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What is Deoxyribonucleic Acid? - Definition & Structure

Instructor: Darla Reed

Darla has taught undergraduate Enzyme Kinetics and has a doctorate in Basic Medical Science

In this lesson, you will learn what deoxyribonucleic acid is and where it gets its name. Also this lesson will help you discover the major components and basic structure of deoxyribonucleic acid.

Defining Deoxyribonucleic Acid

'The deoxyribonucleic acid matched ... suspect number 1.'

Imagine your favorite crime or courtroom drama revealing the perp like this- what a mouthful! Instead of DeoxyriboNucleic Acid, it's easier to say DNA. But what is DNA?

DNA is the language of genetics. Just like a language is composed of words, nucleotides are the words used to write DNA. If nucleotides are the words, then a sequence of nucleotides would be a sentence. Many of the nucleotide sequences are conserved (the same) between species and people. So how can the police nab criminals based on DNA if much of it is the same?

Like some words in other languages are unique, a small portion of sequences in DNA are unique, and it is this uniqueness that differentiates humans from apes and one person's DNA from another. To further understand DNA, let's look at what makes a nucleotide and how they are put together to form the three dimensional structure of DNA.

Building a Nucleotide

The elements Carbon (C), nitrogen (N), hydrogen (H), oxygen (O) and phosphorous (P) are necessary for building nucleotides. These elements compose the three major parts of a nucleotide: a phosphate group, a sugar and a base. (Just the sugar + base are known as a nucleoside.)

The bases can be categorized as either a pyrimidine (single carbon-nitrogen ring) or purine (two rings). There are four main bases that are found in DNA- adenine (purine), guanine (purine), cytosine (pyrimidine) and thymine (pyrimidine). These four bases (A,G,C,T) are the letters in the language of DNA. Purines A and G most favorably pair via hydrogen bonds with pyrimidines T and C, respectively. It is the distinctive arrangement of the bases and their pairing that creates the genetic language (or code) of DNA and is partly responsible for its 3-D structure.

Base pairing in DNA with hydrogen bonds.
DNA bases

The sugar can be considered the 'middle' of the nucleotide since it attaches to both the phosphate group and the base. The sugar also gives us a part of DNA's name deoxyribose. What a complicated sounding term. Why not call it SNA (sugary nucleic acid)?

What Is in a Name?

There is a very good reason not to call it sugary nucleic acid. The name deoxyribose is important for scientists to distinguish it from other sugars and also is very informative about what the sugar looks like and how it behaves chemically. It also helps to distinguish it from other molecules that contain sugar. There are actually much more complicated terms scientists use to refer to this sugar (like Beta-D-2-deoxyribofuranose) so deoxyribose isn't so bad.

How about we break down the term, so it'll be easier to understand and perhaps remember. The prefix 'de' means not or without, while 'oxy' refers to oxygen, therefore 'deoxy' means without oxygen. But what is without oxygen? Let's keep looking at the name to find out. The ending -ose indicates a sugar. In this case, 'ribose' is a 5 carbon sugar where four of the carbons form a furan ring (4 carbons connected to an oxygen in a circle). In ribose, all of the carbons are attached to an oxygen from a hydroxyl (OH) group.

Structure of a furan ring.
Simplified furan ring

Therefore 'deoxyribose' has the same structure as ribose but is missing an oxygen that ribose has.This missing oxygen is very important because it allows for the movement of the molecule in a way not possible if the oxygen were present and helps give DNA its 3-D structure.

Difference between ribose and deoxyribose is an oxygen.
Ribose and deoxyribose structure

3-D Structure of DNA

So, now we know the parts of a nucleotide: a phosphate + a sugar+ a base. We know there are four bases that compose DNA and pair pyrimidine to purine. We know the sugar is in the 'middle' of the nucleotide, but how does it all stack up into a 3-D space?

Connected nucleotides of DNA.
Basic chemical structure of three nucleotides of DNA

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