Cells are the fundamental units of living organisms. Those that function similarly make up the tissue, and a collection of tissues working together forms an organ. A collection of organs makes up the organism. The codes or instructions as to how each cell should function is contained in the nucleus of the cells, as DNA. Units of DNA form genes and the collection of all genes is called the genome.

DNA is systematically packed in different numbers of chromosomes in different species. Human beings have 23 that are found in almost every cell, and one sex chromosome is found in gametes or cells responsible for reproduction. All the cells initially begin as embryonic stem cells in the young embryo.

The unique feature of these stem cells is that they have the flexibility to become any type of cell. Though the genome is identical in every cell, studies of genes tell us that the activation of a different combination of genes induces the stem cell to specialize in a particular way, forming an organism with different types of cells.

From the second half of the 20th century, there have been astounding breakthroughs in the fields of genetics, genetic engineering and biotechnology. The discovery of DNA structure in 1953 by Watson and Crick has since led to significant advancement in these fields, namely the synthesis of genes, discovery of restriction enzymes, cloning of animals, sequencing the genome of organisms and finally the human genome project, which began in 1997 and concluded in 2003.

The history of modern genetics started from the garden of the Austrian monk Gregor Mendel (1882-1884). Patiently experimenting in the breeding of diverse kinds of pea plants, he demonstrated how their visible characteristics could be foreseen according to simple mathematical probabilities as they were passed on from one generation to the next. He proposed that hereditary information was passed from parent to offspring in discrete packets, which he called “factors.”

Different factors were responsible for distinct aspects of a pea plant’s appearance, such as seed shape or flower color. However, the secret of genetic inheritance was unlocked in April 1953 when Watson and Crick proposed a double-helical structure of DNA. The era of molecular genetics in the field of biochemistry thus began.

Scientists, now knowing the molecular structure of the genetic molecule, could begin both to elucidate and manipulate its function. These new studies were, however, dependent on the discovery and use of the many enzymes that are able to modify or join existing DNA molecules, or to aid in the synthesis of new DNA molecules.

As methods of visualizing DNA were being developed in the 1950s, a new tool was discovered: restriction enzymes. Werner Arber identified the first restriction enzyme in 1968. Restriction enzymes are protein molecules that cut DNA chains into defined fragments. During an attack of an invading bacteriophage (virus that attacks/infects bacteria), the bacterium releases a so-called restriction enzyme that recognizes the DNA of the invading bacteriophage and cuts the DNA into pieces, thereby disabling it.

Simultaneously, the bacterium releases another enzyme that defends and protects its own DNA from being cut by the restriction enzyme. It seems that even microorganisms have some sort of built-in intelligent system. The restriction enzyme of a bacterium cuts the viral DNA of foreign origin to safeguard and preserve its own identity. What a beautiful system nature has! Life displays such unique qualities even at the level of microorganisms.

This article is an excerpt from the late Dr. T.D. Singh’s book Life, Matter and Their Interactions.