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DNA nucleotides are the small repeating units that, joined together by the millions into a long spiral ladder shape, form the DNA strand, also called a DNA molecule or double helix or, simply, DNA. But what exactly are DNA nucleotides and why are they important to DNA sequencing?

Each nucleotide segment in a DNA strand is actually a pair of nucleotides, but is commonly called, collectively, a nucleotide, for ease of use. I’ll describe this breakdown visually again so you can see a clear picture: if you take a DNA strand and cut it crosswise so that you now have one rung’ with a short section of pole on either side, you’ll have a single unit that looks like a capital ‘H’; each of those side poles is a short section of what is called the DNA Backbone, and the horizontal rung joining them in the middle is called a Base. We’ll separate and discuss the backbones and the bases on the next two pages.

First, this single H shape, though generally called a nucleotide, is actually a PAIR of nucleotides, joined in the middle. If you take that H shape and cut the rung (base) in the middle, you are now left with two pieces, each looking like a capital T turned on its side. NOW you have a single nucleotide. Let’s take that single DNA nucleotide and zoom in for yet a closer look.

That single piece of DNA nucleotide is made up of two parts. For visual clarity let’s keep looking at it as a sideways T, with the vertical bar being a part of backbone and the horizontal part being the base, the half-rung of the ladder. That’s all we need to cover for this page; remember, I’m taking you smaller and smaller, we’ve cut it down like this so far: the long strand of DNA you see in pictures is shaped as a double helix, actually two long helixes joined down the middle by their half-rungs; if you take that double helix and cut out a single section of it, with one rung and a short section of each backbone, you have a section of the DNA strand known as a nucleotide in general language, and it looks like a capital H. Technically you slice that H in half so it now looks like two capital T’s turned sideways. Each of these T’s is called, correctly, a single nucleotide.

Now we take a nucleotide and take it apart: we take the half-rung of the ladder, called a base, apart from the side if the ladder, the backbone.