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Nucleotides

Nucleotides

You might have heard of DNA and RNA: these molecules contain genetic information that determine the characteristics of living things (including us humans!). But do you know what DNA and RNA are actually made of?

DNA and RNA are nucleic acids, and nucleic acids are made up of building blocks called nucleotides. Here we will describe what a nucleotide is, elaborate on its components and structure, and discuss how it bonds to form nucleic acids and other biological molecules.

Nucleotide Definition

First, let's look at the definition of a nucleotide.

Nucleotides are the building blocks of nucleic acids: when nucleotides bond together, they form what are called polynucleotide chains which, in turn, make up segments of biological macromolecules called nucleic acids.

Nucleotide vs. Nucleic Acid

Before we proceed, let's make things clear: nucleotides are different from nucleic acids. A nucleotide is considered a monomer, while a nucleic acid is a polymer. Monomers are simple molecules that bond with similar molecules to form large molecules called polymers. Nucleotides bond together to form nucleic acids.

Nucleic acids are molecules that contain genetic information and instructions for cellular functions.

There are two main types of nucleic acids: DNA and RNA.

  • Deoxyribonucleic acid (DNA): DNA contains genetic information needed for the transmission of heritable traits and instructions for the production of proteins.

  • Ribonucleic acid (RNA): RNA plays a vital role in the creation of protein. It also carries genetic information in some viruses.

It is important to distinguish between the two because the components and structure of nucleotides of DNA and RNA are different.

Components and Structure of a Nucleotide

We will first discuss the main components of a nucleotide before elaborating on its structure and how it bonds together to form nucleic acids.

3 parts of a Nucleotide

A nucleotide has three major components: a nitrogenous base, a pentose sugar, and a phosphate group. Let’s look into each of these and see how they interact to form a nucleotide.

Nitrogenous base

Nitrogenous bases are organic molecules containing one or two rings with nitrogen atoms. Nitrogenous bases are basic because they have an amino group that tends to bind extra hydrogen, which leads to a lower hydrogen ion concentration in its surroundings.

Nitrogenous bases are classified either as purines or pyrimidines (Fig. 1):

Purines

Pyrimidines

Adenine (A)

Guanine (G)

Thymine (T)

Uracil (U)

Cytosine (C )

Figure 1. Adenine (A) and guanine (G) are purines, while thymine (T), uracil (U), and cytosine (C) are pyrimidines.

Purines have a double ring structure wherein a six-membered ring is attached to a five-membered ring. On the other hand, pyrimidines are smaller and have a single six-membered ring structure.

The atoms in nitrogenous bases are numbered 1 through 6 for pyrimidine rings and 1 through 9 for purine rings (Fig. 2). This is done to indicate the position of bonds.

Nucleotides Figure 1: Purines and pyrimidines | StudySmarter

Figure 2. This illustration shows how purine and pyrimidine bases are structured and numbered. Source: StudySmarter Originals.

Both DNA and RNA contain four nucleotides. Adenine, guanine, and cytosine are found in both DNA and RNA. Thymine can be found only in DNA, while uracil can be found only in RNA.

Pentose sugar

A pentose sugar has five carbon atoms, with each carbon numbered 1′ through 5′ (1′ is read as “one prime”).

Two types of pentose are present in nucleotides: ribose and deoxyribose (Fig. 2). In DNA, the pentose sugar is deoxyribose, while in RNA, the pentose sugar is ribose. What distinguishes deoxyribose from ribose is the lack of hydroxyl group (-OH) on its 2’ carbon (which is why it is called “deoxyribose”).

Nucleotides Figure 3: Ribose and deoxyribose | StudySmarterFigure 3. This illustration shows how ribose and deoxyribose are structured and numbered. Source: StudySmarter Originals.

The nitrogenous base of a nucleotide is attached to the 1’ end, while the phosphate is attached to the 5’ end of the pentose sugar.

Primed numbers (such as 1’) indicate atoms of the pentose sugar, while unprimed numbers (such as 1) indicate atoms of the nitrogenous base.

Phosphate group

The combination of nitrogenous base and pentose sugar (without any phosphate groups) is called a nucleoside. The addition of one to three phosphate groups (PO4) turns a nucleoside into a nucleotide.

Prior to being integrated as part of nucleic acid, a nucleotide usually exists as a triphosphate (meaning it has three phosphate groups); however, in the process of becoming a nucleic acid, it loses two of the phosphate groups.

The phosphate groups bond to 3’ of ribose rings (in RNA) or 5’ of deoxyribose rings (in DNA).

