DNAs are the most important biochemical molecule of human body. After invention of the microcoscope it has been known that body of an organism is made up of cells. Cell of an organism consists of a small piece of information holding matter that contains the blueprint of formation of cell inside the body. In 1944, this small encyclopedia of cell is discovered as a chemical compound which is denoted as Deoxyribo Nucleic Acid (DNA). DNAs can be regarded as the instruction manual to build  life. DNAs are responsible for the formation of cells. It is a molecule that carries hereditary information from one generation to another. It is found in almost all organisms.  Nuclear DNA and mitochondrial DNA are two types of DNA. The DNA present in the cell nucleus is known  as nuclear DNA and the DNA found in the mictochondria is known as mitochondrail DNA  1.  DNA molecule is a combination of deoxyribose sugar, phosphate and nitrogenous bases. DNA stores information as a code in terms of chemical bases namely Adenine (A), Guanine (G), Cytosine (C) and Thymine (T) 5. The DNA molecule of higher organisms consists of two strands. These strands are linear arrangement of some similar units known as nucleotides. Nucleotides are composed  of one  sugar, one phosphate and a nitrogenous base  5. Strand  is wrapping around each other like a twisted ladder. Sides of these strands are made up of sugar and phosphate molecule. These are connected by bases which are nothing but nitrogen containing chemicals. Adenine (A), Thymine (T), Guanine (G) and Cytosine (C) are four different bases that comprises of DNA molecule. By forming base pairs the two strands are held together. There is a fixed set of rules for base pairing. The two rules are—Adenine always pairs with Thymine (A-T) and Guanine always pairs with Cytosine (G-C). The total number of base pair is known as Genome size. The human genome contains roughly 3 billion bp. The particular order of the bases arranged along the sugarphosphate backbone is called the DNA sequence. In other words a DNA sequence can be described as a very long word over a four letter alphabet A = A, C, G, T, a letter  for every base. Various order or sequence of these bases is responsible for building up of different organisms. The process of determining the precise order or sequence of these chemical bases is known as DNA sequencing 2. This method is extensively used  in the field of medical biology for diagnosis of various diseases, in forensic biology especially in crime forensic, in molecular biology for identification of genes and in evolutionary biology for determination of relationship between different organisms  6. With a small piece of DNA we should be able to know the physical appearance of a person which helps in crime forensics. Since DNA carries traits from one generation to another so in case of medical diagnosis from a small piece of 

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