THE STRUCTURE OF THE HUMAN GENOME
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 (picture provided by David McDonald, Fred Hutchinson Cancer Research Center, Seattle and Tim Knight, University of Washington)
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 that 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. A draft sequence was produced in 2001 that covered 90% of the euchromatic DNA [3, 4], this was followed by later versions that described the sequence of 99% of the euchromatic DNA with an accuracy of 99.99% . The genome can be divided into different categories of DNA based on the structure and function of the sequence (Figure 2.4).
Coding and regulatory sequence
The regions of DNA that encode and regulate the synthesis of proteins are called genes; at the latest estimate the human genome contains only 20000-25000 genes and only