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A line of cars and their headlights: a reminder of higher DNA organization. (Source) |
by Jeanne Garbarino, DXS biology editor
My commute can often cut through some interesting scenery, leading me to occupy myself by staring through the windows. However,
it was only the other night that I noticed the highway running parallel to the
train. It was packed with cars as New Yorkers
were fleeing to their homes in order to brace for the imminent blizzard. Needless to say, the road traffic moved at a
snail’s pace.
My vision of
the highway and all of the beaming headlights reminded me of when I was a kid,
stuck in traffic as a passenger in the back seat of my parents’ car. I would try to take my mind off of my
seemingly infinite confinement by pretending that the glowing headlights coming
toward our car were diamonds, strung up on the most amazing necklace ever made. Truly extravagant beads on a string. But, seeing that sight again no longer
conjured up thoughts of priceless neckwear. Instead, these brightly lit “beads on a
string” immediately reminded me of one thing and one thing only: the cellular
organization of DNA. (Talk about science on the brain.)
In addition
to my (sad?) association of consecutive headlights with DNA organization, this
imagery also summoned previously dormant areas of my brain, bringing to my
current consciousness pictures of my college genetics professor and her lessons
on DNA packaging. All of a sudden, I was
thinking about histones, chromatin, and solenoids, which are all terms to describe the higher organizational structures of DNA.
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Actual "beads on a string," aka DNA being organized for packaging. |
Because we've been able to capture images of these structures using microscopes, the “beads on a string” analogy is the classic description used by scientists and teachers to talk about how DNA is packaged in our
cells. It essentially describes an
elegant solution that has evolved to keep DNA both compact and easily accessible. To
fully appreciate this, one must consider the scale of this molecular
machinery. If placed end-to-end, the DNA
found in a single human cell would be approximately 2 meters (6.6 feet) in
length. Now imagine having to package
the DNA into a compartment within the cell, called the nucleus, that is so
small it can only be visualized with the aid of a microscope. Put another way, this is equivalent of
packaging 40km (or 24 miles) of super fine thread into something the size of a
tennis ball.
How is it
possible to do this in such a way that prevents the DNA from becoming a tangled
mess while still allowing the cellular machinery to easily access the important
information kept in our genetic code? The answer starts with the enlistment of specialized proteins called
histones. A length of DNA will wrap
around a histone core (1.75 times to be exact; equivalent to 146 base pairs),
essentially giving the illusion of a “bead” with each bead separated by a short,
linear segment of DNA approximately 60 base pairs long, called a “linker.” Together, one bead plus its neighboring
linker sequence is called a nucleosome. When examined on a microscope, the serial organization of each nucleosome
unit resembles “beads on a string” and, hence, the analogy has been forever
implanted in my mind.
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(Source) |
More
appropriately termed chromatin, the “beads on a string” is only the first level
of organization required for DNA packaging with our cells. Chromatin can be further condensed into
higher-order structures accomplished by interactions formed between
nucleosomes. What has helped me to understand
this process is the following: Imagine you have a long, thin stick, around which you begin to tightly wrap a chromatin fiber. Then, after all of the chromatin has been
spiraled around the stick, you slide the stick out leaving a tube-like structure. The tube, which is held together through the
interactions formed between nucleosomes, is called a solenoid. The solenoid can be further condensed by
looping, ultimately resulting in a super tight and compact packaging of DNA.
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The hierarchy of DNA organization. Source. |
So, will the
next time you see headlights in traffic make you think of DNA
organization? For my sake, I sure hope
so!
For more information, check out this video:
Also, check out this interactive site from HHMI.
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