Nucleic Acid

Nucleic Acid Definition

Nucleic acid is the chemical name for the molecules RNA and DNA. The name comes from the fact that these molecules are acids – that is, they are good at donating protons and accepting electron pairs in chemical reactions – and the fact that they were first discovered in the nuclei of our cells.

Nucleic acids are large molecules made up of strings, or “polymers,” of units called “nucleotides.” All life on Earth uses nucleic acids as their medium for recording hereditary information – that is nucleic acids are the hard drives containing the essential blueprint or “source code” for making cells.

For many years, scientists wondered how living things “knew” how to produce all the complex materials they need to grow and survive, and how they passed their traits down to their offspring.

Scientists eventually found the answer in the form of DNA – deoxyribonucleic acid – a molecule located in the nucleus of cells, which was passed down from parent cells to “daughter” cells.

When DNA was damaged, or passed on incorrectly, the scientists found that cells did not work properly. Damage to DNA would cause cells and organisms to develop incorrectly, or be so badly damaged that they simply died.

Later experiments revealed that another type of nucleic acid – RNA, or ribonucleic acid – acted as a “messenger” that could carry copies of the instructions found in DNA. Ribonucleic acid was also used to pass down instructions from generation to generation by some viruses.

Today, scientists know that the source code for cells is quite literally written in nucleic acids. Genetic engineering changes organisms’ traits by adding, removing, or rewriting parts of their DNA – and subsequently changing what “parts” the cells produce.

A sufficiently skilled genetic “programmer” can create the instructions for a living cell from scratch using the nucleic acid code. Scientists did exactly that in 2010, using an artificial DNA synthesizer to “write” a genome from scratch using bits of source code taken from other cells.

All living cells on Earth “read” and “write” their source codes in almost exactly the same “language” using nucleic acids. Sets of three nucleotides, called triplets, can code for any given amino acid, or for the stop or start of protein production.

Other properties of nucleic acids may influence DNA expression in more subtle ways, such as by sticking together and making it harder for transcription enzymes to access the code they store.

The fact that all living cells on Earth “speak” almost the same genetic “language” supports the idea of a universal common ancestor – that is, the idea that all life on Earth today started with a single primordial cell whose descendants evolved to give rise to all modern living species.

From a chemical perspective, the nucleotides that are strung together to create nucleic acids consists of a five-carbon sugar, a phosphate group, and a nitrogen-containing base. The image below shows structural drawings of the four DNA and the four RNA nitrogenous bases used by living things on Earth in their nucleic acids.

It also shows how the sugar-phosphate “backbones” bond at an angle that creates a helix – or a double helix in the case of DNA – when multiple nucleic acids are strung together into a single molecule:

Difference of DNA and RNA

Difference of DNA and RNA

DNA and RNA are both polymers of nucleotides. The term “polymer” comes from “poly” for “many” and “mer” for parts, referring to the fact that each nucleic acid is made of many nucleotides.

Because nucleic acids can be made naturally by reacting inorganic ingredients together, and because they are arguably the most essential ingredient for life on Earth, some scientists believe that the very first “life” on Earth may have been a self-replicating sequence of amino acids that was created by natural chemical reactions.

Nucleic acids have been found in meteorites from space, proving that these complex molecules can be formed by natural causes even in environments where there is no life.

Some scientists have even suggested that such meteorites may have helped create the first self-replicating nucleic acid “life” on Earth. This seems possible, but there is no firm evidence to say whether it is true.

Function of Nucleic Acids

By far the most important function of nucleic acids for living things is their role as carriers of information.

Because nucleic acids can be created with four “bases,” and because “base pairing rules” allow information to be “copied” by using one strand of nucleic acids as a template to create another, these molecules are able to both contain and copy information.

To understand this process, it may be useful to compare the DNA code to the binary code used by computers. The two codes are very different in their specifics, but the principle is the same. Just as your computer can create entire virtual realities simply by reading strings of 1s and 0s, cells can create entire living organisms by reading strings of the four DNA base pairs – A, T, C, and G.

As you might imagine, without binary code, you’d have no computer and no computer programs. In just the same way, living organisms need intact copies of their DNA “source code” to function.

The parallels between the genetic code and binary code has even led some scientists to propose the creation of “genetic computers,” which might be able to store information much more efficiently than silicon-based hard drives. However as our ability to record information on silicon has advanced, little attention has been given to research into “genetic computers.”

Because the DNA source code is just as vital to a cell as your operating system is to your computer, DNA must be protected from potential damage. To transport DNA’s instructions to other parts of the cell, then copies its information are made using another type of nucleic acid – RNA.

It’s these RNA copies of genetic information which are sent out of the nucleus and around the cell to be used as instructions by cellular machinery.

