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Review
DNA and chromosomes. As
a class, review the following definitions:
DNA: a double-stranded molecule that carries a cell's genetic
information and is found in the cells of all living organisms.
Chromosome: A
tightly coiled macromolecule of DNA and its associated proteins. Chromosomes contain many
genes. Sexually reproducing organisms have two of each chromosome, one from
each parent. Organisms vary in the number of chromosomes they have. For
example, humans have 46, dogs have 78, mosquitoes have 6, a potato has 48, and
a pea plant has 14.
Genome: The entire set of hereditary factors in
a haploid set of chromosomes.
Show
the class a three-dimensional DNA model. Have students identify the two strands
that make up DNA, the sugar-phosphate backbone, the
individual nucleotide bases, and the nucleotide base pairs. To help students
better understand the way DNA affects a person's traits, have students
look at sets of paired chromosome from individuals with genetically caused
conditions, such as Down syndrome, Huntington's disease, Klinefelter's
syndrome, Tay-Sachs disease, and Jacobs Syndrome. Compare these to a set
of paired chromosomes from an unaffected individual.
(Suggested resource: pathology.washington.edu/galleries/Cytogallery/ main.php?file=human%20karyotypes)
Play
a word game exploring the meaning of "epi." Have students use a standard or medical
dictionary to identify words beginning with the prefix epi, which means "above," "in
addition to," or "outside." Give student pairs vocabulary terms
and definitions (all mixed up). Have pairs work together and match each word to
its correct definition. Review the terms as a class, and then have students
brainstorm the meaning of epigenome
and epigenetics.
Epigenome: Above or outside the genome. In a
sense, the epigenome is like clothing around the DNA. For example, chemical
groups alter the expression of the DNA by binding to the DNA as well as to the
histones around the DNA.
Epigenetics: The study of changes in gene expression
that do not involve changes in the genetic code (i.e., the DNA sequence)
because they lie outside of the genome (i.e., the DNA). In a sense, epigenetics
is the study of the genome's "clothing".
Write similes and
analogies that compare the genome to the epigenome. Brainstorm similes that show the
relationship of the genome to the epigenome. For example, scientists in the
program refer to the genome (i.e., DNA) as being "like the hardware of a
computer" and the epigenome as being "like computer
software." The DNA needs the epigenome to function properly, just as the
computer needs the software to function properly.
Divide
the class into teams, and write on the board, "______is/are to the genome
(i.e., DNA) as ______ is/are to the epigenome." As a class,
brainstorm analogy examples such as:
Keysof a piano are to the genome as fingers are to the epigenome.
Wires
in an electric circuit
are to the genome as switches are to the epigenome.
Then
ask teams to generate other ways of showing the relationship of the genome to
the epigenome. Have teams share their work.
Discuss
"nature versus nurture" questions about identical and fraternal
twins. On the board,
diagram how identical and fraternal twins compare genetically. Explain that
with identical twins, a single fertilized egg splits into two genetically
identical parts, each having the same DNA. With fraternal twins, two separate
eggs are fertilized. Point out that identical twins are genetically identical,
while fraternal twins are only as genetically similar as typical siblings are.
Divide the class into groups and have groups discuss, research, and prepare
responses to the following nature/nurture-related questions:
Which
type of twins, identical or fraternal, always shares the same blood type? (Identical)
Should
blood type be used to determine whether a set of twins is identical or
fraternal? Why or why not? (No, fraternal twins may have the same blood
type.)
Do
identical twins always have the same handedness? Why or Why not? (Not always, right or left handedness isn't entirely
determined by genetics.)
Do
identical twins always have the same eye color? (Almost always, though some
diseases can change eye color.)
Hair color? Explain your answer. (Natural hair color is almost always the
same, but hair can be dyed.)
Do
identical twins always have the same fingerprints? Why or why not? (Not
necessarily, some identical twins have mirror-image prints.)
Would
identical or fraternal twins prefer the same type of music? Explain your
answer. (Not necessarily. Answers will vary.)
With
which type of twins will both individuals always be born as the same sex? (Identical)
Would
twins always weigh the same at birth? Why or why not? (No, they may have
received differing amounts of nutrients.)
Is it best for
scientists to study identical or fraternal twins to learn about the impact of
the environment on genes? (Identical, because they have the same genome.)
After Watching
Analyze
models for epigenetic factors.
The segment displays an animation of the relationship between DNA and the
epigenome, such as the histone proteins and methyl groups. Replay the animation
for students and discuss how DNA's structure can be altered by epigenomic
factors that expose (often by acetylation) or hide (often by methylation)
different areas of the genome.
