Biology Reference
In-Depth Information
Figure 2.3
The male human karyotype pictured contains 46 chromosomes, 22 autosomes and the
X and Y sex chromosomes - the female karyotype has two X chromosomes. The chromosomes have
been labelled with fluorescent probes allowing them to be identified. (Image provided by Duncan
Holdsworth, Westlakes Research Institute, University of Central Lancashire, UK)
Each chromosome contains one continuous strand of DNA, the largest - chromo-
some 1 - is approximately 250 000 000 bp long whereas the smallest - chromosome
22 - is approximately 50 000 000 bp [3 - 5]. In physical terms the chromosomes range
in length from 73 mm to 14 mm. The chromosomes shown in Figure 2.3 are in
the metaphase stage of the cell cycle and are highly condensed - when the cell
is not undergoing division the chromosomes are less highly ordered and are more
diffuse within the nucleus. To achieve the highly ordered chromosome structure, the
DNA molecule is associated with histone proteins, which help the packaging and
organization of the DNA into the ordered chromosome structure.
The structure of the human genome
Great advances have been made in our understanding of the human genome in
recent years, in particular through the work of the Human Genome Project, which
was officially started in 1990 with the central aim of decoding the entire genome.
It involved a collaborative effort involving 20 centres in China, France, Germany,
Great Britain, Japan and the United States. Draft sequences were produced in 2001,
