Nucleic Acids : Definition, Structure, Functions, and FAQs

Nucleic Acids

Like a computer where you can store the data in the storage device, your body stores your information in the cells. You can also transfer this information to future generations, just as your ancestors shared theirs with you.

This information is encoded in everyone’s body through  DNA and RNA. And you surely have heard of these terms or might have used them sometimes. But do you know their other functions apart from transferring heredity?

This article is all about nucleic acids. You will learn about the nucleic acid definition, its structure, function, and some nucleic acid examples. Have a look.

Nucleic Acid Definition

In 1868, nucleic acids were discovered by Swiss physician Friedrich Miescher. It was isolated from the nuclei of white blood cells. In the beginning, Miescher named this compound “nuclein” because he had isolated it from the nuclei of cells.

Nucleic acids are the basic material that stores genetic information. Nucleic acids are, in fact, polynucleotides that they yield upon hydrolysis. They are the biomolecules found in the nuclei of all living cells in the form of nucleoproteins or chromosomes.

Nucleoproteins are the proteins that contain nucleic acids as the prosthetic (implanted) group. Nucleic acids are the macromolecules present in living organisms. They are the prime information-carrying molecules of the cell, and by directing the process of protein synthesis, they regulate the inherited characteristics of every living thing.

Examples of nucleic acids

1. Ribonucleic acid (RNA)
2. Deoxyribonucleic acid (DNA)

These two nucleic acids are found in living organisms and are of much importance. 

Chemical Components of Nucleic Acids

A nucleic acid is a polymer of smaller molecules called nucleotides. They are the structural units of nucleic acids. The basic structure of nucleic acids consists of a polymer chain (polyester) where phosphoric acid forms an ester linkage with sugar.

There are two polynucleotides that nucleic acid yields upon hydrolysis, i.e., DNA and RNA. Each of them consists of three basic components:

1. Nitrogenous base (base which has nitrogen):

Each nucleic acid contains four nitrogenous bases out of five. They are divided into two classes:

• Purines: The most common purines in nucleic acid are adenine (A) and guanine (G). They are double-ring structured nitrogenous bases.
• Pyrimidine: Three most commonly occurring pyrimidines are found in nucleic acid, i.e., cytosine (C), thymine (T), and uracil (U). They are single-ring structured nitrogenous bases.

2. Phosphate (or phosphoric acid):

Phosphoric acid units are present in nucleic acids in the form of diphosphate (pyrophosphate) and triphosphates. For example, ATP and ADP.

3. A pentose sugar (ribose in RNA and deoxyribose in DNA):

Only two sugars, both pentose sugars, have been isolated from the hydroxylating of nucleic acids. These are D-Ribose and 2-Deoxy-D-Ribose.

The basic subunit or monomeric units of nucleic acids are nucleotides. But they also have their subunit in the form of nucleosides. The hydrolysis of nucleotides obtains nucleosides, and nucleotides consist of a base and sugar.

Therefore, hydrolysis of nucleic acids proceeds in three steps:

Nucleic acid → Nucleotides

Nucleotides → Nucleosides + Phosphoric acid

Nucleosides → Purine or Pyrimidine bases + Sugar

In short, when any nitrogenous base gets bound with sugar, they are called a nucleoside. When this nucleoside gets bound with phosphate, it is called a nucleotide.

Hence, depending upon the type of sugar present, nucleosides and nucleotides are categorised into two classes:

1. Nucleosides:

• Ribonucleosides
• Deoxyribonucleosides

2. Nucleotides:

• Ribonucleotides
• Deoxyribonucleotides

Nucleic Acids Structure

The nucleic acid structure has the following two levels:

1. Primary nucleic acids structure:

Nucleic acids are formed by the polymerisation of thousands of nucleotide molecules. During this process, the -CH2-OH hydroxyl group at the C5 carbon atom of the sugar residue of one nucleotide merges with one of the -OH groups of the phosphoric acid group present at C3 of the other nucleotide to form a long polynucleotide chain.

Therefore, the primary nucleic acid structure is the sequence in which the four nitrogen bases are connected to the sugar-phosphate mainstay of a nucleotide chain.

2. Secondary nucleic acids structure:

The double-helical structure of DNA is explained based on secondary nucleic acid structure. In 1953, Wilkins and his co-workers revealed that DNA has two intertwined strands through X-ray studies.

Later, Watson and Crick proposed that the secondary structure of DNA has two DNA chains wound as a right-handed helix around a common axis but heading in opposite directions.

