The nucleic acids are one of the six important biomolecules. They are also known as biopolymers. Friedrich Mischer discovered nucleic acids for the first time. Later on, various people found the role of nucleic acids in heredity and evolution. DNA, also known as Deoxyribonucleic acid, is mainly responsible for inheritance. DNA is not only found in chromosomes but also present in the mitochondria. It is known as the blueprint of life. A DNA molecule carries important instructions for the growth, reproduction and the development of an organism. Its structure looks like a double helix. RNA, also known as ribonucleic acid, is mainly found in the nucleolus. It is also present in the cytoplasm and the ribosomes. RNA is a single-stranded structure. It codes for a protein in a common cellular process known as translation. Nucleotides arranged in the RNA are linear. The nucleotides are small building blocks of DNA and RNA. They are the basic structural units of DNA and RNA.





The genetic material:

A wide storage of an individual’s information related to the characteristic traits is known as the genetic material. The cells are the storage houses of the genetic material. It possesses an ability to get transferred from the parent to the progeny. The genetic material, mainly consisting of DNA, is present in most of the organisms. Certain viruses possess only RNA as their genetic material. There are differences among the genetic materials of the prokaryotes and the eukaryotes. Very few chromosomes are present in the prokaryotes. In most of the prokaryotes, the cells contain double-stranded DNA. The eukaryotic cells have many chromosomes. The DNA is compact. Just like folding a long thread into several small folds, the DNA fits into the chromosomes. The genes are parts and parcels of the DNA. They are involved in protein-coding and other activities. The DNA looks like a long chain. Unlike DNA, RNA is not responsible for inheritance. Both the nucleic acids are known as polymers. A polymer is a large molecule consisting of many similar small molecules with specific linkage. These similar molecules are known as monomers. The scientists studied different aspects of the genetic material and structured various experiments.





Image 1: The genetic material





Structure of the nucleic acids:

The DNA and RNA polymers consist of monomers known as nucleotides. A nucleotide contains three main parts such as a pentose sugar, nitrogenous base, and a phosphate bond. The nucleotides are linked together by covalent linkages. These bonds form linkages between phosphate group of one nucleotide and the third carbon of the sugar present on the other nucleotide. The 5’-3’ phosphate linkages are known as phosphodiester bonds. They are strong stabilizers of the nucleic acid backbone. The DNA consists of a pentose sugar known as deoxyribose. RNA has a ribose sugar.

There are two classes of nitrogenous bases. They are known as purines and pyrimidines respectively. Purines are nine-membered structures. They have double rings. Pyrimidines are six-membered structures. They are single ringed. Examples of purines are adenine and guanine. Examples of pyrimidines are thymine, cytosine, and uracil.





The chemical composition of DNA:

A DNA consists of three important chemical compounds as follows:

1. Sugar: As discussed earlier, the DNA consists of a deoxyribose sugar. It is a pentose sugar. It consists of five carbon atoms. A deoxyribose sugar is a monosaccharide. Loss of an oxygen atom from the ribose sugar gives rise to deoxyribose sugar. An enzymatic reaction involving ribonucleotide reductase synthesizes the sugar.

2. Nitrogenous base: They consist of one or more nitrogen atoms. There are four types of nitrogenous bases in a DNA mainly adenine, guanine, thymine, and cytosine. A nitrogenous base bonds two nucleotides. These bases form hydrogen bonds between the two DNA strands and look like a twisted ladder. Adenine pairs with thymine and Cytosine pairs with Guanine.

3. Phosphoric acid or a phosphate group: The sugar and the base combine to form a nucleoside. A phosphate group gets added to a nucleoside and results in the formation of a nucleoside phosphate. A nucleotide is also known as nucleoside phosphate. The 5’-group of the sugar is attached to the phosphate group. There is a covalent bond between the phosphate group of one nucleotide and 3’ carbon of the sugar present on the other nucleotide. The phosphodiester bonds are very strong.





Image 2: Nucleic acids





The molecular structure of the DNA:

The double helical structure consists of nitrogenous bases paired with hydrogen bonds for maintaining the complementary structure. The adenine base pairs with the thymine using two hydrogen bonds. The guanine and cytosine pair using three hydrogen bonds. The helix involves specific turn, measured in nanometers. Each turn measures 3.4 nanometres giving a diameter of 0.34 nanometers for the double helix.

Watson and Crick published their works in 1953. Their double helix model of DNA had the features describing the structure and orientation of the double helix. Watson and Crick deduced that the DNA molecule consisted of two polynucleotides wound around each other. Their orientation was clockwise or right-handed. The strands were anti-parallel to each other. The orientation of the bases is toward the central axis. The hydrogen bonds between the bases help the strands to get separated easily. They also found an unequal spacing between the two sugar-phosphate backbones. The unequal spacing results into two types of grooves. Major groove is wide. The narrow groove is known as a minor groove. The nitrogenous bases can make contacts with the proteins for further packing of DNA into structures known as chromosomes.





Structure of RNA

The nucleolus and the cytoplasm are the places where the RNA resides. DNA synthesizes RNA. This process is known as transcription. Just like DNA, an RNA molecule consists of the three main chemical components.

a. A ribose sugar is present in the RNA. It is a five-carbon sugar. It is mainly involved in generating the energy molecule known as ATP.

b. An RNA molecule consists of Adenine, Guanine, Cytosine, and Uracil.

c. The third component includes the phosphate groups.

The messenger RNA or the mRNA gets synthesized from the DNA inside the nucleus. It consists of both introns and the exons. After splicing, the RNA gets matured to form a mature mRNA consisting of only the exons. The process of translation or protein synthesis occurs outside the nucleus. The ribosomal RNA or the rRNA occurs in the ribosomes. These components play a crucial role in translation. The rRNA plays a crucial role in the organization of the nucleolus. The transfer RNA (tRNA) is an important component of translation.





References:

[1] Principles of Nucleic Acid Structure, Stephen Neidle, 2010, Preview

[2] Medical Genetics, G.P. Pal, first edition



