How Do Cells Form New Cells?

The way that body cells divide and become organized groups of cells is called mitosis.


When a human sperm and an egg come together, they combine all of their genes to form the first cell of a new human being. This first cell then must become all of the cells that make up a human: nose, hair, skin, teeth, heart, bones, and everything else! Mitosis is the process where one body cell becomes two body cells. The two cells that result are called daughter cells. Even though the two daughter cells have exactly the same genetic information as the mother cell, they may be slightly different from each other.

Two homologous chromosomes: together they have four chromatids

Every cell in the human body has 46 chromosomes of DNA in the nucleus. Those 46 chromosomes are all paired up, so we often say that humans have 23 pairs of chromosomes. You get one set of chromosomes from your mother and one set from your father. In the diagram, we can say that the dark chromosome came from your father and the light chromosome from your mother. This is called a pair of homologous chromosomes because they contain similar genetic information. What’s interesting is that every cell in your body (except one special place which we’ll see in a minute) has the same exact chromosomes!

When we talk about mitosis, we’re concerned with what the chromosomes do in each cell. The trick is that each of those 46 chromosomes needs to make a copy of itself so the two daughter cells can have the same chromosomes. The first thing that the cell needs to do is to grow in order to prepare for splitting into two cells. This first phase is called interphase, or the growth phase.

While the cell is growing, the chromosomes are not organized: they are like strands of spaghetti spread throughout the nucleus. During the next phase of mitosis, prophase, the chromosomes get organized and ready for mitosis. Then, all of the chromosomes line up in the middle of the cell in metaphase. In anaphase, the chromosomes split apart at the centromere, which is the button-like structure in the middle, into their sister chromatids. Those sister chromatids go to opposite sides of the cell, and then the cell begins to split into two separate cells. This final phase is called telophase and results in two identical cells. During the next interphase, each sister chromatid now makes a copy of itself in order to be complete. After that, the whole process happens again!

Normally, mitosis stops in parts of the body where cells are crowded together. In fact, certain genes in your cells tell them when they should stop reproducing. Sometimes, these genes get damaged and the cells keep reproducing; often, these cells that are growing out of control form cancers. There are many causes of cancer, because there are many reasons that the genes can become damaged: age, viruses, tobacco smoke, diet and disorders that you inherit from your parents can all cause cancer.

1. In which cells does mitosis happen?
2. What are homologous chromosomes?
3. Identify one word for each phase of mitosis that will help you remember what happens in that phase.
Put it together
4. Why should it be useful that most cells stop growing and reproducing at some point?
5. When would it be necessary for the two daughter cells to be different than the mother cell?
Think about it
6. Design a small flip book of approximately 20 pages to demonstrate mitosis. Label all illustrations and include a small description of what is occurring on each page. Each part of the cell should be colored the same throughout the book.
7. What is climate, in your own words?
8. What is the biotic relationship between the yellow and black butterflies and the blue and black butterflies that feed on the same plants? Explain.
9. Identify the role of a cell to an organism.
10. What is the role of chloroplasts?
Cancer Graphs
  1. In figure 4, which shade of line stands for African-Americans?
  2. What does the y-axis stand for? Explain in your own words.
  3. When was the highest death rate for all cancers in African-American males? Females?
  4. For two sites of cancer, the death rate was first higher for whites, then over time it began to be higher for African-Americans. Respond:
    1. Which sites of cancer was this for?
    2. Which gender was this for?
    3. When did it happen?
  5. Which cancer kills more males? More females?
  6. Which cancer kills the most whites?
  7. Now look at figure 5. What does this graph show?
  8. What does the y-axis stand for?
  9. Which two groups are being compared?
  10. Which cancer has the smallest difference in survival between African-Americans and whites? The largest difference?
  11. If 200 African-Americans developed esophageal cancer, approximately how many would die (from all stages)?
  12. Which cancer site do African-Americans survive more often than whites (from all stages)?
  13. What 4 groups is figure 9 comparing?
  14. According to figure 9, who smokes more, African-Americans or whites?
  15. What is the difference between graphs 9 and 10?
  16. Which year did the biggest difference between white and African-American smoking rates occur? The smallest?
  17. Do the statistics in graph 10 agree with the statistics in graph 9? Why?
  18. In which year did whites smoke more than African-Americans?Why do you think these statistics show such a big difference between the health of African-Americans and whites? What contributes to the health of African-Americans?
Insect Classification


The Manduca sexta, like all insects, has certain characteristics. Those characteristics are:

  1. A body divided into three parts (head, thorax and abdomen)
  2. Three pairs of legs
  3. Usually one pair of antennae and a pair of compound eyes (a few exceptions to these characteristics are found)
  4. Usually two pairs of wings (absent in many insects such as lice, fleas, ants; flies have one pair of wings)

In this lab, you will be identifying all of the different parts of both the caterpillar and the moth, in addition to any other insects present in class. We will have a collection of insects borrowed from the Willis lab at Case Western Reserve University.


