In this explainer, we will learn how to describe the structure of nucleotides and nucleic acids and outline their importance in living organisms.
Nucleic acids are a type of macromolecule adapted to storing and transferring information. Nucleic acids got their name because they were initially discovered in the nucleus of the cell. There are two types of nucleic acids: DNA, or deoxyribonucleic acid, and RNA, or ribonucleic acid. Even though they were initially discovered in the nucleus of eukaryotic cells, nucleic acids exist in all living things including prokaryotes, which do not possess a nucleus at all.
Key Term: Nucleic Acid
DNA and RNA are nucleic acids. They are polymers made of nucleotide monomers. These macromolecules are adapted to storing and transmitting genetic information.
Nucleic acids are polymers. This means that they are large molecules that are made up of several repeating molecular subunits, or monomers. The monomers of nucleic acids are called nucleotides. A nucleotide is made of three parts: a phosphate group, a pentose sugar, and a nitrogen-containing base. The basic structure of a nucleotide is shown in Figure 1.
Key Term: Nucleotide
A nucleotide is a monomer of a nucleic acid polymer. Nucleotides consist of a pentose sugar, a phosphate group, and a nitrogen-containing base.
Example 1: Identifying the Monomer Units in Nucleic Acid Polymers
Nucleic acids are polymers. What are the monomer units of nucleic acids?
A polymer is a large molecule made up of several smaller, similar molecules bonded together. Nucleic acids include DNA and RNA, and while they have some differences, they are both large molecules formed from strands of smaller molecules called nucleotides. A nucleotide consists of a phosphate group, one of 5 different nitrogenous bases, and a pentose sugar. Different types of nucleotides have different pentose sugars: deoxyribose in DNA and ribose in RNA. The diagram provided shows a basic outline of the structure of DNA, with one of its nucleotides highlighted.
Using this information and the diagram, we can conclude that the monomer units of nucleic acids are nucleotides.
Pentose sugars are sugar molecules that possess 5 atoms of carbon (“pent-” is a prefix that means “five”). There are two types of pentose sugars found in nucleic acids: deoxyribose sugar and ribose sugar. We can tell by the names that deoxyribose sugar is in DNA and ribose sugar is in RNA. A diagram of deoxyribose and ribose sugar is shown in Figure 2.
Key Term: Pentose Sugar
A pentose sugar is a sugar molecule that contains five atoms of carbon. The pentose sugar in DNA is deoxyribose sugar, and the pentose sugar in RNA is ribose sugar.
There are 5 types of nitrogenous bases: adenine, thymine, cytosine, guanine, and uracil. These are often represented by their initials: A, T, C, G, and U. The base thymine is only found in nucleotides of DNA and the base uracil is only found in nucleotides of RNA.
In the bond that exists between a nitrogenous base and a pentose sugar, either ribose sugar or deoxyribose sugar, the nitrogenous base is bonded to carbon number 1. The phosphate group is bonded with carbon number 5. A diagram of the 5 nitrogenous bases is shown in Figure 3.
The polymerization of nucleotides joins them together into a nucleic acid. Adjacent nucleotides bond in a chemical reaction called a condensation reaction, also referred to as a dehydration synthesis reaction. A covalent bond forms between the phosphate group of one nucleotide and the pentose sugar of another, releasing a molecule of water in the process. The covalent bond that is formed between a phosphate group and two sugars is called a phosphodiester bond. These strong bonds form a stable structural chain that is referred to as a sugar–phosphate backbone. A diagram illustrating this process is shown in Figure 4.
Key Term: Sugar–Phosphate Backbone
The sugar phosphate backbone describes the strand of alternating, bonded pentose sugars and phosphate groups which give a nucleic acid its structural basis.
Definition: Phosphodiester Bond
A phosphodiester bond is the chemical bond that forms between a phosphate group and two sugar molecules.
Nucleic acids are responsible for storing and transferring genetic information. Since the sugar–phosphate backbone of a nucleic acid is always the same, the genetic information is in the sequence, or order, of the different nitrogen-containing bases.
DNA is specifically adapted to storing information and to passing information on to offspring cells or organisms. In fact, one chromosome can carry almost 250 MB of data. That may not seem like very much, but the data in DNA is what makes you who you are. This means that the data stored in DNA has to be stable, accurate, and easy to copy.
DNA is made of two strands of nucleotides bonded together at their nitrogen-containing bases. The bases are held together with hydrogen bonds. Because of the structure of the nucleotides, DNA forms a twisted ladder shape called a double helix. The sugar–phosphate backbones make up the sides of the ladder, and two hydrogen-bonded bases make up each rung. A diagram of the double helix structure is shown in Figure 5.
Key Term: Double Helix
A double helix is a “twisted ladder” shape, specifically the shape of a molecule of DNA.
