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Cell Cycle

The life of a eukaryotic cell can be divided into 4 distinct phases, ending with cell division.

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Cell Cycle

Where do cells come from?

No matter what the cell, all cells come from preexisting cells through the process of cell division. The cell may be the simplest bacterium or a complex muscle, bone, or blood cell. The cell may comprise the whole organism, or be just one cell of trillions.

Cell Division

You consist of a great 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 divide 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 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.

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. Eukaryotic cells, in contrast, have multiple chromosomes contained within a nucleus, and many other organelles. All of these cell parts must be duplicated and then separated when the cell divides. A chromosome is a molecule of DNA, and will be the focus of a subsequent concept.

Cell Division in Eukaryotes

Cell division is more complex in eukaryotes than prokaryotes. Prior to dividing, all the DNA in a eukaryotic cell’s multiple chromosomes is replicated. Its organelles are also duplicated. Then, when the cell divides, it occurs in two major steps:

  1. 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 diploid number (2 sets) of chromosomes. In humans, that number of chromosomes is 46 (23 pairs). Mitosis is described in greater detail in a different section.
  2. The second major step is cytokinesis. As in prokaryotic cells, the cytoplasm must divide. Cytokinesis is the division of the cytoplasm in eukaryotic cells, resulting in two genetically identical daughter cells.

What is a cell's life like?

The eukaryotic cell spends most of its "life" in interphase of the cell cycle, which can be subdivided into the three phases, G1, S and G2. During interphase, the cell does what it is supposed to do. Though cells have many common functions, such as DNA replication, they also have certain specific functions. That is, during the life of a heart cell, the cell would obviously perform certain different activities than a kidney cell or a liver cell.

The Cell Cycle

Cell division is just one of several stages that a cell goes through during its lifetime. The cell cycle is a repeating series of events that include growth, DNA synthesis, 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.

The Eukaryotic Cell Cycle

The diagram in Figure below represents the cell cycle of a eukaryotic cell. As you can see, the eukaryotic cell cycle has several phases. The mitotic phase (M) actually includes both mitosis and cytokinesis. This is when the nucleus and then the cytoplasm divide. The other three phases (G1, S, and G2) are generally grouped together as interphase. During interphase, the cell grows, performs routine life processes, and prepares to divide. These phases are discussed below. You can watch a eukaryotic cell going through these phases of the cell cycle at the following link: http://www.cellsalive.com/cell_cycle.htm.

The Eukaryotic Cell Cycle. This diagram represents the cell cycle in eukaryotes. The G1, S, and G2 phases make up interphase (I). The M phase includes mitosis and cytokinesis. After the M phase, two cells result.


Interphase of the eukaryotic cell cycle can be subdivided into the following three phases, which are represented in Figure above:

  • Growth Phase 1 (G1): during this phase, the cell grows rapidly, while performing routine metabolic processes. It also makes proteins needed for DNA replication and copies some of its organelles in preparation for cell division. A cell typically spends most of its life in this phase. This phase is sometimes referred to as Gap 1.
  • Synthesis Phase (S): during this phase, the cell’s DNA is copied in the process of DNA replication.
  • Growth Phase 2 (G2): during this phase, the cell makes final preparations to divide. For example, it makes additional proteins and organelles. This phase is sometimes referred to as Gap 2.
Control of the Cell Cycle

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 key checkpoints, which are shown in Figure below. There are a number of main checkpoints.

  • The G1 checkpoint, just before entry into S phase, makes the key decision of whether the cell should divide.
  • The S checkpoint determines if the DNA has been replicated properly.
  • The mitotic spindle checkpoint occurs at the point in metaphase where all the chromosomes should have aligned at the mitotic plate.

Checkpoints (arrows) in the eukaryotic cell cycle ensure that the cell is ready to proceed before it moves on to the next phase of the cycle.


  • Cell division is part of the life cycle of virtually all cells. Cell division is the process in which one cell divides to form two new cells.
  • Most prokaryotic cells divide by the process of binary fission.
  • In eukaryotes, cell division occurs in two major steps: mitosis and cytokinesis.
  • 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.

Practice I

Use this resource to answer the questions that follow.

  1. Cell division has how many steps? What are they?
  2. How do prokaryotic cells divide? How do eukaryotic cells divide?
  3. Describe the process of binary fission.
  4. Compare the cells before and after the mitotic division.
  5. What is cytokinesis?

Practice II

Practice III

Use this resource to answer the questions that follow.

  1. What is interphase?
  2. What occurs during the S-phase?
  3. What is G0?
  4. How long can nerve cells and liver cells remain in G0?
  5. Describe the outcomes of density-dependent inhibition.
  6. What is a growth factor? Give an example.


1. Describe binary fission.

2. What is mitosis?

3. Identify the phases of the eukaryotic cell cycle.

4. What happens during interphase?

5. How might the relationship between cancer and the cell cycle be used in the search for causes of cancer?

6. Cells go through a series of events that include growth, DNA synthesis, and cell division. Why are these events best represented by a cycle diagram?

