Nucleic Acids – Definition, Examples & Functions

Nucleic Acids

Nucleic Acids Definition: The nucleotide polymers that are responsible for determining and regulating the genetic characteristics of an organism, are called nucleic acids. Nucleic acids are present in all living cells.

They derive their name due to their existence inside the nucleus and their acidic nature.

There are two nucleic acids—DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

They are polymers of nucleotides linked in a chain through phosphodiester bonds.

In biological systems, they especially serve as genetic information-carrying molecules. In some cases, RNA molecules serve as catalysts. Nucleic acid was first isolated by Friedrich Miescher (1859) from nuclei of pus cells.

He named this substance nuclein. Later this compound was renamed nucleic acid, due to its acidic properties, by Altmann (1889).

Nucleic acids definition, types, and examples 

Chemical Composition Of Nucleic Acids

Nucleic acids are polymers of monomeric subunits of nucleic acids called nucleotides. Nucleotides join with each other through phosphodiester bonds, to form the chain.

Nucleotides have a distinctive structure composed of three components, covalently bound together.

They are— A nitrogen-containing “base”—either a pyrimidine (one ring) or purine (two rings),

A 5-carbon sugar—ribose or deoxyribose,  A phosphate group.

The combination of a base and a pentose sugar is called a nucleoside. Nucleotides also exist in activated forms containing two or three phosphates, called nucleotide diphosphates or triphosphates.

If the sugar in a nucleotide is deoxyribose, the nucleotide is called a deoxynucleotide. If the sugar is ribose, it is termed a ribonucleotide.

There are five common bases, and four of them are generally represented in either DNA or RNA.

Purines include adenine and guanine, and pyrimidines include uracil, cytosine and thymine.

Structure and function of nucleic acids in biology 

Chemical components of nucleic acids: The nucleic acid is made of several nucleotide units arranged together to form polynucleotide strands. Nucleotides are made up of the following units.

Pentose sugar:

1. The 5-carbon sugar of nucleic acid is known as pentose sugar.
2. The 5th carbon atom of the sugar molecule lies outside the ring structure.
3. The pentose sugar is of two types—ribose and deoxyribose sugar.
4. The nucleic acid with ribose sugar is called ribonucleic acid or RNA and the one with deoxyribose sugar is called deoxyribonucleic acid or DNA.
5. The chemical formula of ribose sugar is C5H10O5.
6. The carbon atoms in the pentose sugar are numbered 1′, 2′, 3′ and so on. Deoxyribose sugar lacks one oxygen atom at the second carbon atom in the ring. The molecular formula of this sugar is C5H10O4.

Phosphate group: It is an important part of a nucleotide, present in both DNA and RNA. A molecule of orthophosphoric acid remains attached to the pentose sugar as a phosphate group.

Examples of nucleic acids and their biological role 

Nitrogenous base: These are nitrogen-containing organic molecules. They are heterocyclic molecules, i.e., they have a ring structure.

Based on the aromatic ring structure, two types of nitrogenous bases are found in nucleic acids—

1. A nitrogenous base with a single ring is known as a pyrimidine ring and
2. A nitrogenous base with a double ring is known as a purine ring.

Pyrimidine:

1. It is comprised of a single heterocyclic ring with four carbon and two nitrogen atoms. The nitrogen atoms are present in the first and third positions in the ring.
2. Pyrimidine bases are of three types, namely thymine (T), cytosine (C) and uracil (U).
3. In DNA, pyrimidines are thymine and cytosine, whereas in RNA, thymine is replaced by uracil.
4. Chemically, thymine is 5-methylpyrimidine-2,4-dione, cytosine is 4-aminopyrimidine-2-one and uracil is pyrimidine-2,4-dione.

Purine: They are made of two heterocyclic rings—

• One being a 6-carbon pyrimidine ring and the other being a 5-carbon imidazole ring. Both the rings share fourth and fifth carbon atoms between them.
• The nitrogen atoms are present in the first, third, seventh and ninth positions.
• Purine bases are of two types, namely adenine (A) and guanine (G).
• Chemically, adenine is purine-6-amine and guanine is purine-2-amine-6-one.

Types of nucleic acids—DNA and RNA

Nucleic acids are generally of two types—

• Deoxyribonucleic acid (DNA)
• Ribonucleic acid (RNA). These are described under separate heads.

