DNA is a very thin, string-like object which is present in each body cell. Each body cell contains a complete sample of DNA. In a person, the DNA sample in every cell is identical with the other. DNA is present in most of the body cell from our muscles, brain, liver, sperm etc. There are some exceptions though; for example, DNA is not present in the red blood cells.
A single DNA strand is made up of tiny bases or building blocks, which are usually referred using four simple letters – A, T, G and C. Specific DNA pieces are described as a sequence of these base letters, for example, AATTGCCTTTTAAAAA and on the other hand, a DNA synthesizer can easily produce the same piece of DNA from the information code AATTGCCTTTTAAAAA.
Some of the DNA sequences encode essential information for the cell which are called ‘coding DNA’. Non-encoding DNA is very often referred to as ‘junk DNA’.
The DNA code, which is also called the genetic code, is passed through the sperm and egg to the progeny.
A locus (plural of ‘loci’) is a location in DNA. Such locations or loci are found at specific area on chromosomes. When a body cell is prepared for cell division for production of two daughter cells, it packs its DNA into bundles known as ‘chromosomes’. There are 23 pairs of chromosomes in our body cells, which are of consistent sizes and shapes. Such chromosomes are numbered. The first chromosome pair is the largest and the second is smaller, and so on.
Among the 23 pairs of chromosomes, one pair is called ‘sex chromosome’, which is responsible for inheritance of different traits from parents to children. In female, the ‘sex chromosomes’ pair has got two ‘X’ chromosomes. Males have one ‘X’ and one small ‘Y’ chromosome.