Dosage Compensation Mechanism

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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] The result is equal transccription of the X chromosome genes in males and females.[5]

In nematode worms, there are two sexes - male and hermaphrodite. Males possess one X chromosome, whereas hermaphrodites contain two X chromosomes. Dosage compensation is brought about through the two-fold decrease in transcirption in each of the X chromosomes in the hermaphrodite. A dosage compensation complex forms on each X chromosome, dissimilar to that in Drosophila, and results in chromosome changes and decreased X chromosome transcription. The exact mechanism is unknown.[6][7]

References

  1. Lodish, et al. 2008. (Molecular Cell Biology) 6th edition. pg 959 Fig22-7
  2. Hartl.D.L, Ruvolo.M, Genetics: analysis of genes and genomes, eighth edition, Jones and Bartlett Learning, 2012 page 262
  3. Whitehall Dr S.K, Genetics Lecture 13, BGM1004 lecture, accessed 27/10/14,
  4. 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. 475
  5. 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.
  6. 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.
  7. 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. 475-476
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