The complete set of instructions for making an organism is called its genome. It contains the master blueprint of all cellular structures and activities for the life time of the cell or organism. Found in every nucleus of a persons many trillions of cells, the human genome consists of tightly coiled threads of deoxyribonucleic acid (DNA) and associated protein molecules, organized into structures called chromosomes.In humans, as in other higher organisms, a DNA molecule consists of two strands that wrap around each other to resemble a twisted ladder whose sides are made up of sugar and phosphate molecules. They are connected by rungs of nitrogen containing chemical called bases. Each strand is a linear arrangement of repeating similar units called nucleotides, which are composed of one sugar, one phosphate, and a nitrogenous base. There are four different bases present in DNA, adenine (A), thymine (T), cytosine (C), and guanine (G). The particular order of the bases arranged along the sugar phosphate backbone is called DNA sequence. The sequence specifies the exact genetic instructions required to create a particular organism with its own unique traits.
Each time a cell divides into two daughter cells, its full genome is duplicated; for humans and other complex organisms, this duplication occurs in the nucleus. During this cell division the DNA molecule unwinds and the weak bonds that are between the weak base pairs break, allowing the strands to separate. Each daughter cell receives one old and one new DNA strand.
Genes
Each DNA molecule contains many genes, the basic physical and functional units of heredity. A gene is a specific sequence of nucleotide bases, whose sequences carry the information required for constructing proteins. They provide the structural components of cells and tissues as well as enzymes for essential biochemical reactions. The human genome consists of 100,000 genes.
Human genes vary widely in length and often extending over thousands of bases, but only about 10% of genome is known to include the protein- coding sequences of genes. All living organisms are composed largely of proteins; humans can synthesize at least 100,000 different kinds. Proteins are large and complex molecules made up of long chains of sub units called amino acids. Twenty two different kinds of amino acids are usually found in proteins. Within each gene specific sequence of three DNA bases directs the cells protein -synthesizing machinery to add specific amino acids.
Chromosomes
The 3 billion blue prints in the human genome are organized into 24 distinct, physically separate microscopic units called chromosomes. All the genes are arranged linearly along the chromosomes. The nucleus of most human cells contains 2 sets of chromosomes, 1 given by each parents. Each set has 23 single chromosomes, 22 autosomes and an X and Y sex chromosomes.
Chromosomes can be seen under a light microscope and when stained with certain dyes, reveal a pattern of light and dark bands reflecting regional variations in the amounts of A and T vs. C and G. Differences is size and banding pattern allow the 24 chromosomes to be distinguished from each other, an analysis called a karyotype. A few types of major chromosomal abnormalities, including missing or extra copies of a chromosome or gross breaks and rejoining, can be detected by microscopic examinations. Most changes in DNA are too subtle to be detected by this technique and require molecular analysis. These subtle DNA abnormalities are responsible for many inherited diseases such as cystic fibrosis and sickle cell anemia or may predispose an individual to cancer and other complex diseases.