The key difference between random and imprinted X inactivation is that random X inactivation is the inactivation of a paternal or maternal X chromosome with equal probability during the time of gastrulation in the epiblast, while imprinted X inactivation is a nonrandom X inactivation of the paternally derived X chromosome in the extraembryonic tissues of mammals.
X inactivation is a process seen in female mammals. Generally, females have two X chromosomes. And, in female mammals, one X chromosome becomes inactive. In other words, one X chromosome becomes a transcriptionally inactive structure. Once this occurs, this particular X chromosome remains inactive throughout the lifetime of the cell and its descendants in the organism. Furthermore, the inactivated X chromosome condenses into a compact structure called a Barr body, and it is stably maintained in a silent state. Also, the X inactivation process depends on the control of two non-coding complementary RNAs. However, the X inactivation does not happen in males, who have only one X chromosome. Moreover, X inactivation can occur randomly or due to imprinting.
CONTENTS
1. Overview and Key Difference
2. What is Random X Inactivation
3. What is Imprinted X Inactivation
4. Similarities Between Random and Imprinted X Inactivation
5. Side by Side Comparison – Random vs Imprinted X Inactivation in Tabular Form
6. Summary
What is Random X Inactivation?
Random x inactivation is the normal X inactivation process in which both maternal and paternal X chromosomes have an equal probability to be inactivated. This takes place in placental mammals including humans during the time of gastrulation in the epiblast. The inactivated chromosome remains inactive throughout the lifetime of the cell and its descendants.
Figure 01: Random X inactivation
Moreover, this is a mechanism for achieving dosage compensation and was first observed in somatic cells of female mammals. X-inactivation centre (Xic) located on the X chromosome regulates the random X inactivation. Thus, due to random X inactivation, genes of the entire X chromosome become transcriptionally silenced.
What is Imprinted X Inactivation?
An offspring inherits two copies of genes from its parents. One copy comes from the mother and the other from the father. And, most of the time, both copies of the gene are functional or in the active form. However, in some instances, only one gene copy remains in its active form while the other is in the inactive form. Therefore, one copy is turned “on” while the other is turned “off”. And, this process is called genomic imprinting. Genomic imprinting primarily takes place during the initial stages of development of an organism and it does not obey the rules of traditional Mendelian inheritance. In extraembryonic lineages of mice, the paternally derived X is chosen to be inactivated. Hence, it is a type of nonrandom X inactivation. Here, imprinted X inactivation of the parental X chromosome takes place. All mouse cells undergo this imprinted X inactivation in 4-8 cell stage embryos.
What are the Similarities Between Random and Imprinted X Inactivation?
- Random and imprinted X inactivation are two forms of X inactivation occurring in female mammals.
- Both processes lead to transcriptional silencing of most genes on one of the two X-chromosomes.
- X-inactivation centre (Xic) located on the X chromosome regulates both random and imprinted X inactivation.
- The noncoding RNA Xist is required for the initiation of both random and imprinted X inactivation.
What is the Difference Between Random and Imprinted X Inactivation?
There are two types of X inactivation: imprinted and random. In random X inactivation, maternal and paternal X chromosomes have equal probability to be inactivated. In imprinted X inactivation, a paternal X chromosome is preferentially silenced in the placenta of eutherian mammals, as well as in all cells of earlier marsupial mammals. So, this is the key difference between random and imprinted X inactivation. Imprinted X inactivation is less complete and less stable than random X inactivation.
The below infographic summarizes the difference between random and imprinted X inactivation.
Summary – Random vs Imprinted X Inactivation
X inactivation is the turning off of one X chromosome. It can happen either by random inactivation or by imprinted X inactivation. Furthermore, it occurs in some female mammals. In random X inactivation, both maternal and paternal X chromosome has an equal probability to inactivate. In contrast, in imprinted X inactivation, primarily paternal derived X chromosome is chosen to be inactivated. Therefore, it is not a random process. Moreover, random X inactivation takes place during the time of implantation while imprinted X inactivation takes place in all cells, early in preimplantation development. Besides, random X inactivation occurs in embryonic lineages while imprinted X inactivation occurs in extraembryonic lineages. So, this is the summary of the difference between random and imprinted X inactivation.
Reference:
1. “X Chromosome: X Inactivation” Nature News, Nature Publishing Group, Available here.
2. “X-Inactivation.” Wikipedia, Wikimedia Foundation, 19 Mar. 2020, Available here.
Image Courtesy:
1. “Human X-Inactivation” By Lilymclaughlin01 (CC BY-SA 4.0) via Commons Wikimedia