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Nucleic acids are macromolecules composed of monomeric units called nucleotides joined together by phosphodiester bonds. These nucleotides constitute DNA and RNA.

Learning Objective: Describe the basic structure of nucleic acids and compare & contrast the structure of DNA and RNA.

In phosphodiester bonds, one phosphoric acid molecule forms bonds with the 3′ carbon of one pentose molecule and 5′ carbon of a second pentose molecule.

This is what you will learn:

  • Characteristics and structure of nucleic acid
  • To identify DNA and RNA
  • Compare and contrast various characteristics of DNA and RNA.

Characteristics of Nucleic Acids

Nucleic acids are the genetic material for all living cells. They play numerous important roles within the cell.

For example, they serve as energy stores for future use in phosphate transfer reactions, predominantly carried out by adenosine triphosphate (ATP).

They also form part of coenzymes, such as NAD, NADP, FAD and coenzyme A.  These coenzymes serve as mediators of important cellular processes and activated intermediates in numerous biosynthetic reactions.

Structure of Nucleic Acids

Each nucleotide  consists of a nitrogenous heterocyclic base (purine or pyrimidine), a pentose sugar, and a phosphate group.


All nucleotides contain a 5-carbon sugar (pentose); the pentose ribose is found in RNA while deoxyribose is found in DNA.

Sugars and phosphates in nucleic acids are connected to each other in an alternating chain.


Phosphoric acid  is attached to each sugar at the 3′ and 5′ C positions to give rise to the sugar-phosphate backbone.

During the phosphodiester bond formation, two phosphate groups are removed from one of the two participating nucleotides.

Nitrogenous Bases

Nitrogenous bases are adenine, guanine, thymine, and cytosine in DNA, whereas, thymine is replaced by uracil in RNA (figure 1 and 2).

Figure 1. Purine
Figure 1. Purine
Figure 2. Pyrimidines
Figure 2. Pyrimidines

Deoxyribonucleic Acid (DNA)

Deoxyribonucleic acid (DNA), the blueprint of life (Figure 3), is a nucleic acid that carries the genetic information in the cell. It is capable of self-replication and synthesis of RNA.

Figure 3. DNA structure
Figure 3. DNA structure

Characteristics of DNA

DNA bases pair up with each other, A with T and C with G, to form units called base pairs.

Each base is also attached to a sugar molecule and a phosphate molecule. Together, they are called a nucleotide.

Nucleotides are arranged in two long strands that form a spiral ladder like structure called a double helix.

One of the most important properties of DNA is to replicate, i.e., making copy of itself (Figure 4).

Figure 4. Process of DNA replication
Figure 4. Process of DNA replication

Ribonucleic Acid (RNA)

Ribonucleic Acid (RNA) is a biologically important molecule that consists of a long chain of nucleotide units (figure 5).

Figure 5. RNA structure
Figure 5. RNA structure

It is necessary for protein synthesis (mRNA, tRNA, and rRNA), and other cellular activities.

Characteristics of RNA

RNA, stands for ribonucleic acid, is a single stranded polymeric molecule made up of one or more nucleotides.

Each nucleotide is made up of a base (adenine, cytosine, guanine, and uracil, typically abbreviated as A, C, G and U), a ribose sugar, and a phosphate.

RNA is synthesized from DNA by an enzyme known as RNA polymerase during a process called transcription.

The new RNA sequences are complementary to their DNA template, rather than being identical copies of the template.

RNA is then translated into proteins by structures called ribosomes.

Types of RNA

There are three types of RNA involved in the translation process

  • messenger RNA (mRNA)
  • transfer RNA (tRNA)
  • ribosomal RNA (rRNA)

It is also the genetic material for some organisms that do not have DNA. Some viruses contain DNA; many only contain RNA.

RNAs are used in some cancer gene therapies to reduce the expression of cancer-causing genes. It is also used to suppress the expression of fruit ripening genes.

Differences between DNA and RNA

Structurally, DNA and RNA are almost identical. However, there are fundamental differences (Figure 6) that account for the different functions of these two molecules.

Figure 6 Difference between DNA and RNA
Figure 6. Difference between DNA and RNA


  • Nitrogenous Bases: Adenine, Guanine, Cytosine, and Thymine
  • Five-Carbon Sugar: Deoxyribose
  • Structure: Double-stranded


  • Nitrogenous Bases: Adenine, Guanine, Cytosine, and Uracil
  • Five-Carbon Sugar: Ribose
  • Structure: Single-stranded

Table showing differences between DNA and RNA

Features Deoxyribonucleic acid

Ribonucleic acid

Structure Double-stranded, consisting of phosphate group, sugar, and four nitrogen containing bases (adenine, thymine, cytosine, and guanine) Single stranded and consists of phosphate group, sugar, and four nitrogen containing bases (adenine, uracil, cytosine, and guanine)
Function Blueprint of biological system or life as it transmits genetic information from one generation to another Transfers genetic code needed for the creation of proteins from nucleus to the ribosome
Location DNA is found in the nucleus of a cell and in mitochondria Depending on the type of RNA, this molecule is found in a cell’s nucleus, its cytoplasm, and its ribosome
Stability Less reactive because of C-H bonds also stable in alkaline conditions More reactive because of C-OH bonds and not stable in alkaline conditions
Propagation Self-replicating Formed from the DNA by transcription
Base pairing Adenine links to thymine (A-T) and cytosine links to guanine (C-G) Adenine links to uracil (A-U) and cytosine links to guanine (C-G)
Effect of UV rays Can be damaged by exposure to ultra-violet rays More resistant to damage by ultra-violet rays than DNA

Things to Remember

  • Nucleic acids are molecules made up of nucleotides responsible for various activities, such as, cell division and protein synthesis.
  • Nucleic acids are macromolecules composed of monomeric units called nucleotides joined together by phosphodiester bonds.
  • Each nucleotide is made up of a pentose sugar, a nitrogenous base, and a phosphate group. There are two types of nucleic acids: DNA and RNA.
  • DNA is called blueprint of life as it passes genetic informationfrom parents to the offsprings.
  • RNA is involved in protein synthesis and its regulation.

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