Nucleic acids represent one of the four major categories of biomolecules, which are the substances that make up cells. The others are proteins, carbohydrates and lipids (or fats). Nucleic acids, which include DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), differ from the other three biomolecules in that they cannot be metabolized to supply energy to the parent organism. (That's why you don't see "nucleic acid" on nutrition information labels.)
The function of DNA and RNA is to store genetic information. A complete copy of your own DNA can be found in the nucleus of almost every cell in your body, making this aggregation of DNA – called chromosomes in this context – rather like the hard drive of a laptop computer. In this scheme, a length of RNA of the sort called messenger RNA contains the coded instructions for only one protein product (i.e., it contains a single gene) and is therefore more like a "thumb drive" containing a single important file.
DNA and RNA are very closely related. The single substitution of a hydrogen atom (–H) in DNA for a hydroxyl group (–OH) attached to the corresponding carbon atom in RNA accounts for the entire chemical and structural difference between the two nucleic acids. As you'll see, though, as so often happens in chemistry, what seems like a tiny difference at the atomic level has obvious and profound practical consequences.
Structure of Nucleic Acids
Nucleic acids are made up of nucleotides, which are substances that themselves consist of three distinct chemical groups: a pentose sugar, one to three phosphate groups and a nitrogenous base.
The pentose sugar in RNA is ribose, while that in DNA is deoxyribose. Also, in nucleic acids, nucleotides only have one phosphate group. One example of a well-known nucleotide that boasts multiple phosphate groups is ATP, or adenosine triphosphate. ADP (adenosine diphosphate) participates in many of the same processes that ATP does.
Single molecules of DNA can be extraordinarily long and can extend for the length of an entire chromosome. RNA molecules are far more limited in size than DNA molecules but still qualify as macromolecules.
Specific Differences Between DNA and RNA
Ribose has a five-atom ring that includes four of the five carbons in the sugar. Three of the others are occupied by hydroxyl (–OH) groups, one by a hydrogen atom and one by a hydroxymethyl (–CH2OH) group. The only difference in deoxyribose is that one of the three hydroxyl groups (the one at the 2-carbon position) is gone and is replaced by a hydrogen atom.
Also, while both DNA and RNA have nucleotides with one of four possible nitrogenous bases included, these vary slightly between the two nucleic acids. DNA features adenine (A), cytosine (C), guanine (G) and thymine. whereas RNA has A, C and G but uracil (U) in place of thymine.
Types of Nucleic Acids
Most of the functional differences between DNA and RNA relate to their markedly different roles in cells. DNA is where the genetic code for living – not just reproduction but day to day life activities – is stored. RNA, or at least mRNA, is responsible for collecting the same information and bringing it to the ribosomes outside the nucleus where proteins are built that allow the carrying out of those aforementioned metabolic activities.
The base sequence of a nucleic acid is where its specific messages are carried, and the nitrogenous bases can thus be said to be ultimately responsible for differences in animals of the same species – that is, different manifestations of the same trait (e.g., eye color, body hair pattern).
Base Pairing in Nucleic Acids
Two of the bases in nucleic acids (A and G) are purines, while two (C and T in DNA; C and U in RNA) are pyrimidines. Purine molecules contain two fused rings, while pyrimidines have only one and are smaller in general. As you'll soon learn, the DNA molecule is double-stranded because of bonding between the nucleotides in adjacent strands.
A purine base can only bond with a pyrimidine base, because two purines would take up too much space between strands and two pyrimidines too little, with a purine-pyrimidine combination being just the right size. But things are actually more tightly controlled than this: In nucleic acids, A bonds to and only to T (or U in RNA), whereas C bonds to and only to G.
Structure of DNA
The complete description of the DNA molecule as a double-stranded helix in 1953 by James Watson and Francis Crick eventually earned the duo a Nobel Prize, although the X-ray diffraction work of Rosalind Franklin in the years leading to this achievement was instrumental in the pair's success and is often understated in history books.
In nature, DNA exists as a helix because this is the most energetically favorable form for the particular set of molecules it contains to take. The side chains, bases and other portions of the DNA molecule experience the right blend of electrochemical attractions and electrochemical repulsions so that the molecule is most "comfortable" in the shape of two spirals, slightly offset from each other, like interwoven spiral-style staircases.
Bonding Between Nucleotide Components
DNA strands consist of alternating phosphate groups and sugar residues, with the nitrogenous bases attached to a different part of the sugar portion. A DNA or RNA strand elongates thanks to hydrogen bonds formed between the phosphate group of one nucleotide and the sugar residue of the next. Specifically, the phosphate at the number-5 carbon (often written 5') of the incoming nucleotide is attached in place of the hydroxyl group on the number-3 carbon (or 3') of the growing polynucleotide (small nucleic acid). This is known as a phosphodiester linkage.
