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Human Chromosomes




Legend:

Representation of the 23 paired chromosomes of the human male.

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):
A chromosome 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 replication origin; a centromere to attach the DNA to the mitotic spindle.; a telomere located at each end of the linear chromosome.

The DNA molecule is highly condensed. The human DNA helix occupy too much space 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) base pair 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 nucleotide pairs 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 human haploid genome contains 3,000,000,000 DNA nucleotide pairs, divided among twenty two (22) pairs of autosomes and one pair of sex chromosomes.


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