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
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
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
Adapted from K.F. Jorgenson, J.H. van de Sande, and C.C. Lin, Chromosoma
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.