Hello! Today’s video is on DNA structure. My website, https://www.sciencewithsusanna.com/ has the blank drawing to accompany this video as well as practice materials to quiz yourself.

DNA is a nucleic acid. So is RNA. Nucleic acids are one of the four kinds of “biomolecules”. The other three are lipids, proteins, and carbohydrates. Each DNA strand contains the genetic information of one chromosome. Each chromosome contains 50 - 250 million nucleotide pairs.
The monomers of DNA are called nucleotides. Since there are 46 strands of DNA, there are over 3 billion nucleotide pairs in each body cell.

These colored strands represent the 46 strands of DNA in a body cell. If you were to look more closely at one, you would see that the shape is that of a helix, or spiral. But indeed, it is 2 helices wound together - a double helix.

These spirals have a phosphate-sugar backbone - I’ll use purple for phosphate and blue for sugar. See how it goes phosphate-sugar-phosphate-sugar all the way along the helix? The other helix has the same pattern, but the strands are anti-parallel, like this: 1)2)

Nitrogenous bases connect the helices are the “code” of DNA.

So a nucleotide is composed of a Phosphate, a Sugar, and a Nitrogenous Base.

Let’s look more closely at how the nitrogenous bases can form a code. There are only 4 different kinds, each with a different chemical structure so that they can only fit together according to their code.

Nitrogenous bases attach to the sugar of the nucleotide, shown here in blue.

The four nitrogenous bases of DNA are: adenine, and guanine, which are the bigger ones and they are classified as purines because of their double ring structure; and thymine and cytosine. These two are smaller and they are classified as pyrimidines because they only have one ring.

Now, if you know one side’s sequence, you can predict the other side’s sequence.

Adenine can only bind to thymine (drawn upside down to remind you that the other strand is anti-parallel!). And guanine only can bind cytosine. So this T will bind an A, and this C will bind a G. You can remember this by Apples on Trees and Gas for Cars. If you always say it this way, you’ll also remember that adenine and guanine are the big purines, and thymine and cytosine are the smaller pyrimidines.

The order of the nitrogenous bases is the genetic code; segments called genes determine which proteins are made.

An example of a mutation is hwhen the nucleotides are paired incorrectly when replicating the DNA and the sequence is changed.

So let’s look more closely at the actual molecules. This is the phosphate group, here is the sugar, and it's specifically called a deoxyribose sugar; that’s how DNA gets its name of deoxyribonucleic acid. Next is a big purine nitrogenous base. So a phosphate bound to a sugar, bound to a nitrogenous base is called a nucleotide, and this is considered the monomer of the biomolecules called nucleic acids.

This particular nitrogenous base is adenine, which always binds to thymine. Hydrogen bonds form between this electronegative oxygen and hydrogen, and between this electronegative nitrogen and hydrogen. Hydrogen allow a place to “unzip” the DNA in order to use the code for making protein, or to use the code to replicate an identical strand of DNA. We call the adenine and thymine match a “base pair”.

Next we’ll finish this nucleotide on the anti-parallel strand: Nitrogenous base, sugar, and phosphate.

We can look at another pair of nucleotides: the big purine Guanine has to bond with the smaller cytosine.
These are able to form 3 hydrogen bonds - here, here, and here. Let’s complete our drawing of the antiparallel strand.

Now spend a few minutes reviewing this information; once you think you’re ready, use my Quizlet flashcards to practice and review! See you in the next video!