Correct option is B
The correct combination of statements is B, C, and E for the following reasons:
B: Both X-chromosomes are activated during the process of oogenesis.
This statement is true because in oogenesis (the formation of eggs), both X-chromosomes in females are initially active in the primary oocytes. Later, during embryonic development, one of the X chromosomes in each cell will be randomly inactivated in a process called X-inactivation.C: The XIST gene encodes for a single, long non-coding transcript, which binds with the X chromosome and helps in its inactivation.
This is also true. The XIST gene (X-inactive specific transcript) produces a long non-coding RNA that binds to the X chromosome and is crucial in initiating and maintaining X-inactivation. This process ensures that only one X chromosome in each cell remains active, while the other is silenced.E: Tsix transcription affects the abundance of Xist RNA in cis.
This statement is true. Tsix is an antisense transcript of XIST. The transcription of Tsix affects the levels of XIST RNA, ensuring that XIST RNA is not produced from the active X chromosome. Tsix plays a regulatory role in controlling X-inactivation by reducing XIST levels on the active X chromosome, thereby maintaining the proper inactivation pattern.
Information Booster
Oogenesis and X-chromosome activation (B): During early stages of oogenesis, both X-chromosomes in females are initially active. However, in the zygote after fertilization, one of the X chromosomes is randomly inactivated in each cell, which is crucial to balance gene dosage between males (XY) and females (XX).
XIST and X-chromosome inactivation (C): The XIST gene is critical for X-inactivation. It produces the XIST RNA, which coats the X chromosome, leading to the silencing of most of its genes. This is a central aspect of dosage compensation in female mammals, ensuring that one X chromosome is inactivated in each cell.
Tsix and XIST regulation (E): The Tsix transcript works as an antisense RNA, meaning it binds to the XIST RNA and prevents its expression from the active X chromosome. This regulatory feedback is crucial for the proper balance of X-inactivation and ensures that only one X chromosome is silenced in females.
Additional Information
(A) Maternally-derived X-chromosome has a greater chance of becoming inactivated in any given cell:
This statement is not fully accurate because, in X-inactivation, the process is random. While there might be a slight bias toward the maternal X chromosome being inactivated in some cells, it’s largely a random process. Thus, this is not a fundamental rule, and hence it is not correct for the given combination.(D) The XIST gene expression is required to initiate inactivation of X-chromosome and also to maintain inactivation from one cell generation to the next:
While this statement is partially true, it doesn't fully explain the complexity of the process and the regulatory mechanisms like Tsix. The expression of XIST is essential, but the statement is incomplete in explaining the entire process of X-inactivation, including the role of Tsix and other regulatory elements.
