DNA : Pathway to Heredity

DNA (deoxyribonucleic acid) is a complex hereditary molecule that is found in all eukaryotes and prokaryotes, and even some viruses. It is the building block of life that inherits genetic material and passes it down from one generation to the next. Let us understand the DNA structure to gain a better perspective of its chemistry.

Features of DNA

The DNA is made up of two antiparallel strands that are complementary to each other. The two polynucleotide strands are composed of a sugar, a nitrogenous base and phosphate group. DNA was first discovered by Freidrich Meischer in 1869, when he isolated a new compound from the cell nucleus and called it nuclein. Later in 1953, James Watson and Francis Crick described the DNA structure as a double helix. This was a breakthrough in science that led to various advances in the understanding of heredity, DNA replication and other cellular activities.

Components of DNA

The DNA is composed of a deoxyribose sugar, a nitrogenous base and a phosphate group. The deoxyribose sugar and the nitrogenous bases make up the nucleoside. When the nucleoside attaches to a phosphate group, a nucleotide is formed. A nucleotide can be defined as the repetitive block out of which the DNA is made.

Deoxyribose sugar is a five-carbon sugar, and it is a monosaccharide (C5H10O4). It is named as deoxyribose because it is devoid of one hydroxyl group at the 2’ carbon as compared to a ribose sugar. The absence of this hydroxyl group gives more flexibility to the DNA molecule and easy transformation into a double helix.

The nitrogenous bases, also called nucleobases, are simple nitrogen-containing biological compounds. The five nucleobases found are Adenine (A), Cytosine (C), Guanine (G), Thymine (T) and Uracil (U). All these five nucleobases are heterocyclic six-membered rings. Only A, T, C and G are found in a DNA strand. Uracil is replaced by Thymine in RNA strands. Adenine and Guanine have a double fused ring structure and are hence classified as purine bases.

Cytosine, Thymine and Uracil are single ringed structures and are hence referred to as pyrimidine bases. Each of these bases forms hydrogen bonds and stacks up in the double strands of DNA to give an appearance of ladder rungs. Adenine forms a double bond with Thymine, and Cytosine forms a triple bond with Guanine.

The repeating units of nucleotides are attached to each other by covalent bonds. The covalent bond is formed between the phosphate group of one nucleotide and the sugar group of another nucleotide. This gives a strong sugar-phosphate backbone to the DNA structure, out of which the nitrogenous bases project outwards. As described earlier, the two strands are held together by hydrogen bonds between the nitrogenous bases.

Structure of DNA

The structure of DNA can be seen in three different stages, wiz., primary, secondary and tertiary. The difference between DNA and RNA is quite notable. An RNA strand is usually single-stranded and has a Uracil base in the place of Thymine. All three structures of DNA are described below in detail.

  • Primary structure: The primary structure of DNA can be described as the sequence of nucleotides. The four nitrogenous bases A, T, C, and G attached with the sugar and phosphate group form a DNA sequence and can be identified as the primary structure of DNA.
  • Secondary structure: The secondary structure can be identified by how the two strands are connected to each other. The nitrogenous bases are bonded to each other on the complementary strands by weak hydrogen bonds. These hydrogen bonds are broken at the time of recombination, which brings about variation in organisms.
  • Tertiary structure: The tertiary structure of DNA is how the two strands are packed by histone proteins into a double helix. Histones are highly basic proteins that provide structural support to the chromosomes. Deacylation and acylation of the protein molecules bring about unwinding or winding of the DNA strands, respectively. This activity determines the level of transcription in the DNA molecule.

This was an overview of the features, structure and components of DNA. Stay tuned to BYJU’S Biology for more information.

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