The living organisms divide into two major groups: prokaryotes, which are single-celled organisms with no cell nucleus, and eukaryotes, which are higher level organisms, and their cells have nuclei.
Eukariotic cell nuclei contain chromosomes - the contiguous structures
in which DNA is stored. Every cell usually includes two copies of each
chromosome (excluding, in males, the sex X,Y
chromosomes). Sex cells include only one copy of each chromosome. The number of chromosomes varies
among species. Humans have 22 pairs of chromosomes, plus the
pair of sex
chromosomes. In humans, the size of the chromosomes varies between
bp.
With contemporary knowledge of the biochemical basis of
heredity, Mendel's abstract concept of a gene can be
redefined as a physical entity, in terms of actual matter. A gene is a specific sequence of nucleotide bases along a
chromosome, and most of the times it
carries information for constructing a protein. Genes are parts of the chromosomes.
In humans genes constitute approximately 
of the DNA, leaving 
of nongenic "junk DNA". The role of the latter is
as yet unknown, but it is speculated to be very important. Several
theories have been suggested, such as physically fixing the DNA in its compressed
position, preserving old genetic data, etc. The total number of genes in
humans is estimated to be between 30,000 and 80,000.