Mouse genome much more complex than expected

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Mouse genome much more complex than expected

http://www.medicalnewstoday.com/medicalnews.php?newsid=30050

03 Sep 2005

More than 100 scientists from Australia, Asia, Europe and the US have

been probing the genome of the mouse in a joint study lasting several

years. Their results in some aspects have completely overturned

geneticists' traditional assumptions. The findings are available in

the prestigious journal Science on 2nd September. The general

conclusion of the study is that the genome of mammals is much more

complex than was hitherto supposed.

The genetic material of mammals, the DNA, can be compared to an

enormous encyclopedia containing the complete blueprint of the animal

in question. Yet this comparison is misleading: over the past few

years it has been realised that on most pages of the encyclopedia

there is no information at all: they contain a chaotic sequence of

letters. Mixed in among these pages there are intelligible pages from

time to time, the genes.

The DNA encyclopedia is stored in the nuclei of the cells. If the

body is to produce a specific protein, the appropriate page of the

encyclopedia is copied (rewritten or 'transcribed'). Only the copies

can leave the cell nucleus. They consist of a DNA-like material known

as mRNA. Each mRNA contains the blueprint for precisely one specific

protein - 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 scientists

has been attempting since then to isolate and analyse the entire mRNA

transcripts in the mouse. Their most astonishing finding is that more

than 60 per cent of all mRNAs are not protein blueprints at all. 'We

don't know what the function of these RNAs is,' the Bonn

neurobiologist Professor s Zimmer admits. However, they seem to

be extremely important: even in such different organisms as hens and

mice these ostensibly so unimportant RNAs are very similar. If they

really had no function they would have mutated during the course of

evolution so quickly that there would nowadays be hardly any

similarity between them.

The scientists came across an additional interesting phenomenon when

they tried to find the 'original sources' of the mRNA copies in the

DNA encyclopedia: information and nonsense are apparently not

distributed randomly. Instead there are entire chapters with many

different protein blueprints, which are separated by long passages

devoid of meaning - Professor Zimmer talks of transcription 'forests'

and 'deserts'.

Although the DNA encyclopedia only has a few tens of thousands of

intelligible 'pages', the researchers counted more than 180,000

different mRNAs. 'The genetic information is arranged on the DNA in a

very complex way,' Professor Zimmer concludes. For example,

the 'copiers' in the cell nucleus combine the different 'paragraphs'

in the DNA encyclopedia with each other in different ways. Thus there

may be several different mRNA copies derived from the same page,

which in turn serve as a blueprint for different proteins.

Only twice as many genes as a threadworm?

This observation might also explain the big question why mammals only

have about twice as many genes as threadworms, which have a much

simpler structure. 'Our study calls into question the classic view

that one gene contains the information for exactly one protein,'

Professor Zimmer explains. 'Mammals frequently use the selfsame place

on the DNA several times over, as a partial blueprint, so to speak,

for different proteins. It is becoming more and more evident that

mammal genes do not have any clearly defined limits.'

Professor s Zimmer

University of Bonn

http://www.uni-bonn.de