Meanwhile, all of the nucleotides with A bases are lined up with nucleotides with T bases in DNA and nucleotides with U bases in RNA; C pairs uniquely with G in both. The two strands of a DNA molecule are said to be complementary to each other, because the base sequence of one can be determined using the base sequence of the other thanks to the simple base-pairing scheme nucleic acid molecules observe.
The Structure of RNA
RNA, as noted, is extraordinarily similar to DNA on a chemical level, with only one nitrogenous base among four being different and a single "extra" oxygen atom in the sugar of RNA. Obviously, these seemingly trivial differences are sufficient to ensure substantially different behavior between the biomolecules.
Notably, RNA is single-stranded. That is, you will not see the term "complementary strand" used in the context of this nucleic acid. Different portions of the same RNA strand, however, can interact with each other, which means that the shape of RNA actually varies more than the shape of DNA (invariably a double helix). Accordingly, there are numerous different types of RNA.
Types of RNA
- mRNA, or messenger RNA, uses complementary base-pairing to carry the message DNA gives it during transcription to the ribosomes, where that message is translated into protein synthesis. Transcription is described in detail below.
- rRNA, or ribosomal RNA, makes up a sizable portion of the mass of ribosomes, the structures within cells responsible for protein synthesis. The remainder of the mass of ribosomes consists of proteins.
- tRNA, or transfer RNA, plays a critical role in translation by shuttling amino acids destined for the growing polypeptide chain to the spot where proteins are assembled. There are 20 amino acids in nature, each with its own tRNA.
A Representative Length of Nucleic Acid
Imagine being presented with a strand of nucleic acid with the base sequence AAATCGGCATTA. Based on this information alone, you should be able to conclude two things quickly: One, that this is DNA, not RNA, as revealed by the presence of thymine (T); and that the complementary strand of this DNA molecule has the base sequence TTTAGCCGTAAT.
You can also be sure of the mRNA strand that would result from this strand of DNA undergoing RNA transcription: It would have the same sequence of bases as the complementary DNA strand, with any instances of thymine (T) being replaced by uracil (U). This is because DNA replication and RNA transcription operate similarly in that the strand made from the template strand is not a duplicate of that strand, but its complement or the equivalent in RNA.
In order for a DNA molecule to make a copy of itself, the two strands of the double helix must separate in the vicinity of copying. This is because each strand is copied (replicated) separately and because the enzymes and other molecules that take part in DNA replication need room to interact, which a double helix does not provide. Thus the two strands become physically separated, and the DNA is said to be denatured.
Each separated strand of DNA makes a new strand complementary to itself, and remains bound to it. So, in a sense, nothing is different in each new double-stranded molecule from its parent. Chemically, they have the same molecular composition. But one of the strands in each double helix is brand-new while the other is left over from replication itself.
When DNA replication occurs simultaneously along separated complementary strands, the synthesis of the new strands actually occur in opposite directions. On one side, the new strand simply grows in the direction of the DNA being "unzipped" as it is denatured. On the other side, however, small fragments of new DNA are synthesized away from the direction of strand separation. These are called Okazaki fragments, and are joined together by enzymes after reaching a certain length. These two new DNA strands are antiparallel to each other.
RNA transcription is similar to DNA replication in that the unpairing of DNA strands is required for it to start. mRNA is made along the DNA template by the sequential addition of RNA nucleotides by the enzyme RNA polymerase.
One mRNA strand usually includes exactly the base sequence necessary to assemble one unique protein downstream in the translation process, which means that one mRNA molecule typically carries the information for one gene. A gene is a DNA sequence that codes for a particular protein product.
Once transcription is complete, the mRNA strand is exported out of the nucleus through pores in the nuclear envelope. (RNA molecules are too large to simply diffuse through the nuclear membrane, as can water and other small molecules). It then "docks" with ribosomes in the cytoplasm or within certain organelles, and protein synthesis is initiated.
How Are Nucleic Acids Metabolized?
Nucleic acids cannot be metabolized for fuel, but they can be created from very small molecules or broken down from their complete form into very small parts. Nucleotides are synthesized through anabolic reactions, often from nucleosides, which are nucleotides minus any phosphate groups (that is, a nucleoside is a ribose sugar plus a nitrogenous base). DNA and RNA can also be degraded: from nucleotides to nucleosides, then to nitrogenous bases and eventually to uric acid.
Breakdown of nucleic acids is important for overall health. For example, the inability to break down purines is linked to gout, a painful disease affecting some of the joints thanks to urate crystal deposits in those locations.
- LibreTexts Introduction to Biology: Ribonucleic Acid (RNA)
- NCBI Bookshelf: Molecular Cell Biology (4th Edition): Structure of Nucleic Acids
- LibreTexts Microbiology: DNA and RNA
- Northern Arizona University: The Molecules of Life
- Scitable By Nature Education: DNA Transcription
- LibreTexts Chemistry: Nucleotides and Nucleic Acids