X-chromosome (Molecular Biology)

In eukaryotes that utilize sexual reproduction, usually two distinct chromosomes exist, called sex chromosomes. In the majority of plants and animals, the male has one copy of each of these two chromosomes and is called the heterogametic sex. The two sex chromosomes (X and Y in humans) have partial homology. They pair during prophase of meiosis in the male, so that two kinds of gametes are produced, each containing just one of the two types of sex chromosomes. In the female, the homogametic sex, are two identical sex chromosomes (X in humans), so only one type of gamete results. The presence of different numbers of X-chromosomes in different cells, depending on the sex, relative to the constant number of autosomes requires that molecular mechanisms exist to compensate for the presence of two X-chromosomes in each cell of a female mammal. The process of dosage compensation leads to the inactivation of one of the X-chromosomes in humans (see X-Chromosome Inactivation). This process is generally random, in that either maternally or paternally derived X-chromosomes are inactivated (see Random X-Inactivation), and results in the appearance of a Barr body, consisting largely of facultative heterochromatin.

In Drosophila the male has a single copy of the X-chromosome, whereas the female is XX with two copies of the X-chromosome. However, the single X-chromosome in males produces the same amount of gene product as the combination of two X-chromosomes in females (1). The male X-chromosome is twice as transcriptionally active as the average female X-chromosome. This is accomplished by the targeting of an enzyme complex that hyperacetylates the histone proteins in the nucleosomes and chromatin of the male X-chromosome. Histone hyperacetylation promotes the processivity of RNA polymerase through nucleosomal arrays and along the chromatin fiber. Thus one gene in the single male X-chromosome in Drosophila is as active as the two copies of the gene in the female. An important aspect of the Drosophila dosage compensation pathway is the capacity to count the number of sex chromosomes relative to sets of autosomes. If a Drosophila egg has equal numbers of X-chromosomes and autosomes, it develops into a female. If there is an extra copy of autosomes relative to the number of X-chromosomes, it does not follow the female differentiation pathway (2).


In the nematode Caenorhabditis elegans, dosage compensation occurs in XX individuals that are hermaphrodites to normalize gene activity on the X-chromosome with males that are XO. This requires a reduction in transcription from the two X-chromosomes in hermaphrodites. A chromosomal modification is involved again, but unlike the case in Drosophila where gene activation was involved, in C. elegans hermaphrodites a specialized (stability and maintenance of chromosome SMC) chromosomal condensation protein is targeted to the X-chromosome to repress its gene activity twofold (3).

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