Definition: DNA (Deoxyribonucleic Acid)
DNA is the molecule that carries the genetic instructions for life. It is composed of two strands of nucleotide that coil around each other to form a double helix.
The sequence of bases is the genetic information that is stored in a molecule of DNA. Since the sequence is so important, it has to be maintained in the correct order. We read the bases along a strand of DNA from the (five prime) to the (three prime) direction. This direction is determined by the location of the third and fifth carbon atoms in the pentose sugar within each nucleotide, which you can see in Figure 2 and Figure 4. The matching, or complementary, strand faces the opposite direction, from to . We call this arrangement “antiparallel.” We can see the antiparallel strands of DNA in Figure 5 and Figure 6.
The bases follow certain rules when bonding with each other. In DNA, adenine only bonds with thymine, and cytosine only bonds with guanine. We call these the “base pairing rules.” If we look at Figure 5 and Figure 6, we can see that, in the DNA molecule, A is always paired with T, and C is always paired with G. The two strands of base-paired nucleotides are called “complementary,” because they fit together like two puzzle pieces.
Example 2: Recalling the Type of Bond That Forms between Complementary Base Pairs in DNA
What type of bond forms between complementary base pairs in DNA?
DNA is the genetic material of humans and is incredibly important in determining our characteristics. One molecule of DNA is made of two complementary strands that twist into a distinct shape called a double helix. The strands are formed by the polymerization of nucleotides. Nucleotide monomers join to form nucleic acid polymers through condensation reactions that form phosphodiester bonds between the phosphate groups and pentose sugars. This creates a stable structure we call a “sugar–phosphate backbone.” The two complementary strands are held together by chemical bonds between the nitrogenous bases, and only certain pairs of bases are able to bond with each other. In DNA, the nitrogenous base adenine bonds with thymine, and cytosine bonds with guanine—these are the “complementary base pairs.” The nitrogenous bases which are able to bond together form hydrogen bonds that hold the two strands together and allow them to twist into a double helix.
So, the type of bond that forms between complementary base pairs in DNA is the hydrogen bond.
Example 3: Composing a Complementary Sequence to a Strand of DNA
A single strand of DNA has the following sequence:
Reading from to on the complementary strand, what should the sequence of DNA bases be? You do not need to include the or .
The complementary strands of DNA are antiparallel, meaning they face in opposite directions. So, if left to right is from to on one strand, then left to right would be from to on the complementary strand. DNA sequences are based on the order of the nitrogenous bases in the strand of nucleotides. Each nitrogenous base only bonds with its complementary pair. In DNA, adenine (A) only bonds with thymine (T), and cytosine (C) only bonds with guanine (G). So, for each “A” in the sequence given in the question, we should place a “T” on the complementary strand. For each “C” we need to place a “G,” for each “T” an “A,” and for each “G” a “C.”
Therefore, using these base pairing rules, we can determine that the sequence of DNA on the complementary strand should be TAATAACGCG.
The hydrogen bonds between the two nucleotide strands are relatively easy to break, and the nucleotide bases will only bond with a complementary match. These traits of DNA are what enable it to carry large amounts of information and to be copied quickly and precisely. DNA is also an especially stable molecule. This is what makes DNA well adapted to its function of storing hereditary information.
The base pairing rules we have described are often called “Chargaff’s rules” after the scientist who developed them. A chemist named Erwin Chargaff discovered in the 1940s that, in a sample of DNA from any species, the concentration of adenine bases will be equal to the concentration of thymine. Likewise, the concentration of cytosine will be equal to the concentration of guanine. This discovery is what led to the base pairing rules we have already described.
This information can also be used to calculate the percent composition of the different bases of a sample of DNA. When provided with the total number of nucleotides in a sample and the quantity of just one type of base, we can use Chargaff’s rules to determine the composition of all four different varieties of nitrogenous bases.
How To: Calculating Percent Composition Using Chargaff’s Rules (Base Pairing Rules)
The base pairing rules for DNA state that, between the two complementary strands, adenine always pairs with thymine and cytosine always pairs with guanine. This is illustrated in the diagram below.
Relatedly, we can infer that, in any sample of DNA, the number of adenine bases will be equal to the number of thymine bases, and the number of cytosine bases will be equal to the number of guanine bases.
This information can be used to calculate the number of the bases and the percent composition of each type of base in a sample of DNA with very little initial data.
Let’s call the total number of bases .
We will call the number of bases for each type of nucleotide , , , and for the number of bases of adenine, thymine, cytosine, and guanine respectively.
Using Chargaff’s rule, we know that and also and finally that
Let’s apply this knowledge to an example.
In the diagram above, the expanded segment possesses 12 base pairs.
Since there are 12 pairs, there are 24 total bases:
Let’s say that we are told that 4 of these bases are cytosine:
Next, we can figure out the number of bases remaining after we have excluded those that are cytosine and guanine: or
This set of calculations tells us that the number of bases remaining after we have excluded those that are cytosine and guanine is 16.
