Human body has many organs like brain, heart, liver, skins, muscles, bones, blood vessels, nerves, etc. and all of these organs consist of cells. The total number of cells accounts for approximately 60 trillion. However, every cell is originated from a single cell called "Zygote" (fertilized egg cell).
Therefore, all the cells of our body come from the Zygote which is created by fertilization with our father's sperm cell and our mother's ovum (egg cell). The Zygote then cleavages to the stem cell which differentiates further into various tissue cells to form organs.
In the center of a cell, there is a nucleus which is surrounded by membrane. Within nucleus, there are pole-shaped materials called "chromosomes". Chromosomes can be divided into two types - autosomes, and sex chromosomes. In general, a human tissue cell contains 46 chromosomes. Among these 46 chromosomes, there are 22 different types of autosomes, each represents a set of two copies, and two "sex chromosomes". This gives 46 chromosomes in total. Female has two identical sex chromosomes named X-chromosomes, and male has two different sex chromosomes, namely, X- and Y- chromosomes. The following picture shows human male chromosomes.
Chromosome contains a molecule called DNA ( Deoxyribonucleic Acid ). DNA is a long chain molecule with approximately 2 meters long, and it is packed together with proteins called chromatin. DNA was found in 1887, and the structure was discovered by famous Watson and Crick in 1953 as "double helical chains". After this discovery, new development of modern molecular biology has started.
DNA is a long polymer made from repeating units called "nucleotides". The double helical chains consist of phosphate-sugar linkage with the base. There are four different kinds of bases - namely adenine (A), thymine (T), guanine (G), and cytosine (C). These four different bases play a very important role for genetic information. It has become clear that the four different bases act like "code" to create proteins by their combination.
The DNA which produces protein is called "gene". However, only a small fraction of the DNA sequence produces protein. For example, about 2 % of total DNA can produce the specific protein. The rest is called "non-coding DNA. Why there are so many non-coding DNA ? For a long time, the non-coding DNA has been called "junk DNA", but the recent study shows that the junk DNA also plays an important role. The term "genome" means entire DNA including both the genes and the non-coding DNA sequences.
The following picture illustrates schematic drawing of the double helix.
Among four different bases, combinations of A-T and C-G can create the linkage, and sugar is surrounding this layer. On the outer surface of the layer, phosphate groups are attached constructing the double helix structure.