Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the two nucleic acids found in nature. Nucleic acids in turn represent one of the four "molecules of life," or biomolecules. The others are proteins, carbohydrates and lipids. Nucleic acids are the only biomolecules that cannot be metabolized to generate adenosine triphosphate (ATP, the "energy currency" of cells).
DNA and RNA both carry chemical information in the form of a nearly identical and logically straightforward genetic code. DNA is the originator of the message and the means by which it is relayed to subsequent generations of cells and whole organisms. RNA is the conveyor of the message from the instruction-giver to the assembly-line workers.
While DNA is directly responsible for messenger RNA (mRNA) synthesis in the process called transcription, DNA also relies on RNA to function properly in order to convey its instructions to ribosomes within the cells. The nucleic acids DNA and RNA can therefore be said to have evolved an interdependence with each equally vital to the mission of life.
Nucleic Acids: Overview
Nucleic acids are long polymers made up of individual elements called nucleotides. Each nucleotide consists of three individual elements of its own: one to three phosphate groups, a ribose sugar and one of four possible nitrogenous bases.
In prokaryotes, which lack a cell nucleus, both DNA and RNA are found free in the cytoplasm. In eukaryotes, which have a cell nucleus and also possess a number of specialized organelles, DNA is found mainly in the nucleus. But, it can also be found in the mitochondria and, in plants, inside chloroplasts.
Eukaryotic RNA, meanwhile, is found in the nucleus and in the cytoplasm.
What Are Nucleotides?
A nucleotide is the monomeric unit of a nucleic acid, in addition to having other cellular functions. A nucleotide consists of a five-carbon (pentose) sugar in a five-atom interior ring format, one to three phosphate groups and a nitrogenous base.
In DNA, there are four possible bases: adenine (A) and guanine (G), which are purines, and cytosine (C) and thymine (T), which are pyrimidines. RNA contains A, G and C as well, but substitutes uracil (U) for thymine.
In nucleic acids, the nucleotides all have one phosphate group attached, which is shared with the next nucleotide in the nucleic-acid chain. Free nucleotides, however, can have more.
Famously, adenosine diphosphate (ADP) and adenosine triphosphate (ATP) participate in countless metabolic reaction in your own body every second.
The Structure of DNA vs. RNA
As noted, while DNA and RNA each contain two purine nitrogenous bases and two pyrimidine nitrogenous bases, and contain the same purine bases (A and G) and one of the same pyrimidine bases (C), they differ in that DNA has T as its second pyrimidine base while RNA has U every place T would appear in DNA.
Purines are larger than pyrimidines as they contain two joined nitrogen-containing rings to the one in pyrimidines. This has implications for the physical form in which DNA exists in nature: it's double-stranded, and, specifically, is a double helix. The strands are joined by the pyrimidine and purine bases on adjacent nucleotides; if two purines or two pyrimidines were joined, the spacing would be too great or two small respectively.
RNA, on the other hand, is single stranded.
The ribose sugar in DNA is deoxyribose whereas that in RNA is ribose. Deoxyribose is identical to ribose except that the hydroxyl (-OH) group at the 2-carbon position has been replaced by a hydrogen atom.
Base-Pair Bonding in Nucleic Acids
As noted, in nucleic acids, purine bases must bind to pyrimidine bases to form a stable double-stranded (and ultimately double-helical) molecule. But it is actually more specific than that. The purine A binds to and only to the pyrimidine T (or U), and the purine G binds to and only to the pyrimidine C.
This means that when you know the base sequence of a strand of DNA, you can determine the exact base sequence of its complementary (partner) strand. Think of complementary strands as inverses, or photographic negatives, of each other.
For instance, if you have a strand of DNA with the base sequence ATTGCCATATG, you can deduce that the corresponding complementary DNA strand must have the base sequence TAACGGTATAC.
RNA strands are a single strand, but they come in various forms unlike DNA. In addition to mRNA, the other two main types of RNA are ribosomal RNA (rRNA) and transfer RNA (tRNA).
The Role of DNA vs. RNA in Protein Synthesis
DNA and RNA both contain genetic information. In fact, mRNA contains the same information as the DNA from which it was made during transcription, but in a different chemical form.
When DNA is used as a template to make mRNA during transcription in the nucleus of a eukaryotic cell, it synthesizes a strand that is the RNA analog of the complementary DNA strand. In other words, it contains ribose rather than deoxyribose, and where T would be present in DNA, U is present instead.
During transcription, a product of relatively limited length is created. This mRNA strand usually contains the genetic information for a single unique protein product.
Every strip of three consecutive bases in mRNA can vary in 64 different ways, the result of four different bases at each spot raised to the third power to account for all three spots. As it happens, each of the 20 amino acids from which cells build proteins is coded for by just such a triad of mRNA bases, called a triplet codon.
Translation at the Ribosome
After mRNA is synthesized by DNA during transcription, the new molecule moves from the nucleus to the cytoplasm, passing through the nuclear membrane through a nuclear pore. It then joins forces with a ribosome, which is just coming together from its two subunits, one large and one small.
Ribosomes are the sites of translation, or the use of the information in mRNA to manufacture the corresponding protein.
During translation, when the mRNA strand "docks" on the ribosome, the amino acid corresponding to the three exposed nucleotide bases – that is, the triplet codon – is shuttled into the region by tRNA. A subtype of tRNA exists for every one of the 20 amino acids, making this shuttling process more orderly.
After the right amino acid is attached to the ribosome, it is quickly moved to a nearby ribosomal site, where the polypeptide, or the growing chain of amino acids preceding the arrival of each new addition, is in the process of being completed.
Ribosomes themselves are made up of a roughly equal mixture of proteins and rRNA. The two subunits exist as separate entities except when they are actively synthesizing proteins.
Other Differences Between DNA and RNA
DNA molecules are considerably longer than RNA molecules; in fact, a single DNA molecule makes up the genetic material of an entire chromosome, accounting for thousands of genes. Also, the fact that they are separated into chromosomes at all is a testament to their comparative mass.
Although RNA has a more humble profile, it is actually the more diverse of the two molecules from a functional standpoint. In addition to coming in tRNA, mRNA and rRNA forms, RNA can also act as a catalyst (enhancer of reactions) in some situations, such as during protein translation.
About the Author
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.