21Zero to Genetic Engineering Hero - Chapter 1 - Isolating DNA, the Blueprints of Life
Further, a process called diffusion causes water to move away from the DNA throughout the tube of alcohol.
Because there are fewer and fewer water molecules dissolving the DNA, and the DNA molecule does not
like to interact with the uncharged isopropyl alcohol molecules, the DNA begins to fold upon its charged
self with help from the (+) charged salt you added, also known as sodium (Na
). The folding DNA becomes
larger and more dense to the point where it becomes visible to the naked eye and “falls out of solution”.
When this happens, the DNA is said to precipitate.
people who are clapping, singing, and cheering. The singer dives off of the stage into the crowd and begins
included, is a sea of water molecules. The arms of the water people in the audience are the bonds that can
hang onto the DNA singer. As the DNA singer surfs across the crowd, you put your arms up into the air, and
the singer passes over top of you. Your arm bonds can connect to the singer and temporarily hold onto the
singer keeping them in the air. The DNA molecule singer coasts across the arena because all of the water
molecule bonds are keeping the singer held up “dissolved”. This is what is happening in the DNA/salt/water
solution; the DNA is dissolved.
What would happen to the DNA if the water bonds disappeared? What if all of a sudden the crowd of people
was replaced by thousands of cats? Let’s pretend that the cats are the isopropyl alcohol molecules. Cats
have no desire to hold up people, even a world-famous singer at a concert. Of course, the cats wouldn’t be
capable of holding up the singer, even if they wanted to. The singer falls to the ground as the cats scatter
the ground in the sea of cats (isopropyl alcohol molecules).
people. But as more and more cats pour in, they start causing the people to separate. No one likes to step
on a cat, so the people accommodate the cats that are taking over the auditorium and separate from each
nervous because the water audience and the arm bonds are no longer blanketing the auditorium - there
are patches of holes forming which the singer might fall through. The DNA singer curls up into a ball to
brace for a fall. This makes the situation worse because whereas before the singer was spread out and many
arm bonds could hold them up, now they are more compact and dense and fewer arm bonds can touch
them and hold them up. With the high number of isopropyl alcohol cats and the low number of water people,
the curled up DNA singer falls to the ground - they precipitate out of solution.
This happens because:
• The isopropyl alcohol cats don’t have the arm bonds to dissolve the charged DNA singer
• The cats caused the water people to spread apart, lowering the arm bonds that could hold onto the singer
• The DNA singer curled up into a ball and became denser
The cat analogy isn’t a perfect one, but it does drive the point across. The non-charged isopropyl alcohol
molecules are not keen on interacting with partially charged water or the charged DNA. Because DNA is
a charged molecule, and isopropyl alcohol is not charged, only weak interactions occur, and the following
occurs as you pour:
isopropyl alcohol, the water molecules surrounding the DNA
become spread out in the isopropyl alcohol leaving the DNA “naked”. The isopropyl alcohol and the DNA
do not like to interact with one another so the DNA folds up and binds to other parts of itself with the help
of sodium (white salt) and becomes denser.
The large DNA molecule will continue to fold upon itself into large compact clumps of DNA. When the
clumps become large enough, the DNA becomes visible, and it also can no longer be dissolved in the
isopropyl alcohol. It begins to precipitate out of solution.
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