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 )?.
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)?
What could have happenedto cause those differences?
Let’s take a closer look
at those chromosomes…
“Extra” in chimps
ANY IDEAS that
might EXPLAIN the “missing” part of the chimp’s #2 chromosome,AND the chimp’s “extra”chromosome?
Maybe the chimp’s“extra” chromosomewas once part of itsshort #2.
Could the “extra”chromosome match the upper part ofour #2?
LET’S TRY IT…
Turn the “extra” oneupside down?!
Let’s try it…
NOW, the next question:“How could this happen?”
“How did this difference happen?”
And, we have two hypotheses(possible explanations):
1. Onesplit to make two, OR
2. Twofused (joined) to make one
Let’s try the second one (fusion). How can we TEST that hypothesis?
But, what kind of evidence can we look for?
Well, it so happens that ALL chromosomes have special tip ends, called “telomeres”…
All Chromosomes have telomeres at bothends(like shoelace aglets!)
Telomeres have a special DNA sequence…
DNA Sequence for Telomeres:
Tandem Repeats in Telomeres:
“ttaggg” is repeated 800-1600 times in each Telomere
Here’s another view ofa chromosome,showing the telomeresuntwisted, and their typicalDNA sequence
It also shows that theupper (shorter) armabove the centromereis called the “p-arm”, andthe lower (longer) arm iscalled the “q-arm”
Here are ends of the uppertelomeres of thechimp’s “short”chromosome (left)…
and its “extra”chromosome (right)
When we turn the “extra”chromosome upside-down,and try to connect it to the“short” chromosome, it onlyFITS one way (left)…
They do NOT fitwhen one telomere istwisted 180o (right)
FURTHERMORE…When we lay the fusion area on its side,we can see more clearly how the DNA sequencechanges at the fusion point.
Reading the top strand only, see:T T A G G G C C C T A A
THAT’S WHAT YOU WILL BE LOOKING FORWhen you are searching the DNA for theFusion Point, you will be lookingat only one strand of DNA (since the “lower” strand is the predictablecomplement of the “upper” strand).Look for something like this:
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?
What would this point be called?(where multiple g’s stop, andmultiple c’s begin)
This would be the FUSION POINTRaise your hand when you see that point in this actual DNA strand below:
On which line does the change happen?
THERE’S the FUSION POINT !
GOT THE PICTURE?
YES!Right in the MIDDLEof our chromosome #2,where the two matchingchimp chromosomesoverlap !
This would be BELOW theCENTROMERE, in the “q-arm” of the chromosome,in the region known as“2q13”,shown in red.
(Can you figure out wherethe number “2q13”comes from?)
2 = chromosome #2
q = the q-arm
1 = region 1 of that arm
3 = sub-part 3 of that region
I have gone to an online DNA databaseand printed out the DNA in that region.
You could do this yourself, but, to savetime, I’ve done that for you…
gives us 52 pages of DNA!
This is what a page looks like…
On this page, there
are 57 lines,
each line with60 bases (letters),and that gives us…3,420 bases per page!
If these 52 pageswere attachedend-to-end,they would stretchabout 14 meters(16 yards) aroundyour room!AND…
If ALL the DNA from ourENTIRE #2 chromosomewas printed out like this,it would stretch about16 km (10 miles)!
Each number on the left edge equals the
number of the first base (letter)on that line.
And, a space has been inserted
after every 10th base (letter)to make counting easier.
when you are
the “ttaggg” pattern
is not perfect!
An occasional “c”
slips in here and there,
and you will see other
If you said
you would be right.
You get to SEARCH those 52 pages!
Are you ready???
Just kidding!Actually, you will formteams of 3-4, andeach team gets thesame 4 pages (from the “2q13” region)
Each person looks forthat “Fusion Area” ona different page.
When one of you findsit, show your partners.
Discuss your discovery with your partners,
and answer the questions
on your “SEARCH” worksheet
PROBLEM: How did our #2 chromosomecome to look identical to two chromosomesin chimpanzees”
HYPOTHESIS: our #2 chromosome was formed by the fusion oftwo chromosomes in an ancestor, after chimps branched off.
TEST: Look for fusionevidencein the form of telomere DNAin the middle of our #2chromosome
PREDICTIONS: If hypothesis is true, we
should find two telomeres there;If NOT true, should be NO telomeres there.
Students get into teams now,and pick up team folder with 4 DNA pages per team, and1 worksheet per student.
Go to MMM2 for followup slides, to show after teams find fusion points, or to show optional DNA models of telomeres.