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STUDENT PAGES FOR LESSON 1B: BEAR SPECIATION USING BEAR DNA
In Part 1 of this activity, you will learn about the structure of chromosomes and the
terms used by scientists studying chromosomes. Then, in Part 2, you will apply this
information and compare sets of chromosomes from different bear species as an
evolutionary geneticist would do.
Several species may appear to be quite similar to one another, and it may be difficult to
determine exactly how they are all related. Since organisms get their characteristics
from the information on their chromosomes (which come from their ancestors), one
way to determine how closely related two species are is to compare not just their
characteristics but their chromosomes themselves. The biologists that do such
comparisons are called evolutionary geneticists.
Chromosomes are microscopic strands of DNA found in the nuclei of the cells of living
things. Similarities in DNA can be used to help determine classification and
evolutionary relationships. Now that scientists can determine, or sequence, the
information coded in DNA, they can compare the DNA of different organisms to trace
the history of genes over millions of years. DNA evidence can also help show the
evolutionary relationships of species and how species have changed. The more similar
the DNA of two species, the more recently they shared a common ancestor, and the
more closely they are related in evolutionary terms. And the more two species have
diverged or changed from one another during evolution, the less similar their DNA will
be. In other words, species that are more closely related should have more similar
chromosomes.
It is difficult even through a microscope to see the chromosomes in the nucleus of a cell,
let alone distinguish one chromosome from another. Geneticists have developed
staining procedures that produce patterns of dark and light bands on each
chromosome. This staining procedure uses a chemical called Giemsa, so the dark bands
produced where the stain adheres are called G bands. Then a camera that attaches to the
microscope is used to take a picture. This picture is called a karyotype.
A chromosome stained with
Giemsa to show the dark and light
banding patterns
A human karyotype showing the
entire set of 23 homologous pairs
of chromosomes
Then these chromosomal patterns from the two species are compared. The similarities
and differences are carefully noted. Geneticists use diagrams or drawings called
ideograms as a standard representation for chromosomes. Ideograms show a
chromosome's relative size and its banding pattern. It is almost like a map of the
chromosome, and bands on the chromosome can be labeled according to their location
on the chromosome.
A chromosome and its ideogram
showing how bands are drawn and
mapped from the original photo
Figure 1 is an ideogram of a giant panda chromosome that shows these bands. The stain
produces band patterns that are unique for each of the different chromosomes. For
example, chromosome #1 for all giant pandas has this same pattern of bands. When
chromosomes are stained with Giemsa, the dark bands are where the chromosome
proteins are tightly condensed, while the light bands are where the proteins are less
condensed, and where the genes are more active.
p
Figure 1. Giant Panda Chromosome #1 (from Nash,
W.G., J. Wienberg, M.A. Ferguson-Smith, J.C. Menninger,
and S.J. O’Brien. 1998. Comparative genomics: tracking
chromosome evolution in the family Ursidae using
reciprocal chromosome painting. Cytogenet Cell Genet
83:182–192.) Used with permission.
q
Notice that the Giant Panda chromosome has a narrow area near the middle. This is
called the centromere. The centromere can be located in the center of the chromosome,
slightly to one side of the chromosome’s center, near one end, or at the very end. The
centromere divides the chromosome into two arms. In this system, the short arm of
each chromosome is designated the “p” arm.
Geneticists compare similarities and differences between chromosomes to determine
how animals are related. Generally, the entire karyotype is used to determine
relatedness. However, in this exercise you will use a subset of 2 chromosomes (Figure
2) to decide how some individuals may be related evolutionarily.
A
A
B
B
C
Chromosome X
Figure 2. Ideograms of chromosomes X and #1
for 3 individual animals. (Images are compiled
from figures in Nash, W.G., J. Wienberg, M.A.
Ferguson-Smith, J.C. Menninger, and S.J.
O’Brien. 1998. Comparative genomics: tracking
chromosome evolution in the family Ursidae using
reciprocal chromosome painting. Cytogenet Cell
Genet 83:182–192.) Used with permission.
Chromosome #1
C
A cladogram is a branch-shaped diagram used to show evolutionary relationships
among species by analyzing certain kinds of physical features in the organisms. A
cladogram can also be constructed using similarities and differences in chromosomes.
A cladogram starts with the main branch, which then splits several times at nodes into
two or more branches, or internodes. The node represents the formation of a new
species or group. Described species (either from the present or from the fossil record)
appear at the tips of the branches. Species on branches that are close together are more
closely related than species that are farther apart, similar to the people on a family tree.
One style of cladogram uses this form:
Flowering Plants
Conifers
Ferns
This cladogram indicates that mosses and
ferns diverged long ago. At a later point,
conifers split away from ferns, and finally
flowering plants diverged from conifers.
Flowering plants therefore are more closely
related evolutionarily to conifers than they
are to mosses or even ferns. A cladogram
like this could be constructed by careful
observation of the physical differences and
similarities among the plants, or by
studying their chromosome structures.
Mosses
Here is a hypothetical cladogram of characters
from Dr. Seuss. Use it to decide:
Hooded Klopfer
Which species are equally related to each other?
Beagle-Beaked-BaldHeaded Grinch
Which species is the most different?
Harp-Twanging Snarp
Ruffle-Necked Sala-ma-goox
STUDENT WORKSHEET: BEAR SPECIATION USING BEAR DNA
Name_________________________________ Date_______
A. Use Figure 1 and the information on your Background sheet to answer the following
questions:
1) How many G-bands are on the chromosome in Figure 1? _______
2) How many G-bands are there with a high concentration of tightly condensed proteins?
