Interesting question. I have personally never
heard of any organism having triple helix DNA.
That would mean there would be three separate
strands of DNA to form this helix. Based on the
way a double-stranded DNA helix is formed, a
triple-stranded helix would not be possible.
me explain, in your cells (and all organisms'
cells) there is lots of double-stranded DNA. When
it is time for the cell to divide and make a copy
of itself it has to duplicate all of the DNA
inside of its nucleus. Here is the way DNA is
copied: First, the double stranded helix is
separated into two separate single strands of DNA.
Second, each single strand is read and a mirror
image of each strand is created. Third, the mirror
image that was created binds to the original
single strand that was read and a new double helix
is formed. Because this happens to both single
strands of the original double-stranded helix, you
end up with two new double-stranded helices when
you started with just one.
To help you
understand the structure of DNA and how the two
single strands fit together to form the helix,
here is an analogy I thought up (hope it helps):
Imagine that each strand of DNA is composed of
four different structures, such as: square pegs,
round pegs, square holes and round holes.
Obviously, square pegs match with square holes and
round pegs match with round holes. When DNA is in
its double-stranded configuration, the two strands
have to fit together as you would imagine the pegs
fitting into their appropriate shaped holes. So if
you had one strand that had a sequence like: round
peg-square hole-square hole-square peg-round hole.
Then the strand of DNA that would fit together
with this strand would be the exact opposite:round
hole-square peg-square peg-square hole-round peg.
See how these two strands if lined up end to end,
would fit together because they are the opposite
of each other? One these two strands are fit
together it is not possible for another strand to
fit in because all of the pegs and holes are
occupied by the perfect fit with the other strand.
When DNA coils up to form a double-stranded helix
it is a very tight coil and it requires little
energy to stay in this shape. Any other
configuration would require more energy and is
therefore not as desirable (cells, just like
humans like to spend the least energy they can on
When it's time to make a
copy of the DNA, a special molecule, called an
cutting enzyme, comes along and separates the two
strands of DNA (pulls the pegs out of the holes,
in my example). After the two strands are
separated, another special molecule (another
enzyme) comes along and, beginning at one end of
one of the strand, the enzyme "reads" the strand
one piece at a time (to find out whether each
piece is a round hole or a square peg or a round
peg or a square hole), then it places the
appropriate match on a growing new strand of DNA.
So if this special enzyme reads the existing
single strand of DNA has a square peg at one
position, then it gets a square hole and puts it
on the new, growing strand of DNA that will bind
with the one it is reading.
This process continues
along until the entire strand of DNA has been read
and a new matching, but opposite strand has been
created right next to it. Immediately after the
new strand is made it binds to the strand that was
read to form a new double-stranded helix that is
an exact copy of the original double-stranded
helix.Can you see how a triple helix would be
impossible with this mode of DNA duplication and
Do you know when and
how the double-stranded helix configuration of DNA