Dosage Compensation Mechanism
As human females have two X chromosomes, this means they have the ability to produce twice as much X-linked gene product, this Dosage Compensation Mechanism is responsible for keeping levels of the X-linked gene product similar in both males and females. Therefore, such a mechanism only exists in females. Dosage compensation requires RNA polymerase, Tsix transcript and Xist transcript as well as the X chromosomes of developing embryonic cells. Note that each transcript is transcribed on one chromosome each. Xist acts as to ensure inactivation of X chromosome whereas Tsix ensures X chromosome remains active. Such activity is carried out from the X inactivation centre [1]., also known as XIC. The XIC is located at the centromere and is where decativation of the X Chromosome begins[2] This mechanism follows the process of X-inactivation, also termed Lyonization or single active X principle, in which regardless of the amount of X chromosomes are present, all but one are inactivated. However, although one of the X chromosomes is said to be inactivated, there is still around 15% of the genes on the inactivated X chromosome which aren't inactivtaed. [3]
In other organisms
Dosage compensation in other sexually-reproducing organisms is achieved through other mechanisms. Drosophila transcribe X chromosome genes in males at twice the level of the female X chromosomes. This increase in transcription is caused by changes in chromatin over the male X chromosome. A dosage-compensation complex forms and brings about this up-regulation. It is associated with histone-modifiying enzymes and two non-coding RNAs transcribed from the male X chromosome.[4][5][6]
- ↑ Lodish, et al. 2008. (Molecular Cell Biology) 6th edition. pg 959 Fig22-7
- ↑ Hartl.D.L, Ruvolo.M, Genetics: analysis of genes and genomes, eighth edition, Jones and Bartlett Learning, 2012 page 262
- ↑ Whitehall Dr S.K, Genetics Lecture 13, BGM1004 lecture, accessed 27/10/14,
- ↑ Conrad, T. and Akhtar, A. (2012). Dosage compensation in Drosophila melanogaster: epigenetic fine-tuning of chromosome-wide transcription. Nat Rev Genet, 13(2), pp.123-134.
- ↑ Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K. and Walters, P. (2007) Molecular Biology of the Cell 5th ed., New York: Taylor & Francis. pp. 275fckLRfckLRIn the nematode worm, sexes are male and hermaphrodite. Males possess one X chromosome, and hermaphrodites contain two X chromosomes. Dosage compensation comes about through a reduction in transcription on each X chromosome in the hermaphrodite. This down-regulation is two fold and, as in the examples above, results in equal transcription in the two sexes. Here, a dosage-compensation complex, dissimilar to that in Drosophila, forms and each gene is effected. The exact mechanism is unknown.<ref>Ercan, S. and Lieb, J. (2009). C. elegans dosage compensation: A window into mechanisms of domain-scale gene regulation. Chromosome Research, 17(2), pp.215-227.
- ↑ Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K. and Walters, P. (2007) Molecular Biology of the Cell 5th ed., New York: Taylor & Francis. pp. 275-276fckLRfckLR=== References ===fckLRfckLR<references />