7. Explain how the cell cycle is regulated.

8. Why is DNA replication essential to the cell cycle?

Real World Applications

Mice and Alzheimer's Disease

Student Exploration

Looking For A Cure

Alzheimer's Disease is a devastating illness which affects many families. Go here to learn how scientists are using mice to help find a cure

Fortunately, with the continuing advancement of techniques in molecular biology scientists at the Max Planck have made tremendous advances. Go here to see a video of the advancements they've made: http://www.youtube.com/watch?v=fIHOHV4ZUO4

You can read more about research at the Max Planck Institute here:http://www.mpg.de/1161288/

You can find out here how genetically engineered animals are tailored to study disease

Extension Investigation

Use the below resources to answer the following questions

  1. Why are scientists interested in finding compounds that will dissolve or prevent the formation of TAU deposits?
  2. How do TAU deposits cause nerve cell death?
  3. What is the relationship between the hippocamus and memory?
  4. How common is Alzheimer's Disease in people over 65 years old?

Resources Cited

Parkinson's Disease


Neurological Disorders

Student Exploration

A Way To A Cure

Thanks to Michael J. Fox, many people who do not have friends or family affected are now familiar with Parkinson's Disease. Watch this video to find out even more.

Here's some more information about specific symptoms and progression of the disease

Here you can find out more about the interaction of dopamine and Parkinson's disease.

Here you can find out more about stem cells and processes the people at the Parkinson's Institute hope to use in their pursuit of a cure.

Here's more information about how stem cells in your body work.

Extension Investigation

Use the resources below to answer the following questions:

  1. What are the symptoms of Parkinson’s disease?
  2. What area of the brain is the source of Parkinson’s disease?
  3. What chemicals allow neurons to talk to each other?
  4. What neurotransmitters are involved with Parkinson’s disease? How do they change when someone has Parkinson’s disease? Be as specific as you can in your answer.
  5. Does Parkinson's disease affect everyone in the same way? Explain your answer.
  6. What is one of the functions of stem cells in the adult body?
  7. How do scientists think stem cells may lead to a cure or treatment of Parkinson's Disease? Be as specific as you can.
  8. How many cell types have scientists identified in the body?
  9. What is the difference between embryonic stem cells and adult stem cells?
  10. What are "Induced Pluripotent Stem Cells" (iPSCs)? How are they produced? Explain your answers as fully as you can.
  11. How are researchers hoping to use stem cells to cure or treat heart disease?

Resources Cited



Bacterial Biofilms

Student Exploration

Already In A Theater Near You

Many of you are familiar with antibiotics. If you're lucky, doctors have prescribed them to you to to fight off bacterial infections like strep throat (despite what some people think antibiotics are not very helpful for viruses like the flu (influenza)). Some of you may have heard of "drug resistant" strains of bacteria. Bacterial strains that have changed to counter the effects of the drug, natural selection has even been invoked to explain how resistant bacteria survive the antibiotics and lead to the changes in future generations. But is it really that simple? Maybe not, maybe it's all about biofilms, groups of different bacteria working together.

Extension Investigation

use the below resources to answer the following questions

  1. Why are biofilms more resistant than planktonic bacteria?
  2. What are cytokines?
  3. What kind of bacteria tend to cause acute bacterial infections?
  4. What kind of bacteria tend to cause chronic bacterial infections?
  5. How does multi-layering in the biofilms increase antibiotic resistance?

Resources Cited

Who's Enza?



Student Exploration

Well shut the window!!

"I had a little bird, and its name was Enza, I opened the window, and in flew Enza". This tune (which was sung to the melody of "ring around the rosie") is a bit before your time. It's a nursery rhyme from 1918, something kids would sing while jumping rope. At the time, there was a worldwide pandemic of a type of avian influenza which was dubed the Spanish Flu. People theorize that little diddies like this are made up at times of extreme stress to help children manage, but no one knows for sure. Now, you may be thinking "What? Stress from the flu? The flu comes around every year, what's the big deal?" Well, you can go here to find out the big deal with the Spanish flu.

Here's a video showing how influenza infects organisms

At this point, some of you may be concerned because you have heard of the strains of bird flu making the rounds today. Well, to equal the Spanish flu a new flu strain would have to be highly lethal and readily transmitted. So far, no strains have come around with both these qualities. Not only that but scientists are using biomolecular techniques to study this virus and determine if any strain has a lethal comnbination of genes before the strain has spread into a pandemic and to hopefully have a vaccine ready. Go here to find out more.

Extension Investigation

Use the below resources to answer the following questions

  1. What is the estimate of deaths from the Spanish Flu?
  2. How are biomolecular techniques being used to combat avian flu?
  3. Why would it be useful to identify what gene allows influenza to “jump species”?
  4. How many people worldwide die from influenza every year?
  5. How does the virus enter the body?
  6. What are some other illnesses cause by viruses?
  7. What is the difference between the "lytic" and lysogenic" virus phases?

Resources Cited

HagerDavid from Youtube http://www.youtube.com/watch?v=YSgkoldBNkI&feature=fvwrel

Youtube http://www.youtube.com/watch?v=UjLq9Vf9VQs

BillNyeRulz Youtube http://www.youtube.com/watch?v=rbYwNOcKqqc

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