Mouse Genome Much More Complex Than Expected

The genetic material of mammals, the DNA, can be compared to anenormous encyclopedia containing the complete blueprint of the animalin question. Yet this comparison is misleading: over the past few yearsit has been realised that on most pages of the encyclopedia there is noinformation at all: they contain a chaotic sequence of letters. Mixedin among these pages there are intelligible pages from time to time,the genes.

The DNA encyclopedia is stored in the nuclei of the cells. Ifthe body is to produce a specific protein, the appropriate page of theencyclopedia is copied (rewritten or 'transcribed'). Only the copiescan leave the cell nucleus. They consist of a DNA-like material knownas mRNA. Each mRNA contains the blueprint for precisely one specificprotein -- this at least has been the traditional doctrine.

Three years ago the DNA of the mouse was completely sequenced.An international research team consisting of more than 100 scientistshas been attempting since then to isolate and analyse the entire mRNAtranscripts in the mouse. Their most astonishing finding is that morethan 60 per cent of all mRNAs are not protein blueprints at all. 'Wedon't know what the function of these RNAs is,' the Bonn neurobiologistProfessor Andreas Zimmer admits. However, they seem to be extremelyimportant: even in such different organisms as hens and mice theseostensibly so unimportant RNAs are very similar. If they really had nofunction they would have mutated during the course of evolution soquickly that there would nowadays be hardly any similarity betweenthem.

The scientists came across an additional interesting phenomenonwhen they tried to find the 'original sources' of the mRNA copies inthe DNA encyclopedia: information and nonsense are apparently notdistributed randomly. Instead there are entire chapters with manydifferent protein blueprints, which are separated by long passagesdevoid of meaning -- Professor Zimmer talks of transcription 'forests'and 'deserts'.

Although the DNA encyclopedia only has a few tens of thousandsof intelligible 'pages', the researchers counted more than 180,000different mRNAs. 'The genetic information is arranged on the DNA in avery complex way,' Professor Zimmer concludes. For example, the'copiers' in the cell nucleus combine the different 'paragraphs' in theDNA encyclopedia with each other in different ways. Thus there may beseveral different mRNA copies derived from the same page, which in turnserve as a blueprint for different proteins.

Only twice as many genes as a threadworm?

This observation might also explain the big question why mammalsonly have about twice as many genes as threadworms, which have a muchsimpler structure. 'Our study calls into question the classic view thatone gene contains the information for exactly one protein,' ProfessorZimmer explains. 'Mammals frequently use the selfsame place on the DNAseveral times over, as a partial blueprint, so to speak, for differentproteins. It is becoming more and more evident that mammal genes do nothave any clearly defined limits.'