WHAT do left or right-handedness, schizophrenia and the asymmetry of our organs have in common? They could all be linked to the way chromosomes are apportioned between the cells in our body.
Textbooks say chromosomes duplicate prior to cell division, forming two identical sister chromatids. They are pulled apart when the cell divides in such a way that each daughter cell receives a random mixture of the newly synthesised chromosomes and the old chromosomes that acted as templates for the copies (see Diagram).
Not so, say Athanasios Armakolas and Amar Klar of the US National Cancer Institute at Frederick, Maryland. They have shown in lab experiments with mouse cells that the distribution of at least one of the chromosomes, chromosome 7, can be non-random. They found that embryonic cells destined to become heart and pancreas tissue do share a random distribution of old and new copies of the chromosome. However, distribution was non-random in embryonic cells destined to form the intestine and liver, which come from the 鈥渆ndoderm鈥� layer at the centre of the embryo, and also in those that will become nerve cells, found in the outermost 鈥渆ctoderm鈥� layer, (Science, vol 311, p 1146).
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This observation could have several implications. Although daughter cells will remain genetically identical, differences will accumulate in the physical structure of the DNA packages, and affect how the genes within them are expressed. Such 鈥渆pigenetic鈥� differences, say the researchers, may affect development (for more on epigenetics, see 鈥淚nheriting a heresy鈥�).
鈥淥n the surface it looks like a silly idea,鈥� says Klar. But it is already known that something similar happens in yeast cells. Sister cells of yeast, which have identical chromosomes, can be sent on different developmental pathways depending on which strands of DNA they inherit.
The same may happen in people, says Klar. Until now, it was thought that genes alone controlled the development of the embryo. But if chromosomes are distributed in a non-random way, it could provide a mechanism that affects future development of all our cells, ultimately producing an asymmetrical arrangement of our organs 鈥� with our heart on the left and our liver on the right, for example. This asymmetry develops at an early stage, in the embryo.
鈥淚f chromosomes are distributed non-randomly, it could provide a mechanism that affects the future development of our cells鈥�
Klar suggests the same process might initiate structural differences between the left and right hemispheres in the brain, which control left or right-handedness. Such structural differences have been implicated in the development of psychosis. Large parts of mouse chromosome 7 and human chromosome 11 have similar genetic sequences, and human chromosome 11 is known to contribute to brain laterality. If chromosome 11 segregates non-randomly in people, as chromosome 7 appears to in mice, this might explain some lateral differences in the brain, Klar says.
鈥淚f there is asymmetry in DNA inheritance, this might offer alternative explanations for the differentiation of cell types,鈥� agrees Phillip Karpowicz, a geneticist at the University of Toronto, Canada. However, he warns that Armakolas and Klar have only observed differences in how chromosomes are inherited, and haven鈥檛 yet done experiments to support their ideas about what this may mean.