Nucleoside, nucleotide, and nucleic acid structure

In a polynucleotide, one nucleotide is joined to the adjacent nucleotide by a phosphodiester linkage. Such bonding between the pentose sugar and the phosphate group creates a repetitive, alternating pattern called the sugar-phosphate backbone.

A phosphodiester linkage is a chemical bond that holds a polynucleotide chain together by linking a phosphate group to 5' in the pentose sugar of one nucleotide to the hydroxyl group at 3' in the pentose sugar of the next nucleotide

The resulting polynucleotide has two "free ends" that are different from each other:

  • The 5’ end has a phosphate group attached.

  • The 3’ end has a hydroxyl group attached.

These free ends are used to indicate a directionality across the sugar-phosphate backbone (such direction can be either from 5’ to 3’ or from 3’ to 5’). The nitrogenous bases are attached along the length of the sugar-phosphate backbone.

The sequence of nucleotides along the polynucleotide chain defines the primary structure of both DNA and RNA. The base sequence is unique for each gene, and it contains very specific genetic information. In turn, this sequence specifies the amino acid sequence of a protein during gene expression.

Gene expression is the process by which genetic information in the form of DNA sequence is encoded into an RNA sequence, which in turn is translated into an amino acid sequence to form proteins.

The diagram below sums up the formation of nucleosides, nucleotides, and nucleic acids from the three major components (Fig. 4).

Nucleotides Figure 4: Nucleoside, nucleotide, and nucleic acid | StudySmarter

Figure 4. This diagram shows how a pentose sugar, a nitrogenous base, and a phosphate group form nucleosides, nucleotides, and nucleic acids. Source: StudySmarter Originals.

The secondary structure of DNA and RNA differ in several ways:

  • DNA consists of two intertwined polynucleotide chains that form a double-helix structure.

    • The two strands form a right-handed helix: when it is viewed along its axis, the helix moves away from the observer in a clockwise screwing motion.

    • The two strands are antiparallel: the two strands are parallel, but they run in opposite directions; specifically, the 5’ end of one strand faces the 3’ end of the other strand.

    • The two strands are complementary: the base sequence of each strand aligns with the bases on the other strand.

  • RNA consists of a single polynucleotide chain.

    • When RNA folds, base pairing can take place between complementary regions.

In both DNA and RNA, each nucleotide in the polynucleotide chain pairs with a specific complementary nucleotide via hydrogen bonds. Specifically, a purine base always pairs with a pyrimidine base as follows:

  • Guanine (G) pairs with Cytosine (C) via three hydrogen bonds.

  • Adenine (A) pairs with Thymine (T) in DNA or Uracil (U) in RNA via two hydrogen bonds.

A hydrogen bond is the attraction between the partially positive hydrogen atom of one molecule and the partially negative atom of another molecule.

Nucleoside and nucleotide naming conventions

Nucleosides are named according to the nitrogenous base and pentose sugar attached:

  • Nucleosides with purine bases end in -osine.

    • When bonded to ribose: adenosine and guanosine.

    • When bonded to deoxyribose: deoxyadenosine and deoxyguanosine.

  • Nucleosides with pyrimidine bases end in -idine.

    • When bonded to ribose: uridine and cytidine.

    • When bonded to deoxyribose: deoxythymidine and deoxycytidine.

Nucleotides are named similarly, but they also indicate if the molecule contains one, two, or three phosphate groups.

Adenosine monophosphate (AMP) has one phosphate group

Adenosine diphosphate (ADP) has two phosphate groups

Adenosine triphosphate (ATP) has three phosphate groups

Additionally, the name of nucleotides can also indicate the position in the sugar ring where the phosphate is attached.

Adenosine 3’ monophosphate has one phosphate group attached to 3’

Adenosine 5’ monophosphate has one phosphate group attached to 5’

Nucleotides in other biological molecules

Besides storing genetic information, nucleotides are also involved in other biological processes. For example, adenosine triphosphate (ATP) functions as a molecule that stores and transfers energy. Nucleotides can also function as coenzymes and vitamins. They also play a role in metabolic regulation and cell signaling.

Nicotinamide adenine nucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) are two coenzymes formed through the attachment of adenosine to a nicotinamide analog nucleotide.

NAD and NADP are involved in oxidation-reduction (redox) reactions in cells, including those in glycolysis (the metabolic process of breaking down sugars) and in the citric acid cycle (a series of reactions that release stored energy from chemical bonds in processed sugars). A redox reaction is a process where electrons are transferred between two participating reactants.

