10.5. Nucleic Acids

Every generation of each and every species resembles its ancestors in many ways. How are these characteristics transmitted from one generation to the next? It has been observed that nucleus of a living cell is responsible for this transmission of inherent characters, also called heredity. The particles in nucleus of the cell, responsible for heredity, are called chromosomes which are made up of proteins and another type of biomolecules called nucleic acids. These are mainly of two types, the deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Since nucleic acids are long chain polymers of nucleotides, so they are also called polynucleotides.

Nucleic acids are vital biomolecules found in the nucleus of cells and play a key role in heredity, which is the way traits are passed from parents to offspring. The main types of nucleic acids are DNA and RNA. DNA contains genetic instructions essential for the development and functioning of living organisms. It is a double-helix structure made up of nucleotides, which are the building blocks of the nucleic acids. RNA is usually single-stranded and is involved in protein synthesis.

Think of DNA as a recipe book for an entire kitchen, where each recipe represents a trait that gets passed down to future generations, ensuring certain characteristics like eye color or height are shared within a family.

Complete hydrolysis of DNA (or RNA) yields a pentose sugar, phosphoric acid and nitrogen containing heterocyclic compounds (called bases). In DNA molecules, the sugar moiety is b-D-2-deoxyribose whereas in RNA molecule, it is b-D-ribose.

When nucleic acids like DNA and RNA are broken down, they yield three main components: a sugar (either deoxyribose for DNA or ribose for RNA), phosphoric acid, and nitrogenous bases which are essential for the formation of the genetic code. These bases can be adenine, guanine, cytosine, and thymine for DNA, with RNA having uracil instead of thymine.

Imagine a train set where each train car represents a component of nucleic acid: the sugar connects the cars, the phosphoric acid acts as the tracks, and the bases are the unique cars themselves that determine the train's destination, which is the expression of traits.

A unit formed by the attachment of a base to 1¢ position of sugar is known as nucleoside. In nucleosides, the sugar carbons are numbered as 1¢, 2¢, 3¢, and so on to distinguish these from the bases. When nucleoside is linked to phosphoric acid at 5¢-position of sugar moiety, we get a nucleotide. Nucleotides are joined together by phosphodiester linkage between 5¢ and 3¢ carbon atoms of the pentose sugar.

Nucleotides are the fundamental units of nucleic acids. A nucleoside is formed when a nitrogenous base is attached to a sugar, and when this nucleoside is attached to a phosphate group, it becomes a nucleotide. The nucleotides are connected through phosphodiester bonds, forming a long chain that creates the backbone of the nucleic acid molecule.

Think of nucleotides like beads on a necklace, where each bead is unique (representing the different bases) and connected by a string (the phosphodiester bond) that keeps them in order to form the DNA or RNA chain.

Information regarding the sequence of nucleotides in the chain of a nucleic acid is called its primary structure. Nucleic acids have a secondary structure also. James Watson and Francis Crick gave a double strand helix structure for DNA. Two nucleic acid chains are wound about each other and held together by hydrogen bonds between pairs of bases.

The primary structure of nucleic acids describes the linear sequence of nucleotides, which determines the genetic information encoded within the molecule. In contrast, the secondary structure refers to the three-dimensional shape that the molecule forms due to interactions between nucleotides. For DNA, this structure is a double helix formed by two strands of nucleotides twisted around each other with specific base pairing (A with T, and C with G) stabilizing the helix.

Visualize primary structure as a sentence made up of letters (nucleotides) that convey a specific message. When those letters are arranged in a certain way (together forming a double helix), they become a beautifully structured story (the DNA molecule) that can be read and interpreted.

In the secondary structure of RNA, single stranded helics are present which sometimes fold back on themselves. RNA molecules are of three types and they perform different functions: messenger RNA (m-RNA), ribosomal RNA (r-RNA), and transfer RNA (t-RNA).

RNA serves several critical roles in the biological processes of the cell. There are three main types of RNA: mRNA, which carries the genetic information from the DNA to the ribosomes for protein synthesis; rRNA, which is a structural component of ribosomes and helps in protein assembly; and tRNA, which transports amino acids to the ribosome for protein synthesis. Each type plays a specific role in translating the genetic code into functional proteins.

You can think of mRNA as the messenger delivering a vital message, while ribosomal RNA is like the factory where the products are made, and transfer RNA acts like delivery trucks that bring the raw materials (amino acids) needed for production.

DNA is the chemical basis of heredity and may be regarded as the reserve of genetic information. DNA is exclusively responsible for maintaining the identity of different species of organisms over millions of years. Another important function of nucleic acids is the protein synthesis in the cell.

The primary function of DNA is to store and transmit genetic information necessary for the development, functioning, and reproduction of all living organisms. Additionally, RNA plays a critical role in the process of protein synthesis, where it helps translate the genetic code into proteins that carry out the functions of the cell.

Imagine DNA as a long library filled with books (genetic information) that tell the story of a living organism, while RNA serves as the librarian that helps interpret the information from the books to create the final story, which are proteins.

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