Use
a skein of yarn to model coiled DNA, and attach a washer to the loose end. Hold
the skein so that the washer pulls the loose end down. Have students identify
the epigenetic factor. (The washer—it's not part of the
"DNA") Which
part of the DNA was hidden and unavailable for transcription but is now
available? (The length pulled from the skein by the washer—the exposed
part)
Brainstorm
models for stem cells, the genome, the epigenome, and cancer. In the program, one explanation for how
some types of cancer arise is through errors when stem cells overwork to
replace tissue. Discuss with students that a stem cell can develop into any of
the body's cell types. One function of stem cells is regeneration and
repair of damaged tissue. Stem cells that keep dividing because they
can't shut down their own growth may be one way cancer develops.
Discuss stem
cells with students, using a well-stocked kitchen could be used as a metaphor
for stem cells. The genome would be all the ingredients; the epigenome would be
opening a cabinet door to retrieve a particular ingredient; and cancer would be
adding too much of one or more ingredients in a recipe. Other metaphor examples
include a cookbook, telephone, CD player, or computer. As a class, brainstorm
similar metaphor models. Then have student teams correlate each metaphor to
stem cells, the genome, the epigenome, and cancer. Have teams share their metaphors and analysis.
Draw
a cartoon or write a poem or song.
Over centuries, many scientists have contributed to our understanding of
heredity and DNA. Have student teams research one of the scientists listed
below and, using a cartoon, poem, or song, present that scientist's
contribution. Have teams include when the scientist lived, where he or she
worked, and at least one significant contribution he or she made to advance our
understanding of heredity, DNA, and/or genetics.
Charles
Darwin (1809–1882): Wrote On the Origin of Species in 1859. He proposed the theory of natural selection
and suggested that adaptive traits were inherited.
Gregor Mendel (1822–1884): Studied
peas and other plants. Found that traits are carried by discrete units (later,
these were termed "genes") and that there are rules of inheritance.
Alfred Hersey (1908–1997) and his
assistant Martha Chase: Showed in 1952 that DNA, rather than proteins, carries
genetic information. He won a Nobel Prize for this work.
Rosalind
Franklin (1920–1958): Isolated and took X-ray crystallography images of
DNA, setting the stage for Watson and Crick to make their breakthrough insight
into the structure of DNA.
James Watson and Francis Crick (Watson 1928–, Crick
1916–2004): In 1953, determined the structure of DNA. Both scientists won
a Nobel Prize for this work.
The Human
Genome Project (Developed in the late 1980s/early 1990s): Many scientists
continue to work on this project, sequencing the human genome to better
understand such diseases as Alzheimer's. Early results include the
discovery that each of the body's cells contains more than three billion
nucleotide units. About one percent of the genome (clustered in as few as
30,000 genes) seems to be used for transcription.
Write
a twin-study proposal.
Review with students why scientists use identical twins to study the effect of
the environment on the epigenome (and ultimately on health). (Because
identical twins have the same genetic makeup, scientists can study how
differences in lifestyle influence twins' epigenomes and their health.) In a way, the environment
"talks" to us through the epigenome. Lifestyle differences can be
reflected in the epigenome. Have students brainstorm lifestyle differences that
identical twins may have that could influence their epigenome, and, by
extension, their health. On the board, record students' ideas. Possible
ways identical twins' epigenomes could be different include:
Twins
live in different geographic locations. One twin lives close to where there is
heavy use of pesticides and one does not. Scientists could study the impact of
pesticides on health (kidney disease) and on the epigenome.
One
twin smokes and one doesn't. Scientists could study the impact of smoking
on health (lung disease) and on the epigenome.
One
twin eats a vegetarian diet and one doesn't. Scientists could study the
impact of a particular diet on health and the epigenome.
One
twin has a particular kind of cancer. Scientists could study the role of the
environment in that type of cancer by carefully looking at differences in where
and how each twin lives.
One
twin works as a police officer and one as a yoga instructor. Scientists could
study the impact of stress on health conditions such as heart attacks or
ulcers; they could also study changes in the epigenome.
As a class, review the core elements of a scientific
proposal. For example:
Have
students carefully define a question.
Isolate
one variable.
Identify
the twin who is the focus of the experiment and the one who is the control.
Have
students consider other factors that could influence data or results.
Make sure
students consider the ethics involved in doing their study.
Have
students review the list of ways identical twins could have different
epigenomes, and choose a health-related question that could be addressed in a
twin study. Divide the class into teams, and have each one choose a question
from this list to further address in a proposal (each team takes a different
question). Have teams write a proposal for studying their question to learn
more about whether the environment could have influenced the epigenome and
could have been the main contributor to a particular health factor. Then have
teams share their completed proposals with the class.
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.
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.
"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.