X Chromosome

‘The X chromosome: not just her brother's keeper - The X chromosome has traditionally been characterized as a conscientious sister to her derelict brother that is the Y. Beyond dutifully maintaining the family heritage, however, the X has developed its own unique identities. Now, the complete sequence of the human X allows us to appreciate its distinctiveness at an unprecedented resolution.’ Nature, 2005

‘In 2005, an international team led by the Wellcome Trust Sanger Institute published in the journal Nature an analysis of the remarkable X chromosome. The study shows how we got an X chromosome and how it has been preserved (while the Y chromosome has degenerated). It also identifies new genes involved in disease and provides a gold-standard platform for studies to understand, to diagnose and, it is hoped, to treat a huge range of human disease. The human X chromosome has a different biology to all others. Whereas females have two X chromosomes, males have only one X chromosome and a Y chromosome, which is an eroded version of the X chromosome, containing only a few genes. The consequences are dramatic; any defects in genes on the X chromosome are often apparent in males because the Y chromosome does not carry corresponding genes to compensate. These remarkable chromosomes evolved from humble beginnings as an 'ordinary' pair of identical chromosomes. It is thought that changes to a gene on one of the pair created the key switch in the pathway to male development and set in train the degeneration of this chromosome. As this emerging Y chromosome eroded, maintaining the integrity of the X chromosome was essential. When the integrity is compromised in human males, disease often results. Human chromosomes. A DNA ‘paint’ identifies sequences on the X chromosome (yellow) and a small region at the tip of the Y chromosome. The new study confirmed the existence of 1098 protein-coding genes on the X chromosome. Only 54 of these genes have functional counterparts on the much smaller Y chromosome, which has only 76 known genes altogether and has been described as an 'eroded' version of the X chromosome. More than 300 diseases have been mapped to the X chromosome - by far the highest proportion of any chromosome - including Duchenne muscular dystrophy and haemophilia. The genome sequence has been used in the isolation of more than 40 genes that are involved in medical conditions, including cleft palate and blindness. Interestingly, almost 10 per cent of the genes on the X chromosome are part of a somewhat mysterious family of 'cancer-testis antigens', which are normally expressed in the testis but also appear in certain cancers, making them possible targets for immunotherapy. "The importance of the sequence goes beyond the biology of individual genes," says Dr Mark Ross, Project Leader at the Wellcome Trust Sanger Institute. "We have also gained a deep insight into the evolution and biology of the whole chromosome. We can see the way evolution has shaped the chromosomes that determine our gender to give them their unique properties”.’ Wellcome Trust, 2005

X Marks the Star Chromosome

X is the star chromosome, marking fundamental difference;

biological treasure among creatures of Earth - preservation

of that star-root, inculcation of light into organic expression,

enshrinement; what ailment afflicted the pair, kissing letters,

turning union into opposition, competition, X into guardian.

What blow or chance for the sexless species in the garden -

mistaken advantage; what curtailment began of muscular Y -

culture of flowers, romance, candlelight to reconcile, sweeten

the two who were once the same; dancing partners, helical

lovers - coaxing togetherness from biological antagonism -

physical difference. X is the home of genetic productivity,

Y clutching at the essential, eroded; fumbling in existence.

She needs him for now, starry X, triggering her processes;

but always she is moving back to completeness, harmony.

Note from the author
exploring the project

    Gene Story
    Romantic Science
    Some Special Genes
    X & Y
        Y Chromosome
        SRY Gene – Master Switch
        Sex Wars
        X Chromosome
        Some notes on the
        Gender of Science

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