On Becoming a Scientist
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Thinking Like a Scientist Activity 3

Scientists themselves believe, at heart, that behind the diversity lies a unity.

--Horace Freeland Judson

Order from Disorder


Objective

Students will investigate the importance of comparison and classification in science.


About the Activity

When scientists describe and classify objects or events, they do so in order to compare and communicate information about the objects or events. By comparing different groups or classifications, scientists can begin to recognize patterns. These patterns help scientists draw conclusions about the properties and relationships between the objects or events.

In this activity, students will create a karyotype--an ordered display of chromosomes--to see one of the patterns that occurs in living organisms.

Background Information

To study a person's chromosomes, scientists take a photograph of a sample of white blood cells under a microscope. Water and fixatives are added to the cell during the metaphase cycle of mitosis, when the chromosomes are spread out. This allows the scientists to clearly see each chromosome as well as the location of the centromere of each chromosome. The chromosomes are often treated with a chemical stain, which stains some segments more darkly than others. This results in a distinct banding pattern. Scientists then cut up the photograph and make a chart called a karyotype that shows the individual chromosomes grouped in pairs. The chart helps scientists see defects in the chromosomes and/or any missing or extra chromosomes.


Materials

For each group:

  • Handout 1
  • Scissors
  • Tape or glue stick
  • Paper


Preparation

Duplicate the handout to distribute to the students. You may also want to provide a second copy, or make an overhead transparency, of the handout so that students have a completed karyotype to examine.


Instructions:

 

  1. Have students begin by cutting out the individual chromosomes on the handout. They should then place all the cutout chromosomes in a pile, and mix them up.
  2. Have students create the karyotype by regrouping the chromosomes into pairs based on length, banding pattern, and position of centromere. Ask them to tape or glue each pair onto the sheet of paper. Then have them label the pairs with the names of the chromosomes.


Discussion Questions:

 

  1. What is the normal number of chromosomes in a human body cell? How do you think scientists could have determined this?
  2. How might scientists be able to identify possible genetic defects by creating a karyotype?
  3. Is this a karyotype for a male or a female? How can you tell?
  4. There are normally 23 chromosomes in human sperm and egg cells. Why is the number of chromosomes in these cells different from those in human body cells?



Handout 1: Human Chromosomes

Activity 4: Seeking Solutions


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