Mystery of the
Matching Marks
part 2
Let’s look at our two sets of
chromosomes again, side-by-side.
This time, Focus on their DIFFERENCES:
What do you see in the
chimp chromosomes (on the right)
that is DIFFERENT from
the human chromosomes (on the left)?
How are they different?
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GOOD EYES!
- Chimp’s #2 is shorter than our #2
-Chimp has an extra unmatched chromosome
What could have happened
to cause those differences?
Let’s take a closer look
at those chromosomes…
“Missing” part
“Extra” in chimps
ANY IDEAS that
might EXPLAIN the
“missing” part of the
chimp’s #2 chromosome,
AND the chimp’s “extra”
chromosome?
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Maybe the chimp’s
“extra” chromosome
was once part of its
short #2.
Could the “extra”
chromosome match
the upper part of
our #2?
LET’S TRY IT…
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Nope!
They don’t seem
to match.
What else could
we try?
Turn the “extra” one
upside down?!
Let’s try it…
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WOW !
IT WORKED! They DO MATCH!
NOW, the next question:
“How could this happen?”
- Was there ONE #2 in our
common ancestor, that split
to make TWO in chimps, OR
-Were there TWO short
chromosomes in our ancestor that
fused (joined) to make ONE in humans?
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We DO have a PROBLEM:
“How did this difference happen?”
And, we have two hypotheses
(possible explanations):
1. One split to make two, OR
2. Two fused (joined) to make one
Let’s try the second one (fusion).
How can we TEST that hypothesis?
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We could look for evidence of
fusion in the middle of our
#2 chromosome…
But, what kind of evidence
can we look for?
Well, it so happens that
ALL chromosomes have special
tip ends, called “telomeres”…
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CHROMOSOME PARTS
Head
Telomer
e
All Chromosomes have
telomeres at both ends
(like shoelace aglets!)
Centromere
Tail
Telomere
Telomeres have a
special DNA sequence…
ttagggttagggttagggttagggttagggttaggg…
||||||||||||||||||||||||||||||||||||
aatcccaatcccaatcccaatcccaatcccaatccc… 12
Did you notice the repeated sequence: ttaggg?
Head
Telomer
e
Centromere
DNA Sequence for
Telomeres:
ttagggttagggttaggg…
||||||||||||||||||
aatcccaatcccaatccc…
NOTICE:
Tandem Repeats in Telomeres:
ttagggttagggttaggg…
||||||||||||||||||
aatcccaatcccaatccc…
Tail
Telomere
“ttaggg” is repeated
800-1600 times in each Telomere
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Here’s another view of
a chromosome,
showing the telomeres
untwisted, and their typical
DNA sequence
It also shows that the
upper (shorter) arm
above the centromere
is called the “p-arm”, and
the lower (longer) arm is
called the “q-arm”
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TELOMERE DNA CLOSE-UP
Here are ends
of the upper
telomeres of the
chimp’s “short”
chromosome (left)…
and its “extra”
chromosome (right)
Short #2
“Extra”
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NOTICE!
When we turn the “extra”
chromosome upside-down,
and try to connect it to the
“short” chromosome, it only
FITS one way (left)…
They do NOT fit
when one telomere is
o
twisted 180 (right)
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FURTHERMORE…
When we lay the fusion area on its side,
we can see more clearly
how the DNA sequence
changes at the fusion point.
Reading the top strand only,
see:
T TA G G G C C C TAA
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THAT’S WHAT YOU WILL BE LOOKING
FOR
When you are searching the DNA for the
Fusion Point, you will be looking
at only one strand of DNA
(since the “lower” strand is the predictable
complement of the “upper” strand).
Look for something like this:
…ttagggttagggttagggccctaaccctaaccctaa…
Read this like lines of text in a book…
Do you see where the multiple g’s (and no c’s) END,
and multiple c’s (and no g’s) BEGIN?
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What would this point be called?
(where multiple g’s stop, and
multiple c’s begin)
This would be the FUSION POINT
Raise your hand when you see that point
in this actual DNA strand below:
On which line does the change happen?
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Maybe this will show it
more clearly:
THERE’S the FUSION POINT !
GOT THE PICTURE?
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NOW…
WHERE should we LOOK
for the
FUSION POINT?
2a
YES!
Right in the MIDDLE
of our chromosome #2,
where the two matching
chimp chromosomes
overlap !
2b
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This would be BELOW the
CENTROMERE, in the
“q-arm” of the chromosome,
in the region known as
“2q13”,
shown in red.
2a
2b
(Can you figure out where
the number “2q13”
comes from?)
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(OPTIONAL)
For “2q13”…
2 = chromosome #2
q = the q-arm
1 = region 1 of that arm
3 = sub-part 3 of that region
2a
2b
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So, where can we see the DNA
from this region of our
#2 chromosome
to examine?
I have gone to an online DNA database
and printed out the DNA in that region.
You could do this yourself, but, to save
time, I’ve done that for you…
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This 2q13 region
gives us
52 pages of DNA!
This is what a page
looks like…
On this page, there
are 57 lines,
each line with
60 bases (letters),
and that gives us…
3,420 bases per page!
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If these 52 pages
were attached
end-to-end,
they would stretch
about 14 meters
(16 yards) around
your room!
AND…
If ALL the DNA from our
ENTIRE #2 chromosome
was printed out like this,
it would stretch about
16 km (10 miles)!
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By the way…
Each number on the left edge equals the
number of the first base (letter)
on that line.
241
181
121
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1
ttctgcaaat
tttcaaataa
agtgtctact
atgaaaggta
gaattcttgt
caatacacaa
attaatttca
cttgtgtgtt
tttcttctag
tctgtattta
tgagctcata
tccccatagg
cacaagtgtg
gaagcagtga
gaaacccact
ttcagataaa
agccagttta
cacaaagttt
ttctaaagtg
c acgttactt
tataatagtt
tcagataatt
ttggtaaatt
tatgcttaac
gatatttggg
tttctttatt
catttttcat
aagaatatta
cagtcagttg
tatttaagtc
And, a space has been inserted
after every 10th base (letter)
to make counting easier.
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You may notice
when you are
searching, that
the “ttaggg” pattern
is not perfect!
An occasional “c”
slips in here and there,
and you will see other
minor “glitches.”
WHY?
If you said
“MUTATIONS,”
you would be right.
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NOW, it’s YOUR turn!
You get to SEARCH
those 52 pages!
Are you ready???
Just kidding!
Actually, you will form
teams of 3-4, and
each team gets the
same 4 pages (from the
“2q13” region) 29
One of those 4 pages
should have the
“Fusion Area”
Each person looks for
that “Fusion Area” on
a different page.
When one of you finds
it, show your partners.
Discuss your discovery with your partners,
and answer the questions
on your “SEARCH” worksheet
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Chimp
RECAP
Us
PROBLEM: How did our #2 chromosome
come to look identical to two chromosomes
in chimpanzees”
Fusion?
<--Common
Ancestor
HYPOTHESIS: our #2 chromosome
was formed by the fusion of
two chromosomes in an ancestor,
after chimps branched off.
TEST: Look for fusion evidence
in the form of telomere DNA
in the middle of our #2chromosome
PREDICTIONS: If hypothesis is true, we
should find two telomeres there;
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If NOT true, should be NO telomeres there.
GO
SEARCH
for the
Tell-Tale Telomeres !
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Students get into teams now,
and pick up team folder with
4 DNA pages per team, and
1 worksheet per student.
Go to MMM2 for followup slides,
to show after teams find fusion points,
or to show optional DNA models of telomeres.
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