Nucleic acids and similar molecules can also be used by cells for other purposes. Ribosomes – the cellular machines that make protein – and some enzymes are made out of RNA.

The fact that RNA can act both as hereditary material and an enzyme strengthens the case for the idea that the very first life might have been a self-replicating, self-catalyzing RNA molecule.

Nucleic Acid Structure

Because nucleic acids can form huge polymers which can take on many shapes, there are several ways to discuss the “structure of nucleic acid”. It can mean something as simple as the sequence of nucleotides in a piece of DNA, or something as complex as the way that DNA molecule folds and how it interacts with other molecules.

Please refer to our Nucleic Acid Structure article for more information.

Polymer of Nucleic Acids

As mentioned above, a “polymer” is a molecule that is made up of smaller parts. “Poly” means “many,” while “mer” means “part.”

When you hear doctors or scientists talking about “nucleic acids,” they almost always mean polymers. RNA and DNA are both typically strands of many nucleotides strung together.

Single nucleotides are usually referred to as just that – nucleotides. The term “nucleic acid” as well as the terms “RNA” and “DNA” are usually reserved for polymers of many nucleic acid monomers strung together.

Monomer of Nucleic Acids

The monomers of nucleic acids are molecules called nucleotides. These molecules are fairly complex, consisting of a nitrogenous base plus a sugar-phosphate “backbone.”

When our cells join nucleotides together to form the polymers called nucleic acids, it bonds them by replacing the oxygen molecule of the 3′ sugar of one nucleotide’s backbone with the oxygen molecule of another nucleotide’s 5′ sugar.

This is possible because the chemical properties of nucleotides allow 5′ carbons to bond to multiple phosphates. These phosphates are attractive bonding partners for the 3′ oxygen molecule of the other nucleotide’s 3′ oxygen, so that oxygen molecule pops right off to bond with the phosphates, and is replaced by the oxygen of the 5′ sugar. The two nucleotide monomers are then fully linked with a covalent bond through that oxygen molecule, turning them into a single molecule.

Nucleotides are the monomers of nucleic acids, but just as nucleic acids can serve purposes other than carrying information, nucleotides can too.

The vital energy-carrying molecules ATP and GTP are both made from nucleotides – the nucleotides “A” and “G,” as you might have guessed.

In addition to carrying energy, GTP also plays a vital role in G-protein cell signaling pathways. The term “G-protein” actually comes from the “G” in “GTP” – the same G that’s found in the genetic code.

G-proteins are a special type of protein that can cause signaling cascades with important and complex consequences within a cell. G-proteins are turned “on” or “off” by the phosphorylation of GTP.

Quiz

1. Which of the following is NOT a reason why some scientists think the first life might have been made of RNA?
A. RNA nucleotides can be created spontaneously by natural processes.
B. RNA can carry hereditary information, just like DNA.
C. RNA can form enzymes that can catalyze chemical reactions, just like proteins.
D. None of the above.

Answer to Question #1
D is correct. All of the above are reasons why some scientists think RNA may have been the first life form!

2. If there are only four base pairs of RNA and DNA, then why do we list five? (A, G, C, T, and U?)
A. Uracil is not a true nucleotide.
B. Uracil is the RNA equivalent of Thymine.
C. Uracil and Thymine are interchangeable.
D. None of the above.

Answer to Question #2
B is correct. Uracil is the RNA equivalent of Thymine. Although there are tiny chemical differences between the two, U and T can play the same role in base pair hydrogen bonding. The four DNA base pairs are A, T, C, and G, while the four RNA base pairs are A, U, C, and G.

3. Why might the “handedness” of our nucleic acids be important?
A. Left-handed nucleic acids might take up more room in our cells than right-handed ones.
B. The handedness of one’s nucleic acids determines whether you’re left-handed or right-handed.
C. Some enzymes can only interact with molecules that have the correct “handedness” for their active sites.
D. None of the above.

Answer to Question #3
C is correct. While “handedness” in molecules has nothing to do with whether your right or left hand is dominant, it can determine whether your enzymes can interact with the molecule. Enzymes might interact very differently with the right-handed vs. left-handed version of a molecule.

References

  • Lodish, H. F. (2016). Molecular cell biology. New York, NY: Freeman.
  • Mansfield, M. L., & Ballanco, J. (2011). A Model for the Evolution of Nucleotide Polymerase Directionality. PLOS. Retrieved from http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0018881
  • Qin, K., Dong, C., Wu, G., & Lambert, N. A. (2011). Inactive-state preassembly of Gq-coupled receptors and Gq heterotrimers. Nature Chemical Biology, 7(10), 740-747. doi:10.1038/nchembio.642
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