DNA
By Krishna Bhikadiya | 21 August 2024
DNA (Deoxyribonucleic acid) is a special molecule that carries genetic instructions for life. One can think of DNA as an instruction manual for building and running living organisms. It directs the synthesis of proteins and regulates various biological processes. It is present inside the nucleus of the cell (eukaryotic) or in the cytoplasm (prokaryotic), and is responsible for transmission of inherent characters from parents to progeny (heredity).
What is DNA Made Up Of?
DNA is a long chain polymer made up of small units call nucleotides. Hence, it is also called a polynucleotide. A nucleotide has two components - a backbone made from deoxyribose sugar and phosphate groups, and nitrogenous bases.
The nucleotides are joined together by covalent bonds called phosphodiester linkages between the sugar of one nucleotide and the phosphate of another - resulting in alternate sugar-phosphate backbond. The nucleotides contain nitrogenous bases, which in the case of DNA are adenine (A), thymine (T), guanine (G) and cytosine (C). This forms the primary structure of DNA. The sequence of bases is unique for a particular organism, and it is what makes us unique. The sequence of bases is also what stores genetic information, and the proteins encoded by the DNA.
Structure of DNA
In 1953, James Watson and Francis Crick proposed the famous double helix model based on X-ray diffraction data obtained by Rosalind Franklin and Maurice Wilkins. DNA does not usually exist as single strand or chain but instead as a pair of strands that are tightly held together. These two long strands coil around each other in the shape of a double helix.
The DNA double helix is stabilised primarily by two forces -- hydrogen bonds between nucleotides and base stacking interactions among aromatic nucleobases. The two strands are complementary to each other because hydrogen bonds are formed between specific bases only - adenine with thymine, and cytosine with guanine. Both strands of DNA store the same biological information. The DNA chain measures 2.2-2.6nm wide, and one nucleotide unit measures 0.33nm long.
A large part of DNA (more than 98%) in human is non-coding, meaning that these sections do not serve as patterns for protein synthesis. The genetic information carried by DNA is held in sequences of pieces of DNA called genes. These genes regulate protein synthesis resulting in traits of organisms.
DNA replicates itself before cell division into two so each cell gets a copy when they divide. It is due to this fundamental process that genetic inheritance takes place. Very often, mutations occur during DNA replication, leading to variations in the population.
Understanding DNA and genetic mechanisms allows scientists to study evolutionary relationships and trace the lineage of various species. DNA is very special because it serves as a foundation of life. Think of it as a set of recipes that tells your body how to make everything it needs, from your eye colour to how your cells work.
References:
- Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. The Structure and Function of DNA.