Since, according to Chargaff’s rule, we can conclude that half of the remaining 16 bases are adenine and half are thymine:
And, since ,
We can check these values by comparing them to what we see in the diagram. We count the number of A, T, C, and G to check our work.
Now, we know the number of each type of base present in our sample of 24 total bases; let’s calculate the percent composition of each.
To calculate the percent, we will divide the number of a particular base by the total number of bases and multiply that value by .
If we complete this calculation for each of the bases, we get the information in the table below.
If we start with the total number of nucleotides and the number of just one type of base, we can calculate the number and the percent composition of each of the 4 bases in a sample of DNA.
Example 4: Calculating Percent Composition of Nucleotide Bases in DNA
A DNA molecule contains 180 bases. 18 of these bases are adenine.
- What percentage of the bases are thymine?
- What percentage of the bases are guanine?
This question presents us with some information about a section of a DNA molecule. We are first told that the molecule contains 180 bases. We are also informed that of those 180 bases, 18 are adenine.
A molecule of DNA consists of two complementary strands. Complementary means that the two strands fit together according to a pattern. In this case, that pattern is what is known as the “base pairing rules.” The base pairing rules state that where a strand of DNA possesses the base adenine, the complementary strand will have the base thymine. Likewise, where there is a guanine base on one strand, the complementary strand will have cytosine. The illustration of DNA shown below provides an example of these base pairing rules in action.
Knowing this, we can conclude that if there are 18 adenine bases in a section of DNA, there will also be 18 thymine bases, since every adenine would be paired with a thymine. The question asks about the percentage of the thymine bases.
To convert the number to the percentage, we must take the number of a particular base, divide it by the total number of bases, and then multiply that value by :
Now we have the total number of bases, the number of adenine bases, and the number of thymine bases. Using these numbers, we can calculate the number of guanine and cytosine bases:
The same rules apply to the bases cytosine and guanine as the ones we have used for adenine and thymine. Of the 144 remaining bases, since cytosine and guanine are complementary pairs, exactly half will be cytosine and half will be guanine:
Using the above calculations, we can determine that 72 bases will be cytosine and 72 will be guanine.
Again, the question asks about the percentage of guanine, not its number. To obtain this value, we must divide the number of guanine bases by the total number of bases, then multiply the resulting value by :
DNA and RNA molecules serve different functions in living cells. DNA stores genetic information and RNA copies that information to carry it from place to place. RNA specifically carries the genetic code from the DNA in the nucleus to the parts of the cell responsible for protein synthesis.
In order for this to happen, the two strands of DNA separate and RNA nucleotides pair with the exposed DNA bases, forming a single strand of information that can be transferred elsewhere. This is shown in Figure 7.
When RNA copies information from DNA, it follows a similar set of base pairing rules. Cytosine and guanine pair as usual. Wherever DNA has thymine, it will pair with the RNA base adenine, but where DNA has adenine, it will pair with the RNA base uracil instead of thymine. This is illustrated in Figure 8 and Table 1.
Key Term: RNA (Ribonucleic Acid)
RNA is a single-stranded polynucleotide that is specifically adapted for the transmission of genetic information from place to place.
Example 5: Contrasting the Types of Nitrogenous Bases in DNA and RNA
What nitrogenous base in DNA is replaced by uracil in RNA?
Both DNA and RNA are nucleic acids. Nucleic acids are polymers made of nucleotide monomers. A nucleotide is a molecule made of a phosphate group, a pentose sugar, and a nitrogen-containing base. There are 5 kinds of nitrogenous bases: adenine, thymine, cytosine, guanine, and uracil. Both DNA and RNA nucleotides possess adenine, cytosine, or guanine bases. Only DNA possesses the base thymine, and only RNA possesses the base uracil. This means that, during base pairing, an adenine base would have the complementary pair thymine in a molecule of DNA but would pair with uracil in RNA.
Therefore, we can determine that the nitrogenous base in DNA that is replaced by uracil in RNA is thymine.
Nucleic acids are the molecules that tell cells what they are, what cellular machinery to build, and how to build it. The adaptations of nucleic acids make them especially suited to their function in information storage and transfer.
Let’s summarize what we have learned in this explainer.
- Nucleic acids are biological macromolecules adapted to storing and transferring information.
- Nucleic acids are polymers made of monomers called nucleotides.
- A nucleotide is a molecule that consists of a phosphate group, a pentose sugar, and one of 5 nitrogenous bases.
- The nitrogenous bases present in DNA are adenine, thymine, cytosine, and guanine, while RNA has the base uracil instead of thymine.
- DNA is adapted to storing information, is a double-stranded molecule, and possesses deoxyribose sugar.
- RNA is adapted to transferring information, is a single-stranded molecule, and possesses ribose sugar.