Deoxyribonucleic acid: The self-replicating biomolecules, that are present in the chromosomes and carry genetic information are called Deoxyribonucleic acid or DNA.

“function of nucleic acid “

Deoxyribonucleic acid Location: Most of the DNA is present mainly in the nucleus and sometimes in other cell organelles, in the living cell.

Deoxyribonucleic acid Quantity: The unit of measuring the DNA content is a picogram (pg) [lpg = 10’12g]. The DNA content is dependent on the species as well as the cells of the organism.

The DNA content is also dependent on the ploidy (chromosome number) of the cells.

Each chromatid is made up of one DNA molecule. Hence, there are two molecules of DNA present in two chromatids containing chromosomes of the prophase and metaphase stages of cell division. On the other hand,

DNA and RNA nucleic acids – structure and functions 

There is one molecule of DNA present in a single chromatid containing chromosomes of the anaphase stage of cell division. About 1.9m long DNA is present in 23 pairs of chromosomes, in humans.

Structural features: The structural features of DNA are described below.

Deoxyribonucleic acid Shape: In eukaryotic cells, DNA appears as long, thread-like, and unbranched while in prokaryotic cells, mitochondria, plastids etc., it is circular.

In eukaryotes, the nuclear DNA is attached to histone proteins, to form chromatin fibers. Excessive coiling or supercoiling of chromatins gives a distinct chromosome structure.

Deoxyribonucleic acid Length: The length of DNA depends on the species. For example, mitochondrial DNA is about 5 nm long while bacterial DNA is about 1.4 //m long, etc.

Nucleic acids and their role in genetic information 

Chemical structure of DNA: Most DNA exists in the double helix form, in which two linear strands of DNA are wound around each other.

The major force favourable for the formation of this helix structure is complementary base pairing.

Complementary base pairing occurs between A (adenine) and T (thymine) with two hydrogen bonds, and between G (guanine) and C (cytosine) with three hydrogen bonds.

The two strands of DNA are antiparallel to one another. This implies that one strand is aligned in a 5′ to 3′ direction, while the other strand is aligned in a 3′ to 5′ direction.

Nucleotide chains: When nucleotides are bound with each other by phosphodiester bonds, they form polynucleotide chains.

Two such polynucleotide chains form a molecule of DNA. When the number of nucleotide units is less than 20, it is called an oligonucleotide. When such a number exceeds 20, it is called a polynucleotide.

 

“biological importance of nucleic acid “

Double helix structure of DNA: The first information about the structure of DNA came from X-ray diffraction data on DNA structure collected by Rosalind Franklin and Maurice Wilkins in the early 1950’s.

On the basis of Chargaff’s chemical data, Wilkins and Franklin’s X-ray diffraction data and inferences drawn from there, Watson and Crick proposed the double helix model of DNA (1953).

For their contributions, Watson, Crick and Wilkins were awarded the Nobel Prize in Physiology and Medicine in the year 1962 (Franklin died of ovarian cancer in 1958 and the Nobel prize is not awarded posthumously). The features of the double helix model of DNA are given below

The eachdoublein ahelixright-handmodel helix proposes wound two around strands the of same axis.

In a DNA molecule, the two intertwined strands are not parallel, i.e., antiparallel. One strand is aligned 5′-3′ while the other is aligned 3′-5′.

The diameter of the DNA helix is about 20A.

In this structure, the helix makes a turn at every 3.4 nm length of DNA, and the distance between two neighboring base pairs is 0.34 nm. Hence, there are about 10 base pairs per turn.

The intertwined strands make two grooves of different widths, referred to as the major groove and the minor groove, which may facilitate binding with specific proteins.

Each DNA molecule has a sugar-phosphate backbone, with nitrogenous bases attached to them.

Functions of nucleic acids in protein synthesis 

The nitrogenous bases of one DNA molecule form a hydrogen bond with the adjacent base belonging to the other DNA molecule.

A binds with T with a double bond & G binds with C with a triple bond. This keeps the helical structure of the DNA stable, with its diameter constant.

In a single strand of DNA, the amount of A is not equal to that of T, while the amount of G is not equal to that of C. On the other hand, in the case of double-stranded, A + G = T + C.

