Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information. DNA is often compared to a set of blueprints or a recipe, or a code, since it contains the instructions needed to construct other components of cells, such as proteins and RNA molecules. The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information.

Chemically, DNA consists of two long polymers of simple units called nucleotides, with backbones made of sugars and phosphate groups joined by ester bonds. These two strands run in opposite directions to each other and are therefore anti-parallel. Attached to each sugar is one of four types of molecules called bases. It is the sequence of these four bases along the backbone that encodes information. This information is read using the genetic code, which specifies the sequence of the amino acids within proteins. The code is read by copying stretches of DNA into the related nucleic acid RNA, in a process called transcription.

Within cells, DNA is organized into structures called chromosomes. These chromosomes are duplicated before cells divide, in a process called DNA replication. Eukaryotic organisms (animals, plants, fungi, and protists) store their DNA inside the cell nucleus, while in prokaryotes (bacteria and archaea) it is found in the cell's cytoplasm. Within the chromosomes, chromatin proteins such as histones compact and organize DNA. These compact structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed.

Sometime around 6000 BCE a nomadic herding people settled into villages in the Mountainous region just west of the Indus River. There they grew barley and wheat using sickles with flint blades, and they lived in small houses built with adobe bricks. After 5000 BCE the climate in their region changed, bringing more rainfall, and apparently they were able to grow more food, for they grew in population. They began domesticating sheep, goats and cows and then water buffalo. Then after 4000 BCE they began to trade beads and shells with distant areas in central Asia and areas west of the Khyber Pass. And they began using bronze and working metals.

The climate changed again, bringing still more rainfall, and on the nearby plains, through which ran the Indus River, grew jungles inhabited by crocodiles, rhinoceros, tigers, buffalo and elephants. By around 2600 BCE, a civilization as grand as that in Mesopotamia and Egypt had begun on the Indus Plain and surrounding areas. By 2300 BCE this civilization had reached maturity and was trading with Mesopotamia. Seventy or more cities had been built, some of them upon buried old towns. There were cities from the foothills of the Himalayan Mountains to Malwan in the south. There was the city of Alamgirpur in the east and Sutkagen Dor by the Arabian Sea in the west.

One of these cities was Mohenjo-daro (Mohenjodaro), on the Indus river some 250 miles north of the Arabian Sea, and another city was Harappa, 350 miles to the north on a tributary river, the Ravi. Each of these two cities had populations as high as around 40,000. Each was constructed with manufactured, standardized, baked bricks. Shops lined the main streets of Mohenjo-daro and Harappa, and each city had a grand marketplace. Some houses were spacious and with a large enclosed yard. Each house was connected to a covered drainage system that was more sanitary than what had been created in West Asia. And Mohenjo-daro had a building with an underground furnace (a hypocaust) and dressing rooms, suggesting bathing was done in heated pools, as in modern day Hindu temples.

The people of Mohenjo-daro and Harappa shared a sophisticated system of weights and measures, using an arithmetic with decimals. Whether these written symbols were a part of a full-blown written language is a matter of controversy among scholars, some scholars pointing out that this and the brevity of grave site inscriptions and symbols on ritual objects are not evidence of a fully developed written language.

The people of Mohenjo-daro and Harappa mass-produced pottery with fine geometric designs as decoration, and they made figurines sensitively depicting their attitudes. They grew wheat, rice, mustard and sesame seeds, dates and cotton. And they had dogs, cats, camels, sheep, pigs, goats, water buffaloes, elephants and chickens.