Ribose & Deoxyribose: What Is the Function of Sugar in DNA?

Did you know that the DNA molecule is sweet? It sure is. The double helix that contains the most important information in human biology is actually in part made up of sugar. How sweet is that?

However, before we get into the sweeter things, let us first cover some biology. The DNA molecule contains genes that carry information that is needed to make proteins. These proteins then turn into structures that turn into organs that make up an entire human. In other words, DNA is the basis for everything. But you knew that.

How much do you know about the structure of the DNA molecule? Often referred to as a helical structure, it twists and turns, forming grooves on the sides. In fact, it is called a double helix because it is made up of two strands. In other words, the DNA molecule is a double stranded helical structure, or simply a double helix. It twists and it coils, and it also does something called super coiling, which is a term that describes extreme twisting and turning until the structure becomes really dense. Imagine a piece of thread that you twist and turn with your fingers. Twist it enough in one direction, and it begins twisting on its own again, forming thicker structures. The DNA double helix does pretty much the same thing. The thicker structures that it forms are called chromosomes, which contain a lot of genetic information. In even more detail, the DNA double helix twists and turns around little structures that are shaped like little squares, and then the strings of those squares twist and turn again until they form really think structures, which are the chromosomes.

The reason you are getting all of this information is so that you can better appreciate the contents of the DNA helix and how it contributes to the overall structure.

But let us get to the sugars, or the ribose and deoxyribose molecules. The letters DNA stand for deoxyribonucleic acid, which means that the DNA molecule only contains the deoxyribose sugar. The ribose sugar is part of RNA, or ribonucleic acid, which is a similar molecule but also very different.

Each of the two sugars—ribose and deoxyribose

Each of the two sugars—ribose and deoxyribose—are pentose sugars. This means that they are sugar molecules with five carbons. Shape-wise, they are circular molecules or rings. These carbon atoms within the rings are numbered 1’ to 5’. The reason the numbers are primed is so that they can be distinguished from the nitrogenous bases (more on those later). The difference between the ribose and the deoxyribose sugars is that the latter lacks a hydroxyl group (-OH) at its 2’ carbon. This lack of the hydroxyl group means specific things for the DNA molecule, namely that it can only form phosphodiester linkages with other nucleic acids at its 3’ and 5’ carbons. (More on phosphodiester linkages in a bit.)

Ribose, on the other hand, has a hydroxyl group at its 2’ carbon and is thus considered to be a lot less stable than the deoxyribose sugar. This is particularly important as DNA is speculated to be a lot more stable and RNA a lot more reactive and therefore unstable because of that very difference in the sugar molecules.

Remember how we mentioned that the DNA molecule is made up of two strands, that it is a double helix? Well, each of those two strands has backbones that are made up of certain molecules. These strands are held together by certain linkages, and the entire helical structure is strong enough to withstand the pulls and twists and squeezes of the super coiling process.

Remember also how we mentioned that DNA contains genes? These genes are in the form of nucleotides, the basic structural unit of the DNA molecule. Each nucleotide consists of a nucleoside that is linked to a phosphate group and a nitrogenous base. A nitrogenous base is a nitrogen-containing molecule that behaves like a base. There are four different types of nitrogenous bases, namely Adenine, Thymine, Guanine, and Cytosine. (There is also Uracil, which is an RNA-specific nitrogenous base and as such beyond the scope of this post.) Each of the four aforementioned nitrogenous bases is different in molecular structure. The Thymine and Cytosine have a nitrogenous base that is made up of just one cyclic molecule, while the Guanine and Adenine are made up of two rings, one with six carbons and one with five that are fused to one another.

Each of those nucleotides contains the deoxyribose sugar molecule to which are attached the phosphate group on one side and any one of the four nitrogenous bases on the other side. These nucleotides are positioned in such a way that each phosphate binds to a sugar molecule of another nucleotide, via a phosphodiester bond. As such, the DNA helix has a sugar-phosphate backbone. This interaction ensures that the nitrogenous bases are rotated toward the center of the DNA molecule. These then react with the nitrogenous bases of the other strand in such a way that a Cytosine always binds to a Guanine and an Adenine always binds to a Thymine, each of which is via covalent bonds (a specific type of bond that is really strong). Furthermore, a Cytosine—Guanine bond is made up of three hydrogen bonds, while an Adenine—Thymine bond is based on two hydrogen bonds.

This is always the case and nucleotides are never promiscuous. Each nucleotide pairs with its partner, every time. So, the next time you see genes in their raw letter form, you will know what these letters refer to. Also, you are now all set to talk about the sweet side of DNA.

Laura Day
 

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