Types of DNA: Most organisms have regular double-helical forms of DNA. Some bacteriophages and animal viruses have single-stranded DNA.

In prokaryotic organisms (bacteria, etc.), the DNA molecule is not linear, rather it is circular.

DNA can be classified according to the following characters—

1. The number of nucleotides in each turn,
2. The bond angle between the nitrogenous base pairs,
3. The diameter of dna double helix,
4. The direction often of double helix structure, i.e., Right or left.

There are three major forms of DNA helices—A-DNA, B-DNA, and Z-DNA. The structure of B-DNA was proposed by Watson and Crick, which has been discussed in detail in this chapter.

The comparisons between the three forms of DNA.

Ribonucleic acid (RNA): RNA is a type of nucleic acid that is comprised of a single-stranded polynucleotide chain and responsible for protein synthesis.

Occurrence: The non-genetic RNA is present in all prokaryotic and eukaryotic cells. It occurs both in the nucleus as well as in the cytoplasm.

In some viruses, like TMV, influenza virus, retrovirus (HIV), etc., RNA acts as a genetic material and is known as ‘Genetic RNA’.

Quantity: The quantity of RNA depends on the metabolic reactions of the cell. During protein synthesis, the amount of RNA increases in cells.

Structural features:

Cellular RNA, both genetic and non-genetic, is single-stranded and may fold upon itself to form hairpin-like structures entirely or at certain.

1. Regions Rheovirus has double-stranded genetic RNA.
2. The strand is unbranched but may be folded in some cases so that the structure is complex.
3. They are made up of several nucleotides linked by 3′-5′ phosphodiester bonds.
4. Every nucleotide is made up of a ribose sugar, a nitrogenous base, and a phosphate. The nucleotides are called ribonucleotides.
5. The nitrogenous bases present in RNA are adenine, guanine, cytosine and uracil.
6. The main function of RNA is protein synthesis. It is the main genetic material in many viruses.
7. RNA is of different types. They are discussed under separate heads below.

Types of RNA: The different types of RNA have been discussed below.

Genetic RNA: The RNA that acts as genetic material in the absence of DNA, is called genetic RNA.

For example, Tobacco Mosaic viruses (TMV), wound tumor viruses, etc. contain genetic RNA.

This type of RNA is again of two types—

Single-stranded RNA: The RNA molecule made up of a single strand is called single-stranded RNA. E.g., TMV (plant virus), influenza (animal virus).

Double-stranded RNA: The RNA molecule made up of two strands is called double-stranded RNA. E.g., Wound tumor virus (plant virus), Rheovirus (animal virus), etc.

Non-genetic RNA: The RNA presenting the prokaryotic and eukaryotic cells, where DNA is the main genetic material, is called non-genetic RNA. These are synthesized from DNA by transcription.

In both prokaryotic and eukaryotic cells, three principal types of RNA are found. They are messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).

Genetic material: DNA is the biological molecule that stores all the genetic information of the cell. In some viruses, RNA may function as the molecule that stores genetic information.

Transmission of hereditary characters: DNA functions as the molecule, that carries the genetic information from parents to offspring.

Synthesis of protein: Coded genetic information is transferred to the mRNA from DNA by the process of transcription. These codes are used to synthesize proteins by translation with the help of rRNA and tRNA.

Thus, information in DNA or genes is expressed in the form of proteins. Messenger RNA (mRNA) is the nucleic acid that carries information from the nucleus to the cytoplasm on which proteins are made.

Nucleic acids in living organisms – importance and examples

The rRNA is a constituent of ribosome. This type of RNA helps in binding mRNA to ribosome and also in decoding the codons.

They also help in catalyzation of amino acid assembly into a polypeptide chain. The tRNA transports amino acids to the growing end of a polypeptide chain at ribosomes.

Role in metabolic activity: DNA indirectly controls the metabolic activity of the cell through the synthesis of necessary protein molecules, like enzymes.

Role In gene mutation and variation: Changes in the sequences of base pairs of DNA alter the arrangement of nucleotides in mRNA.

This causes alterations of codons which lead to changes in proteins. This induces variations in organisms leading to mutations. Mutation when leads to evolution can give rise to new species.