Chromosome

A chromosome is an organized package of DNA found in the nucleus of the cell. Different organisms have different numbers of chromosomes. Humans have 23 pairs of chromosomes--22 pairs of numbered chromosomes, called autosomes, and one pair of sex chromosomes, X and Y. Each parent contributes one chromosome to each pair so that offspring get half of their chromosomes from their mother and half from their father. 
For most of the life of the cell, chromosomes are too elongated and tenuous to be seen under a microscope.Before a cell gets ready to divide by mitosis, each chromosome is duplicated (during S phase of the cell cycle).As mitosis begins, the duplicated chromosomes condense into short (~ 5 µm) structures which can be stained and easily observed under the light microscope.These duplicated chromosomes are called dadys.
When first seen, the duplicates are held together at their centromeres. In humans, the centromeres contains ~1 million base pairs of DNA. Most of this is repetitive DNA: short sequences (e.g., 171 bp) repeated over and over in tandem arrays.While they are still attached, it is common to call the duplicated chromosomes sister chromatid, but this should not obscure the fact that each is a bona fide chromosome with a full complement of genes.The kinetochore is a complex of proteins that forms at each centromere and serves as the attachment point for the spindle fiber that will separate the sister chromatids as mitosis proceeds into anaphase.The shorter of the two arms extending from the centromere is called the p arm; the longer is the q arm.Staining with the trypsin-giemsa method reveals a series of alternating light and dark bands called Gband.G bands are numbered and provide "addresses" for the assignment of gene loci.