Lesson II: Chromosomes
Key Concepts:
Organization of genes on chromosomes
Genes are located in the nucleus on structures called chromosomes
Sex Chromosomes
DNA structure
Materials Needed
 Steelwool
 Karyotypes/ Chromosome Spreads-Normal and Abnormal
 Pipe cleaners
 Strips from post-it notes (2 different colors)
 Double-sided tape
 Clay
 Writing utensil
Introduction
Remind students that last week we discussed how many of our phenotypes (i.e.: rolling tongue) are
determined by the genes we inherit from our mother and father, which is called out genotype. Genes are located
in all cells of the body. The material that makes up the genes is called DNA, deoxyribonucleic acid. Today we
will be discussing the location of genes within cells. Draw pictures of cells on the board (round, with
projections, etc). Ask how many cells are in the human body (trillions). What are structures within cells. Ask in
what region of the cell is DNA found? (nucleus). If you were to look at the nucleus when a cell is undergoing
division with a very powerful microscope, you would see structures called chromosomes. (Draw picture).
The DNA strands are packed tightly in chromosomes. A brillo-pad/steel wool prop is useful to show
how the same amount of DNA takes up a lot less space when folded up rather than stretched out. (Optional) If
all the DNA you needed to encode for traits (1 meter?) was stretched out, it would not be able to fit inside the
nucleus. Give students cut rubber band and ask them to start twirling the bands. Notice as you keep twirling,
the rubber band begins to compact.
Replicated + condensed
centromere
First Work Station Activity: Chromosome Pictures (Karyotypes)
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Pass out pictures of karyotypes (normal male and female).
Distribute to each student three pictures (
Karyotypes can be made from cells that are undergoing cell
division as the DNA is most condensed and it is possible to
take pictures of the chromosomes.
Have the students examine and count the
chromosomes and discuss what they see.
Humans have 46 chromosomes or 23 pairs (why pairs).
Discuss features used to recognize the different
chromosomes: size, banding pattern and centromere
location.
Optional: if students ask how karyotypes are made. Cells
put in solution that makes them swell with water until they
burst. After the cells have been stained with a dye, the
individual chromosomes are apparent and can be
photographed. The chromosomes can be cut, matched in
pairs, and arranged in a chart according to size and shape.
Ask the students, “How would you determine sex by looking at these chromosome alignments?” Can
have them compare pictures of male and female karyotype. Sex chromosomes XX or XY. Ask “From which
parent did the male inherit his Y chromosome?” After answer can again ask where father inherited his Y.
Next, compare karyotypes of normal and abnormal, i.e. an individual with Down syndrome or some
other chromosome abnormality. Ask the students to compare their two pictures and note any differences.
Genetic counselors have the job of deciphering, based on a karyotype, whether a baby or a fetus has a normal
number of chromosomes or abnormal such trisomy 21. Cells of the fetus are obtained during a procedure called
amniocentesis in which fluid containing fetal cells is removed from the placenta with a needle [like a blood
draw]. They are grown and a karyotype is prepared. “Has anyone heard of Down’s Syndrome?” Explain what
trisomy 21 is and what the consequence is of having more than 2 copies of a set of genes. Trisomy of only a
few of the chromosomes will produce a viable fetus such as 21, 13 and 18 (all have abnormalities). Trisomy of
most chromosomes is lethal (eg #15) resulting in spontaneous miscarriage. Miscarriages are quite frequent
(some estimates 30%). Also, chromosome abnormalities also occur in cancer cells so knowing the karyotype
of a cancer cell helps to understand what went wrong and is used for diagnosis.
(Optional) Turner’s syndrome, XO, affects girls (1/2000-1/2500). Short stature, no ovaries, webbed
neck, etc. Klinefelter is XXY (1/500-1/1000). No secondary sex characteristics, tall.
BACK TO THE LECTURE: Chromosomes are composed of many genes. Ask students to guess how many
genes there are in humans (20,000). Write on the board 200, 2,000, 20,000, 200,000, 2,000,000. Ask students
vote. Answer about 20,000. (Drosophila has 12,000 and yeast 6,000).
Then mention that genes are made up of DNA. Write DNA on the board and ask what it stands for
(deoxyribonucleic acid). Ask the class to say it together with you. Ask why it is called a nucleic acid (because it
is located in the nucleus).
Show model of DNA. Ask studets-what shape is DNA (double helix. Discuss 4 bases: adenine (A),
guanine (G), thymine (T) and cytosine (C). Write on the board. Show model and discuss how bases are paired.
Base-pairing has two rules: guanine always pairs with cytosine and adenine always pairs with thymine. A base
on one strand complements a base on the opposite strand. The order of the bases has INFORMATION. (refer
to letters making words, sentences, etc). There is a code to be able to read the information which you will learn
about in high school .
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[(OPTIONAL) Draw two anti-parallel srands. Next to that, draw a sugar structure on the board, saying that it
is also called ribose (refer to “ribo” in deoxyribo). Draw a phosphate group (P) attached to the left of the ribo,
saying that this gives DNA its negative charge. To the right of the ribose, write the 4 different bases (see
appendix, page 24-25).]
Second Work Station Activity - Build a DNA model:
A each station, give each student (i) two pipe cleaners of the same color, (ii) 6 strips from post-it notes
of the same color (A-T pair) and 6 strips from post-its notes of another color (G-C pair) and ½ block of colored
modeling class.
Have students write a base letter on the left side of a strip and its complement on the right side on 6
strips (3 strips should be of one color, corresponding to A-T, and the other 3 should be another color,
corresponding to G-C). Then turn the strips over, with the written bases facing down towards the table. The 6
strips should be placed as if they make the rungs of a ladder; leave a bit of space in between. Place double-sided
tape that extends the full length of the strip onto each strip. Then place one pipe cleaner over the left side of the
strips and then the 2nd pipe cleaner over the right side of the strips, ensuring that they are sticking to the tape. On
the remaining 6 strips of paper, write the same sequence (make sure that the letter on this strip corresponds to
the same letter written on the same side on the other strip). These strips are then placed on top of the strips that
have the tape and pipe cleaners attached to them. Bring the ends of the two pipe cleaners together and twist
them so that they are closed. Then grab both ends of the pipe cleaners and twist once, so that a node forms in
the middle. Stick one end into the clay, and mold the clay around the pipe cleaners forming a base that can
support the model. Ask students to describe the structure to friends and/or family.
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