Nucleic acids - Definition, Structure, Properties, Types

Nucleic acids are complex biomolecules that play a fundamental role in living organisms. They are responsible for storing, transmitting, and expressing genetic information. Just as proteins are polymers of amino acids, nucleic acids are polymers of nucleotides. These molecules determine the hereditary characteristics of organisms and ensure the continuity of life from one generation to the next.

Composition of Nucleic Acids

Each nucleotide consists of three components: a nitrogenous base, a pentose sugar, and a phosphate group. These nucleotides are linked together through phosphodiester bonds to form the polynucleotide chains of DNA and RNA.

1. Nitrogenous Bases: Nitrogenous bases are heterocyclic aromatic compounds containing nitrogen. They are of two types:

• Purines: Purines have a fused double-ring structure. Adenine (A) and Guanine (G) are examples.
• Pyrimidines: Pyrimidines have a single-ring structure. Cytosine (C), Thymine (T), and Uracil (U) are pyrimidine bases. Thymine is present in DNA, whereas uracil is present in RNA.

2. Pentose Sugar: The sugar present in nucleic acids is a five-carbon (pentose) sugar. The absence of one oxygen atom at the 2′ carbon in deoxyribose distinguishes DNA from RNA.

• β-D-Ribose is present in RNA.
• β-D-2-Deoxyribose is present in DNA.

3. Phosphate Group: A phosphate group derived from phosphoric acid is attached to the sugar molecule. It links adjacent nucleotides through phosphodiester bonds and forms the sugar-phosphate backbone of nucleic acids.

Nucleosides

A nucleoside is formed when a nitrogenous base combines with a pentose sugar. The nitrogenous base may be a purine (adenine or guanine) or a pyrimidine (cytosine, thymine, or uracil).

• The sugar may be ribose, found in RNA, or deoxyribose, found in DNA.
• The nitrogenous base is attached to the C-1′ carbon atom of the sugar through a β-N-glycosidic linkage. Since a nucleoside contains only a base and a sugar, it does not contain any phosphate group.

When these bases are attached to ribose sugar, the nucleosides formed are adenosine, guanosine, cytidine, uridine, and thymidine.

Nucleotides

When a phosphate group is attached to a nucleoside, a nucleotide is formed. The phosphate group is usually attached to the fifth carbon atom (5′ carbon) of the sugar molecule.

• Nucleotides are the actual building blocks of nucleic acids.
• Thousands of nucleotides join together through phosphodiester bonds to form long chains known as polynucleotides.
• These polynucleotide chains constitute DNA and RNA.
• Therefore, a nucleotide consists of three components: a nitrogenous base, a pentose sugar, and a phosphate group.

Types of Nucleic Acids

Nucleic acids are the genetic materials present in living organisms. They are of two main types Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA). Both are polymers of nucleotides and are essential for the storage, transmission, and expression of genetic information.

1. Deoxyribonucleic Acid (DNA)

Deoxyribonucleic acid (DNA) is a nucleic acid composed of nucleotide units containing deoxyribose sugar, phosphate groups, and the nitrogenous bases adenine, guanine, cytosine, and thymine.

• DNA is the principal genetic material in most living organisms.
• It is mainly found in the nucleus of eukaryotic cells and forms an important component of chromosomes. In prokaryotes, DNA is present in the nucleoid region.
• DNA carries hereditary information that is passed from one generation to the next.

Examples: Human chromosomal DNA and the DNA present in bacterial cells such as Escherichia coli.

2. Ribonucleic Acid (RNA)

Ribonucleic acid (RNA) is a nucleic acid composed of nucleotide units containing ribose sugar, phosphate groups, and the nitrogenous bases adenine, guanine, cytosine, and uracil.

• RNA is found in both the nucleus and cytoplasm of cells.
• It is synthesized from DNA and is associated with the transfer and expression of genetic information.
• RNA occurs in different forms within the cell, each having a specific role.

Examples: The major forms of RNA are messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).

Structure of DNA

DNA (Deoxyribonucleic Acid) is the genetic material present in most living organisms. According to the Watson and Crick model, DNA consists of two polynucleotide chains twisted around each other to form a double helix.

• Each nucleotide contains a deoxyribose sugar, a phosphate group, and a nitrogenous base.
• The sugar and phosphate groups form the backbone of the DNA molecule, while the nitrogenous bases are arranged inside the helix.
• The bases pair specifically through hydrogen bonds Adenine (A) pairs with Thymine (T) and Guanine (G) pairs with Cytosine (C).The two strands are complementary to each other.
• The double-helical structure of DNA provides stability and enables the storage and transmission of genetic information.

Structure of RNA

RNA (Ribonucleic Acid) is a nucleic acid composed of ribonucleotides. Each nucleotide contains a ribose sugar, a phosphate group, and a nitrogenous base.

• Unlike DNA, RNA is generally single-stranded.
• It contains adenine (A), guanine (G), cytosine (C), and uracil (U). Uracil replaces thymine, which is present in DNA.
• RNA is synthesized from DNA and is found in both the nucleus and cytoplasm of cells.
• It occurs mainly in three forms: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).

Functions of DNA

DNA serves as the genetic material in most living organisms. Its primary function is to store hereditary information and ensure its transmission from one generation to the next.

• DNA stores the genetic information required for the structure and functioning of an organism.
• It transmits hereditary traits from parents to offspring.
• DNA directs the synthesis of proteins by providing the necessary genetic instructions.
• It controls various cellular activities through the information encoded in genes.
• DNA undergoes replication before cell division, ensuring that genetic information is accurately passed to daughter cells.

Functions of RNA

RNA plays an important role in the expression of genetic information and in the synthesis of proteins. It acts as an intermediary between DNA and proteins.

1. Messenger RNA (mRNA)

• Carries genetic information from DNA to ribosomes.
• Serves as a template for protein synthesis.

2. Transfer RNA (tRNA)

• Transports amino acids to ribosomes during protein synthesis.
• Helps in the correct arrangement of amino acids to form proteins.

3. Ribosomal RNA (rRNA)

• Forms the structural and functional component of ribosomes.
• Participates in the process of protein synthesis.