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NOVA scienceNOW: Epigenetics

Classroom Activity

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Activity Summary
Students make a model of chromatin and use it to show how chemical tags that bond to the chromatin can influence its uncoiling.

Learning Objectives
Students will be able to:

  • explain the difference between genetics and epigenetics.

  • state that the structure of chromatin includes DNA and histones coiled together.

  • demonstrate how chemical tags that attach to the chromatin help the chromatin uncoil.

  • describe how chemical tags (i.e., an epigenetic factor) play an important role in enabling DNA to be "read" by enzymes and transcribed into messenger RNA.

Materials for each student
  • copy of the "Epigenetics" student handout (PDF or HTML)
  • colored, fine-tipped permanent marker
  • ballpoint pen
  • 3 lengths of thin (i.e., small-diameter) rubber surgical tubing, each 24 inches long (1/8-inch rubber bungee cord can be used as a substitute)
  • 2 small binder clips (3/4-inch size)
  • packaging tape or duct tape, 2 inches in width

Background
For students to grasp the basics of epigenetics, they will need an understanding of DNA and its structural chemistry in the chromosome. Consequently, this lesson is appropriate for a high school biology class in which students have had some exposure to the fundamentals of DNA and RNA. However, as the lesson's theme is epigenetics rather than DNA, the lesson's goal is for students to be able to explain the difference between genetics and epigenetics and to describe the role epigenetic factors play in enabling DNA to be "read" by enzymes and transcribed by messenger RNA.

In the field of epigenetics, scientists study how chemical tags attach themselves to DNA or to the structures surrounding the DNA. These chemical tags can control gene expression, silencing or activating genes. Because these chemical tags are independent of the DNA sequence itself, they are considered to be epigenetic factors. Epigenetic researchers examine the role this silencing or activation of genes might play in cell differentiation, cell development, disease, and heredity.

Epigenetics is a highly relevant area, offering scientists new ways to investigate many fundamental questions about life, health, and disease. For example, how does a single fertilized egg cell differentiate into over 200 cell types? How do exposures to nutrients, toxins, pollutants, and other environmental agents affect gene expression? These questions are at the core of much of today's cutting-edge research and technology in such fields as health care, medicine, pharmacology, fertility, and the management of environmental pollutants.

In this activity, students build a model showing that DNA is enclosed in a histone cushion to form chromatin, the basic component of a chromosome. They use the model to demonstrate one way that methyl groups can bind to the chromatin, enabling it to uncoil to expose the DNA. The activity should clarify the meaning of the term epigenetics and suggest why researchers from many fields are so keenly interested in learning more about the interaction between genetics and epigenetics.

Depending on how familiar your students are with DNA and RNA, you might want to review DNA's structure and the processes of transcription and translation. For example, have students list three facts they can recall about DNA and RNA. Then ask students to share their facts and develop a class list on the board or on a flip chart. Also, discuss how the coiling of DNA around the histone proteins enables nearly two meters-worth of DNA to fit into a microscopic cell nucleus!


Procedure
  1. Demonstrate how DNA is coiled into a double helix. Before class, prepare a set of two tubes representing DNA. Mark these tubes as described in Steps 1a and 1b on the Student Handout. Tape the ends. Invite two students to help you in a demonstration. Have each helper take a taped end of the tubing and face one another. Ask them to twist the tubes into a spiral. Point out that, after just a few twists, the tubing forms into a "double helix."

  2. Demonstrate that DNA's double helix is itself arranged into a secondary spiral. Have the students continue to twist. The tubing will begin to knot up. Have the helpers pull lightly, maintaining gentle outward pressure while still slowly twisting the tubing. The knots should begin to organize into a thick spiral. The structure you are aiming for is a neat, tight, thick spiral. Make sure the class understands what the model shows: (1) the DNA double helix; and (2) the double helix itself is twisted into a secondary spiral.

