A DNA molecule is a polynucleotide (poly means many). It is made of units called nucleotides, each of which contains a nitrogenous base, a deoxy ribose sugar, and a phosphate.
There are four nitrogenous bases Adenine, Guanine, Thymine and Cytosine and hence four kinds of nucleotides in a DNA molecule. The various combinations of these nucleotides in a segment of DNA form the different genes.
In physical structure, a DNA molecule is a double helix containing two polynucleotide strands.
Cell division takes place in a manner so that one cell divides into two identical cells with the same number of identical chromosomes. Therefore, prior to cell division, every chromosome should contain two chromatids made of two identical DNA molecules. This is achieved through the process of DNA duplication or DNA replication.
The major steps of DNA duplication are:
The double stranded DNA molecule unwinds with the help of certain enzymes to expose two strands of DNA.
A DNA polymerase enzyme catalyzes the formation of a new daughter strand which can form a double helix with one strand of parental DNA molecule. So two DNA molecules, each with a parental strand and a new strand get generated.
The two identical DNA molecules then become two chromatids which remain attached by a centromere.
Upon DNA replication, each chromosome contains two identical molecules of DNA housed in its two chromatids. During cell division, the two chromatids, separate out as two chromosomes, one each, passing into the two daughter cells.
DNA of an individual is the same in each and every cell of the body and also resembles the DNA of parents. Just like the fingerprint, DNA of every individual is unique and even if a hair or drop of blood or semen of the criminal is left at the site of the crime, it can be used to detect the DNA of the criminal and compared with that of the suspect to ascertain the truth.