tumor.
Summary
Cell division is part of the life cycle of virtually all cells. It is a more complicated process
in eukaryotic than prokaryotic cells because eukaryotic cells have multiple
chromosomes and a nucleus. The cell cycle is a repeating series of events that cells go
through. It includes growth, DNA synthesis, and cell division. In eukaryotic cells, there
are two growth phases, and cell division includes mitosis. The cell cycle is controlled by
regulatory proteins at three key checkpoints in the cycle. The proteins signal the cell to
either start or delay the next phase of the cycle. Cancer is a disease that occurs when
the cell cycle is no longer regulated. Cancer cells grow rapidly and may form a mass
of abnormal cells called a tumor.