_______
3) How many G-bands are there with less condensed proteins, more active genes? _______
4) What is the long arm designated? ______________
B. Use Figure 2 and the information on your Background sheet to answer the following
questions:
This figure shows chromosomes #1 and X from 3 different animals. By comparing the banding
patterns on the chromosome sets, answer the following:
1) How many species do you think are represented by these chromosome sets? __
2) What leads you to this conclusion?
3) Which species are most closely related? Which one is most different? Explain your
answers.
4) Based on figure 2 and your answers above, draw a cladogram showing the most likely
evolutionary relationships among the species represented by A, B and C:
C. Figures 3-5 show 3 different sets of bear karyotypes. Use Figures3-5 to answer the
following questions:
1) The first set (Figure 3) shows side-by-side karyotypes of the 6 bear species in the world that
have 74 chromosomes(36 pairs): polar bear, American black bear, Asiatic black bear, sloth bear,
sun bear, and brown bear. Examine Figure 3 carefully and note what you observe about the
karyotypes of these bear species.
HINT: Write the name of the bear species next to the abbreviation for ease in interpretation.
For example, next to the abbreviation UAR write the word “brown bear”.
Based on their chromosome sizes, shapes, and patterns, do you think they are closely related?
Explain.
2) Now look at Figure 4. Figure 4 shows the karyotypes of the brown bear (UAR) and the
spectacled bear (TOR) side-by-side. Note that not each chromosome in a set from one species is
matched up with the same number chromosome from another. For example, the TOR
chromosome #7 is matched with the p arm of UAR chromosome #20 and the q arm of UAR
chromosome #5. This is because during meiosis, the genetic code on chromosomes may be
rearranged into different configurations (as evolution occurs at the molecular level in the cell’s
DNA!). Some chromosomes may be shortened, parts of them joined with others or even lost
completely. When a karyotype is produced, the distinct banding patterns allow scientists to
match the original pieces with one another. These matched chromosomes are called homologs.
Can you find the homolog for the TOR chromosome #11 in Figure 4?
3) TOR, the spectacled bear, has a chromosome count of 52, compared with the 74 of the first 6
species discussed (figure 3). Figure 4 shows the best chromosome matches made between UAR
and TOR. Based on this information and their karyotypes, how would you compare the
relationship of UAR to TOR? Is TOR as closely related to UAR as the previous species
represented in figure 3?
4) Look at Figure 5, which shows the karyotypes of the brown bear (UAR) and the giant panda
(AME). AME has 42 chromosomes, and Figure 5 shows the best chromosome matches made
between UAR and AME. Again, make comparisons based on the chromosome appearances.
How would you compare the relationship of UAR to AME? Is AME as closely related to UAR
as the previous species represented in figure 3?
5) Draw a cladogram based on your observations and conclusions about the relatedness of the
bear species. Refer back to the cladograms on your Background sheet to see how closely related
species appear on branches compared to those less closely related.
6) In the chapter, “In the Market for Bears”, the author questioned whether the Golden Moon
Bear was a new species or simply a color phase of the Asiatic Black Bear. If you had DNA
samples from a black colored Asiatic Black Bear and a Golden Moon Bear, how could you
determine if the two bears were of the same species?
Figure 3. Karyotypes of the 6 bear species in the world that have 74
chromosomes: polar bear (Thalarctos maritimus or TMA), American black
bear (Ursus americanus or UAM), Asiatic black bear (Selanarctos
thibetanus or STH), sloth bear (Melursus ursinus or MUR), sun bear
(Helarctos malayanus or HMA), and brown bear (Ursus arctos or UAR).
This figure is from Nash WG, O'Brien SJ. 1987. A comparative chromosome
banding analysis of the Ursidae and their relationship to other carnivore.
Cytogenet Cell Genet. 45:206–212. Used with permission.
Figure 4. Comparison of G-banded chromosomes of Tremarctos ornatus (spectacled bear or
TOR) and Ursus arctos (brown bear or UAR). The left chromosome of each pair is the TOR
chromosome. The numbers below each pair refer to TOR chromosomes. The numbers to the
right of each pair identify UAR chromosomes.
This figure is from Nash WG, O'Brien SJ. 1987. A comparative chromosome banding analysis
of the Ursidae and their relationship to other carnivore. Cytogenet Cell Genet. 45:206–212.
Used with permission.
Figure 4. Continued
NOTE: The UAR chromosomes are the same ones shown in Figure 3!
Where there are 2 numbers on the right, it shows that two chromosomes
are being compared to one TOR chromosome. (Chromosomes
sometimes split apart and recombine during mitosis, one way evolution
occurs.) Look at the top left pair of chromosomes in Figure 4. You can
see that the UAR chromosomes 7 and 2 are from the same 7 and 2 in
Figure 3:
This figure is from Nash WG, O'Brien SJ. 1987. A comparative
chromosome banding analysis of the Ursidae and their relationship to
other carnivore. Cytogenet Cell Genet. 45:206–212. Used with
permission.
Figure 5. Comparison of G-banded chromosomes of Ailuropoda
melanoleuca (giant panda or AME) and Ursus arctos (brown bear or
UAR). The left chromosome of each pair is the AME chromosome. The
numbers below each pair refer to AME chromosomes. The numbers to the
right of each pair identify UAR chromosomes. The AME chromosome is
shown by itself when no clear match exists with a UAR chromosome. This
figure is from Nash WG, O'Brien SJ. 1987. A comparative chromosome
banding analysis of the Ursidae and their relationship to other carnivore.
Cytogenet Cell Genet. 45:206–212. Used with permission.