  • Hand Lens
  • 1 Caterpillar
  • 1 Moth
  • Pencil
  • Paper



  1. Using the caterpillar, turn it so that its ventral surface (underside) is facing you. Make a sketch of the caterpillar, showing all of the sections, starting with the head and ending with the terminal proleg. Clearly label:
    • The head, thorax and abdomen
    • The three pairs of legs
    • The antennae
    • Any other significant body part you notice (e.g., the prolegs)

Using the moth, turn it so that its ventral surface (underside) is facing you. Make a sketch of the moth, showing all of the same sections as the caterpillar. With the moth, however, you will also label where the wings join the thorax.

Keeping your two drawings side by side, clearly demonstrate with arrows which parts of the caterpillar develop into which parts of the moth.

Repeat step #1 for all insects available for identification. Make sure that you have at least five observations for each organism.

  1. Make a comparison chart for the different organisms that you observed. You should have a header row and a row for each organism. The columns will be the characteristics that you observed (at least five). Inside the chart, you should put check marks or descriptions that show which organisms demonstrated which characteristics.
  2. Why do you think that you saw the similarities between organisms that you did?
  3. What do these similarities tell us about how related these organisms are to each other?
  4. Add another row to the chart, and use a human as the organism. What are the major differences?
  5. Assume that a scientist finds an organism that has three body segments: head, thorax and abdomen. It also has antennae, two pairs of wings, but only two pairs of legs. What are two possible inferences that scientist could make?
  6. According to the characteristics of insects, create a new insect by sketching something from your own imagination. Make sure that it has all of the necessary parts, then label those parts and name your organism!
  7. Choose six of the insects that you have observed. Make a dichotomous key with at least five steps that classifies these six insects according to the characteristics that you observed.
The Rising Rate of Melanoma

Melanoma is the most dangerous form of skin cancer, and it is becoming more common. Studies link skin cancer to ultraviolet radiation in sunlight. Yet many people still spend too much time in the sun without adequate protection.


Analyze and Conclude

  1. What does the graph show about the incidence of melanoma in men and women from 1976 to 2004?
  2. What was the rate of melanoma for men in 2004? If there are 80 million men in the United States, ages 40–54, how many cases of melanoma does that rate represent?
  3. Use the data in the graph to calculate the percentage increase in the rate of melanoma in men and in women from 1976 to 2004.
  4. Today, despite doctors’ warnings, many people still believe that a tan is a sign of good health. What impact do you think that belief has had on the incidence of melanoma in recent years?
  5. The chance of developing melanoma increases with age. The data in the graph are for people ages 20 to 54. If you looked at similar data for people ages 20 to 75, how do you think the graph would be different? Explain your answer.

Build Science Skills

While the rate of melanoma was rising between 1976 and 2004, the rate of lung cancer was falling—more than 50 percent in men and almost 30 percent in women. Public health officials say this decrease proves that antismoking campaigns work. What if you were in charge of an antitanning campaign? List three methods you would use to educate teens about the dangers of tanning.

Comparing Bones

The term forelimb refers to the front leg or arm of a vertebrate. The bones in the forelimbs of modern vertebrates are homologous. These forelimbs evolved from the forelimbs of an extinct lobe-finned fish that lived more than 380 million years ago. If modern vertebrates all had different origins, it would be unlikely for them to have such similar structures.

In this activity, you will compare the bones in the forelimb of a human, cat, and lizard.


  • scissors
  • tape


Look at the diagram of the human arm. Note how the upper arm (humerus), lower arm (ulna and radius), and hand bones fit together. Use the diagram as a model to help you piece together the forelimbs of a cat and lizard.


  1. Cut out the drawings of bones on the next page.
  2. Move the parts around to form a cat forelimb and a lizard forelimb. NOTE: The drawings are not drawn at actual size.
  3. After you have pieced together both forelimbs, tape the limbs onto a sheet of notebook paper. Label the parts of each forelimb and add a label for the animal name.

Analyze and Conclude

  1. How did the drawing of the human arm help you assemble the cat and lizard forelimbs?
  2. Describe how you decided which bones belonged to which animal.
  3. Describe the similarities among the three forelimbs. How would these compare in actual size?
  4. Use your structures and your experience to compare the ways that the three species use their forelimbs.

Build Science Skills

How do these homologous structures provide evidence to support the theory that vertebrates evolved from a common ancestor?