So let’s take a moment to talk about how the nitrogenous bases
come together to make the steps in this spiral staircase of helical DNA.
We know that purines pair with pyrimidines and this makes sense
because you don’t want the steps to be all wonky as you walk up the staircase.
So a double ring structure will pair with a single ring structure.
A purine will pair with a pyrimidine.
First of all, we’ll see adenine and thymine pair together, making a nice even sized step.
And then, also, we’ll see that guanine and cytosine pair together.
So those will always be the pairs that match up.
Also, you’ll notice that what bonds the nitrogenous bases to each other
is a hydrogen bond or two or three hydrogen bonds.
In the case of guanine and cytosine, you’ll see that there are three hydrogen bonds.
Whereas with adenine and thymine, you’ll see that there are two hydrogen bonds.
These hydrogen bonds as a zipper up the middle make DNA easy to separate,
make the two strands easy to separate
which is really important when we consider DNA replication during cell division.
We need to be able to unzip it
and we need to be able to match up complementary bases
in order to make new strands of DNA.
But that’s the subject of a future lecture.
One more nitrogenous base based molecule that’s important,
just because this is really closely related to DNA,
doesn’t mean that it has a similar function of DNA.
The common theme here though is that it has a five carbon sugar which is ribose,
a nitrogenous base, and phosphate.
However, in this case, we have three phosphate groups.
So this is adenosine, the particular nitrogenous base,
triphosphate, meaning three phosphate groups.
Now, it could be guanine on occasion or thymine or cytosine.
Mostly, we will see adenosine triphosphate.
It is the energy currency for life.
On occasion, we’ll see guanine triphosphate or GTP used as an energy source.
But most commonly again, ATP is the main currency.