X chromosome

Introduction

155 million base pairs
In contrast to the Y chromosome, the X chromosome contains about 5% of the haploid genome and encodes house-keeping and specialized functions.
Genes such as Wnt-4 and DAX-1 necessary for initiation of female pathway ovary development
An early discovery (1961) was that in order to have correct levels of X chromosome gene/protein expression (gene dosage), females must "inactivate" a single copy of the X chromosome in each and every cell. The initiator of the X inactivation process was discovered (1991) to be regulated by a region on the inactivating X chromosome encoding an X inactive specific transcript (XIST), that acts as RNA and does not encode a protein.
The genetic content of the X chromosome has been strongly conserved between species because these genes have become adapted to working as a single dose - Ohno's law
X inactivation occurs randomly throughout the embryo, generating a mosaic of maternal and paternally derived X chromosome activity in all tissues and organs. This can be seen in the fur colour of tortoiseshell cats.

Links: Genital - Female Development | X Inactivation | Genetic abnormality locations chromosomes 21-XY

Human Chromosomes

Human Idiogram: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | X | Y

X Inactivation

Macaque Xi at interphase^[1]

The presence in females of 2 X chromosome raises the issue of gene dosage, in the case of mammals this is regulated by inactivating one of the X chromosomes. To balance expression with the autosomal chromosomes the dosage imbalance is then adjusted by doubling expression of X-linked genes in both sexes. In some other species compensation occurs by increasing the expression of X in males.

Links: X Inactivation

Fragile X

External Links

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Human Chromosomes: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | X | Y

Idiogram Chromosome Banding - The term refers to the light and dark pattern, seen after staining with a dye, of individual chromosomes identified in metaphase. It is only in meiosis and mitosis during metaphase that chromosomes can be easily identified, during the normal cell life (interphase) the chromosomes are unravelled and distributed within the nucleus in chromosome territories. A band is that part of a chromosome which is clearly distinguishable from nearby regions by appearing darker or brighter with one or more banding techniques.

Human Idiogram: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | X | Y

Genetic abnormality locations: 1-4 | 5-8 | 9-12 | 13-16 | 17-20 | 21-XY | sSMC

Links: Genetics | Abnormal Development - Genetic

Cite this page: Hill, M.A. (2024, April 16) Embryology X chromosome. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/X_chromosome

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Glossary Links

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Cite this page: Hill, M.A. (2024, April 16) Embryology X chromosome. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/X_chromosome

What Links Here?

↑ McLaughlin CR, Chadwick BP. Characterization of DXZ4 conservation in primates implies important functional roles for CTCF binding, array expression and tandem repeat organization on the X chromosome. Genome Biol. 2011 Apr 13;12(4):R37. PMID: 21489251 | Genome Biol.

[1] McLaughlin CR, Chadwick BP. Characterization of DXZ4 conservation in primates implies important functional roles for CTCF binding, array expression and tandem repeat organization on the X chromosome. Genome Biol. 2011 Apr 13;12(4):R37. PMID: 21489251 | Genome Biol.

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