DNA - definition and structure

What is DNA? 

• These instructions are found inside every cell and are passed down from parents to their children.

• It is a nucleic acid and is one of the four major types of macromolecules that are known to be essential for all forms of life.

• DNA is found in the nucleus, with a small amount of DNA also present in mitochondria in the eukaryotes.

Structure of DNA

The genetic material in most organisms is DNA or Deoxyribonucleic acid; whereas in some viruses, it is RNA or Ribonucleic acid. A DNA molecule consists of two polynucleotide chains i.e. chains with multiple nucleotides. Let’s understand the structure of this chain in detail.

DNA and Its occurence:-

The DNA molecule is composed of units called nucleotides, and each nucleotide is composed of three different components, such as sugar, phosphate groups, and nitrogen bases. 

Structure Of Polynucleotide Chain

A nucleotide is made of the following components:

Pentose sugar – A pentose sugar is a 5-carbon sugar. In case of DNA, this sugar is deoxyribose whereas, in RNA, it is ribose.

Phosphate group-

Nitrogenous base – These can be of two types – Purines and Pyrimidines. Purines include Adenine and Guanine whereas pyrimidines include Cytosine and Thymine. In RNA, thymine is replaced by Uracil.

Nitrogenous base + pentose sugar (via N-glycosidic linkage) = Nucleoside.

Nucleoside + phosphate group (via phosphoester linkage) = Nucleotide.

Nucleotide + Nucleotide (via 3′-5′ phosphodiester linkage) = Dinucleotide.

Many nucleotides linked together = Polynucleotide.

A polynucleotide has a free phosphate group at the 5′ end of the sugar and this is called the 5′ end. Similarly, the sugar also has a free 3′-OH group at the other end of the polynucleotide which is called the 3′ end. The backbone of a polynucleotide chain consists of pentose sugars and phosphate groups; whereas the nitrogenous bases project out of this backbone.

The DNA molecule consists of 4 nitrogen bases, namely adenine (A), thymine (T), cytosine (C) and Guanine (G) which ultimately forms the structure of a nucleotide. The A and G are purines and the C and T are pyrimidines.

The two strands of DNA run in opposite directions. These strands are held together by the hydrogen bond that is present between the two complementary bases. The strands are helically twisted, where each strand forms a right-handed coil and ten nucleotides make up a single turn.

The pitch of each helix is 3.4 nm. Hence, the distance between two consecutive base pairs (i.e., hydrogen-bonded bases of the opposite strands) is 0.34 nm.

Who Discovered DNA?

DNA was first recognized and identified by the Swiss biologist, Johannes Friedrich Miescher in 1869 during his research on white blood cells.

The double helix structure of a DNA molecule was later discovered through the experimental data by James Watson and Francis Crick. Finally, it was proved that DNA is responsible for storing the genetic information in living organisms.

Functions of DNA:

1. Genetic Information (Genetic Blue Print):

DNA is the genetic material which car­ries all the hereditary information. The genetic information is coded in the arrangement of its nitrogen bases.

2. Replication:

DNA has unique property of replication or production of carbon copies (Autocatalytic function). This is essential for transfer of genetic information from one cell to its daughters and from one generation to the next.

3. Chromosomes:

DNA occurs inside chromosomes. This is essential for equitable distribution of DNA during cell division.

4. Recombination’s:

During meiosis, crossing over gives rise to new combination of genes called recombinations.

5. Mutations:

Changes in sequence of nitrogen bases due to addition, deletion or wrong replication give rise to mutations. Mutations are the fountain head of all variations and evolution.

6. Transcription:

DNA gives rise to RNAs through the process of transcription. It is heterocatalytic activity of DNA.

7. Cellular Metabolism:

It controls the metabolic reactions of the cells through the help of specific RNAs, synthesis of specific proteins, enzymes and hormones.