  3. Add a histone strand and twist the three tubes into a secondary spiral. Uncoil the DNA model and add the third piece of tubing. (It does not matter where the third tube is in relation to the others, as long as it is neatly alongside the other two.) Tape the three tubes together at the ends. Tell students that chromosomes are made of more than just DNA—the DNA is coiled around proteins called histones. Together, the histone proteins and DNA form the chromatin. Chromatin, in turn, forms the chromosome. Once taped together, the three lengths of tubing represent a section of chromatin. Have the helpers repeat the twisting. Invite observations from the class about the appearance of the three tubes. Make sure the class understands what the model shows: (1) chromatin is made of DNA and histones; and (2) the chromatin is twisted into a secondary spiral. (Tell students that in this model, the third surgical tube [i.e., the one with dots or stripes] represents chromatin's histone proteins. As with the two tubes, the three tubes will also twist into the second spiral of knots. Point out how the inclusion of the third tube as well as the twisting makes the sequence of letters [i.e., the DNA's sequence of nucleotides] hard to read.)

  4. Discuss how coiling makes it hard to read the DNA but offers other advantages. Explain that the secondary level of coiling permits the chromatin to be densely packed, enabling two meters-worth of DNA to fit into a cell's nucleus. See if students can identify a major problem with this arrangement. Ask: When your body needs to make a protein, how can the genetic instruction hidden in this coil of chromatin get "read"? Record the ideas on the board. (When coiled, the DNA's sequence of nucleotides cannot easily be read. Consequently, the instructions for protein synthesis are unavailable, and enzymes cannot read the DNA to begin the process of transcription. When the chromatin is in its tightly coiled state, the chromosomes are inactive—no transcription [or protein synthesis] can take place.)

  5. Have students make their own models. Divide your class into groups of three or four students and distribute the materials. Have each group follow the steps on the Student Handout, in which they make their own chromatin model and use it to show how chemical tags (i.e., epigenetic factors) can uncoil segments of the chromatin. Then have them answer the questions at the end of the Student Handout.

  6. Review how epigenetic factors uncoil the chromatin. After students have finished, discuss the Handout questions and students' answers.


Activity Answer

Student Handout Questions

  1. Why can it be difficult for enzymes to "read" DNA base pairs in a coiled nucleosome?

    In the nucleosome the DNA strand is wound onto the histone proteins the way a thread is wound onto a spool. Furthermore, the nucleosomes are themselves coiled. All the winding, twisting, and coiling make it essentially impossible for the transcription enzymes to read a complete sequence of base pairs (i.e., a gene).

  2. In your own words, explain the process of how methyl tags (represented by the binder clips) help chromatin uncoil to reveal the base pairs in a nucleosome.

    When methyl groups attach to particular sites on the tightly coiled chromatin, specific parts of the chromatin (i.e., the nucleosomes) uncoil, revealing segments of base pairs (i.e., the genes). Once the genes are revealed, it is possible for messenger RNA to be transcribed.

  3. How are methyl groups examples of an epigenetic factor?

    Methyl groups originate outside the nucleus. They pass through the nuclear membrane and attach at activation sites on the histones in the chromatin. The chromatin affected by these methylated activation sites unravels, enabling transcription of the genes in this section of the chromatin.

  4. What would happen if methyl groups stayed attached to the nucleosome forever and kept it continuously open?

    There could be overproduction of a particular compound synthesized by that stretch of DNA. Such an overproduction might be related to cancer-like processes.

  5. List some ways that a nucleosome stuck in "continuous reading" mode might become unstuck.

    Remove the methyl groups. If the groups cannot be removed, remove the segment containing the groups (i.e., the strip with the two clips attached).

  6. List some strengths and weaknesses of this activity's model of the DNA–chromatin complex.

    Weaknesses: 1) The relative sizes of the molecules are not accurately represented by the rubber tubing. Specifically, the DNA strand is very fine and narrow compared to the much larger histone protein core molecules. 2) In real life, the substances coil in patterns different from those of the model tubing—the DNA is almost like a fine double thread wound around the much more bulky histone proteins. These histone proteins, in turn, resemble spools that are connected to each other.

