Chromosome

Chromosome: a very long DNA molecule and associated proteins, that carry portions of the hereditary information of an organism.
    a. Structure of a chromosome (Typical metaphase chromosome): 
Achromosome is formed from a single DNA molecule that contains many genes. A chromosomal DNA molecule contains three specific nucleotide sequences which are required for replication: a DNA replicationorigin; a centromere to attach the DNA to the mitotic spindle.; a telomerelocated at each end of the linear chromosome. 

The DNA molecule is highly condensed. The human DNA helix occupy too muchspace in the cell. Small proteins are responsible for packing the DNA into units called nucleosomes.
    b. Stained chromosomes: 
Chromosomes are stained with A-T (G bands) and G-C (R bands) basepair specific dyes. 
When they are stained, the mitotic chromosomes have a banded structure that unambiguously identifies each chromosome of a karyotype. Each band contains millions of DNA nucleotidepairs which do not correspond to any functional structure. 
Adapted from  K.F. Jorgenson, J.H. van de Sande, and C.C. Lin, Chromosoma 68:287-302, 1978.
    c. Karyotype of a male: 
The humanhaploid genome contains 3,000,000,000 DNA nucleotide pairs, divided among twenty two (22) pairs of autosomes and one pair of sex chromosomes.

In a series of experiments beginning in the mid-1880s,Theodor Boveri gave the definitive demonstration that chromosomes are the vectors of heredity. His two principles were the continuity of chromosomes and the individuality of chromosomes.[citationneeded] It is the second of these principles that was so original.[citation needed] Wilhelm Roux suggested that each chromosome carries a different genetic load. Boveri was able to test and confirm thishypothesis. Aided by the rediscovery at the start of the 1900s of Gregor Mendel's earlier work, Boveri was able to point out the connection between the rules of inheritance and the behaviour of the...