Structure of DNA

• It consists of two right-handed polynucleotide strands that are running in opposite directions. It gives birth to a double helix structure in DNA.
• The foundation of each strand comprises sugar-phosphate units, and the nitrogenous base units of each strand are acquainted with the inside of the helix.
• H-bonding connects the base pairs of the two strands.
• A purine base of one strand is always paired with a pyrimidine base of the other strand.
• The twinning of bases in DNA is between C and G along three H-bonds, i.e., C ≡ G and between A and T through two H-bonds, i.e., A = T.

Structure of RNA

• This nucleic acid exists as a single-stranded helix.
• Its structure is similar to the DNA structure.
• The only difference in its structure from DNA is that RNA consists of a ribose sugar, and instead of a thymine nitrogenous base, RNA contains a uracil base.
• There are three types of RNA: tRNA (transfer RNA), mRNA (messenger RNA), and rRNA (ribosomal RNA).
• rRNA synthesises the proteins, and mRNA act as an intermediary to transfer genetic information from DNA to ribosomes.
• tRNA is a part of the interpretive system. It carries amino acids to the site of protein synthesis and specifies amino acids to be incorporated according to information of mRNA.

Nucleic Acid Functions

The two important nucleic acid functions are

1. Replication:

It is a process by which a single DNA molecule produces two identical copies of itself. This process is also called cell division or mitosis.

Replication of DNA is an enzyme-catalysed process. It begins unwinding the double helix by breaking the H-bonds, which mainly connect pairs of bases to form two outlying strands.

DNA replication is a semi-conservative, i.e., only one-half of the parental DNA is conserved, and only one strand is synthesised. It is also the replication process responsible for the hereditary effects transmitted from one cell.

2.Translation:

It is the process by which the nucleotide sequences in mRNA (messenger RNA), formed by transcription, are identified by tRNA (transfer RNA). And accordingly, the order of amino acids in the protein being synthesised is determined.

For each amino acid, there is at least one specific tRNA. The tRNA serves as an amino acid carrier as well as the interpreter of genetic code.

Summary

The simplest nucleic acid definition is that they are the complex units of nucleotides that successively consist of a pentose sugar, a base, and phosphate moiety. The basic nucleic acid function is to transfer the characters from parents to offspring. Besides being a transmitter of heredity effects, main nucleic acid functions help synthesise proteins in the body, replication, and transcription.

There are two examples of nucleic acids — DNA and RNA. In nucleic acid structure, DNA contains a five-carbon sugar molecule known as 2-deoxyribose, while RNA contains ribose. Both DNA and RNA contain guanine, adenine, and cytosine. The fourth base is uracil in RNA and thymine in DNA.

Frequently Asked Questions

Q1. What is DNA fingerprinting? What is it used for?

Answer: As every person has unique fingerprints, their sequence of bases on DNA is also unique. The information concerning this is known as DNA fingerprinting. It is similar for every cell and cannot be changed by any known treatment.

Therefore, DNA fingerprinting is used

• To determine the paternity of a person.
• To recognise racial groups to recast biological evolution.
• To identify criminals in forensic laboratories.
• To recognise the dead bodies in any accident by comparing the DNA of their family members.

Q2. Why does it say that two strands of DNA are like siblings, i.e., complementing each other but not identical?

Answer: The two strands in DNA molecules are complementary because H-bonds are formed between the purine base of one strand and the pyrimidine base of the other and vice-versa. Because of the sizes and fashions of the bases, the only possible coupling in DNA is between C (cytosine) and G (guanine) along three H-bonds and between A (Adenosine) and T (Thymine) through two H-bonds. Therefore, the GC pair base has more stability than the AT base. Therefore, the two strands are not identical but complementary.

Q3. What are the important differences between DNA and RNA?

Answer: The important differences between these two nucleic acids are as given below:

DNA	RNA
The sugar present is 2-deoxy-D-(-)-ribose.	The sugar in RNA is D-(-)-ribose.
It contains cytosine and thymine as pyrimidine bases.	It contains cytosine and uracil as pyramidine bases.
They are very large molecules.	They are comparable to smaller molecules.
Molecular mass varies from six to sixteen million u.	Molecular mass ranges from 20,000 to 40, 000 u.
They have the property of replication.	They do not have such property.
They control the transmission of hereditary effects.	They control the concoction of proteins in the body.
DNA has a double-stranded 𝛼-helix structure.	RNA has a single-stranded 𝛼-helix structure.