    Strengths: 1) Rubber tubing provides a usable representation of the components of chromatin and their respective three-dimensional relationship with each other. 2) The secondary coiling pattern creates a reasonable representation of the nucleosome. 3) Both of these characteristics help to show how more than two meters of DNA can fit into the nucleus of a microscopic cell.

  7. Why might high-level exposures in early life to factors that lead to the accumulation of methyl groups have health consequences much later in life?

    The accumulation of chemical tags, such as methyl groups, that can stick to the histones or DNA might affect cellular repair mechanisms, causing them to break down or become less effective.


Links and Books

Web Sites

NOVA scienceNOW
www.pbs.org/nova/sciencenow/3411/02.html
Offers epigenetics-related resources, including a streamed version of the show, an audio slide show about how the epigenome produces differences, and an Ask the Expert area where site visitors can ask researcher Randy Jirtle questions about epigenetics.

Environmental Health Perspectives
www.ehponline.org/members/2006/114-3/focus.html
Provides a well-written overview, with a clear diagram, of the connection between epigenetic factors and disease in humans.

Epigenome Network of Excellence
epigenome-noe.net/aboutus/epigenetics.php
Offers a brief yet informative overview of the field of epigenetics.

Epigenome Network of Excellence
epigenome.eu/en/1,1,0
Presents a brief, clear overview of epigenetics, with quotes from various researchers, followed by a series of accessible descriptions of different topics in epigenetics.

The Functions of Chromatin Modifications
www.hhmi.org/research/investigators/zhang.html
Explains how epigenetic-mediated dynamic changes in chromatin structure affect gene expression, cell lineage commitment, and cancer development.

Johns Hopkins Epigenetics Center in the Institute for Basic Biomedical Sciences
www.hopkinsmedicine.org/press/2002/November/epigenetics.htm
Provides a basic introduction with an overview of epigenetics presented in lay terms.


Books

Biology: Concepts and Connections
by Neil Campbell, Jane Reece, Martha Taylor, and Eric Simon. Pearson/Benjamin Cummings, 2006.
Provides overview of genetics, DNA, RNA, and other related basic information; written at level appropriate for high school.

Epigenetics
by C. David Allis, Thomas Jenuwein, Danny Reinberg, and Marie-Laure Caparros. Cold Spring Harbor Press. 2007.
Compiles an up-to-date technical scientific collection of papers with useful overviews.


Articles

"A Cell's Second Act" by Richard Saltus. HHMI Bulletin, 19 (1), February, 2006.
www.hhmi.org/bulletin/feb2006/features/cell2.html
Describes researchers' efforts to understand nuclear reprogramming to revert adult cells to medically useful embryonic stem cells.

"DNA Is Not Destiny" by Ethan Watters
discovermagazine.com/2006/nov/
The new science of epigenetics rewrites the rules of disease, heredity, and identity.

"Epigenetics: A historical overview" by Robin Holliday. Epigenetics, 1:2, 76–80, 2006.
cnru.pbrc.edu/pdf/history_of_epigenetics.pdf
Offers a brief history of the field of epigenetics.

"Nurture Takes the Spotlight: Decoding the environment's role in development and disease" by Christen Brownlee. Science News, 169 (25), June 2006.
www.sciencenews.org/articles/20060624/bob8ref.asp
Reviews current research and gives an accessible overview of epigenetics.


Standards

The "Epigenetics" activity aligns with the following National Science Education Standards (see books.nap.edu/html/nses).

Grades 9-12
Science Standard C

Life Science

  • The cell
  • The molecular basis of heredity

Science Standard E
Science and Technology

  • Understandings about science and technology

Science Standard F
Science in Personal and Social Perspectives

  • Personal and Community Health


Classroom Activity Author

Developed by John Glyphis, Ph.D., MPA. Glyphis is a biologist who consults on and writes about science in education and public policy.

Teacher's Guide
NOVA scienceNOW: Epigenetics
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