Nucleotides - Key takeaways

  • Nucleotides are monomers (building blocks) that bond together to form nucleic acids.
  • A nucleotide has three major components: a nitrogenous base, a pentose (five-carbon) sugar, and a phosphate groups.
  • There are two types of nucleic acids formed by nucleotides: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
  • The nitrogenous bases adenine, guanine, and cytosine are found in both DNA and RNA, but thymine is found only in DNA while uracil is found only in RNA.
  • In DNA, the pentose sugar is deoxyribose, while in RNA, the pentose sugar is ribose.

References

  1. Zedalis, Julianne, et al. Advanced Placement Biology for AP Courses Textbook. Texas Education Agency.
  2. Reece, Jane B., et al. Campbell Biology. Eleventh ed., Pearson Higher Education, 2016.
  3. Sturm, Noel. “Nucleotides: Composition and Structure.” California State University Dominguez Hills, 2020, http://www2.csudh.edu/nsturm/CHEMXL153/NucleotidesCompandStruc.htm.
  4. Libretexts. “4.4: Nucleic Acids.” Biology LibreTexts, Libretexts, 27 Apr. 2019, https://bio.libretexts.org/Courses/University_of_California_Davis/BIS_2A%3A_Introductory_Biology_(Easlon)/Readings/04.4%3A_Nucleic_Acids.
  5. Libretexts. “19.1: Nucleotides.” Chemistry LibreTexts, Libretexts, 1 May 2022, https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/The_Basics_of_GOB_Chemistry_(Ball_et_al.)/19%3A_Nucleic_Acids/19.01%3A_Nucleotides.
  6. “Chapter 28: Nucleosides, Nucleotides, and Nucleic Acids.” Vanderbilt University, https://www.vanderbilt.edu/AnS/Chemistry/Rizzo/Chem220b/Ch28.pdf.
  7. Neuman, Robert C. “Chapter 23 Nucleic Acids from Organic Chemistry.” University of California Riverside Department of Chemistry , 9 July 1999, https://chemistry.ucr.edu/sites/default/files/2019-10/Chapter23.pdf.
  8. Davidson, Michael W. “Molecular Expressions Photo Gallery: The Nucleotide Collection.” Florida State University, 11 June 2005, https://micro.magnet.fsu.edu/micro/gallery/nucleotides/nucleotides.html.

Frequently Asked Questions about Nucleotides

A nucleotide is a monomer that bonds with other nucleotides to form nucleic acids.

The three parts of a nucleotide are: a nitrogenous base, a pentose sugar, and a phosphate group.

A nucleotide is a monomer that bonds with other nucleotides to form nucleic acids. Nucleic acids are molecules that contain genetic information and instructions for cellular functions.


Besides storing genetic information, nucleotides also play a significant role in other biological processes, including the storage and transfer of energy, metabolic regulation, and cell signaling.

A nucleotide has three major components: a nitrogenous base, a pentose sugar, and a phosphate group.

Uracil can be found only in RNA. As such, the presence of uracil in a nucleic acid indicates that it is RNA. 

Final Nucleotides Quiz

Question

A nucleotide is a ___.

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Answer

monomer

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Question

Nucleotides bond together to form __.

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Answer

Nucleic acids

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Question

What are the three major components of a nucleotide?

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Answer

Pentose sugar

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Question

What are the two classifications of nitrogenous bases?

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Answer

Purines

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Question

How are purines different from pyrimidines?

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Answer

Purines have a double ring structure wherein a six-membered ring is attached to a five-membered ring. On the other hand, pyrimidines are smaller and have a single six-membered ring structure.  

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Question

Which nitrogenous bases are found in both DNA and RNA?

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Answer

Adenine

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Question

What nitrogenous base is found only in RNA?

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Answer

Uracil

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Question

A pentose sugar has how many carbons?

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Answer

5

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Question

What are the components of a nucleoside?

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Answer

Nitrogenous base and pentose sugar

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Question

The links that form between adjacent nucleotides are called __.

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Answer

Phosphodiester linkage

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Question

The 5’ end of a polynucleotide has ___ attached.

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Answer

a phosphate group

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Question

The 3’ end of a polynucleotide has  ___ attached.

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Answer

a hydroxyl group  

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Question

Where are the nitrogenous bases attached in the polynucleotide?

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Answer

The nitrogenous bases are attached along the length of the sugar-phosphate backbone of the polynucleotide chain.

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Question

Besides storing genetic information, nucleotides are also involved in what other biological processes?

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Answer

Besides storing genetic information, nucleotides are also involved in other biological processes, including the storage and transfer of energy, metabolic regulation, and cell signaling.

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