Q.29 In mammals, females have two X chromosomes and males have one X chromosome. Equal
expression of X–chromosome genes in both sexes is ensured by
(A) Dosage compensation (B) Histone code
(C) RNA silencing (D) Heterochromatin formation

Correct Answer: (A) Dosage compensation

Dosage compensation ensures equal X-chromosome gene expression between mammalian females (XX) and males (XY) through X-chromosome inactivation in females. This mechanism prevents females from having double the gene products from X-linked genes compared to males.​

Option Analysis

(A) Dosage compensation
This is the primary mechanism in mammals where one X chromosome in females is randomly inactivated early in embryonic development, forming a Barr body. The process is mediated by the Xist RNA, which coats the inactive X chromosome, leading to transcriptional silencing and equalizing gene dosage with males.​

(B) Histone code
The histone code refers to specific post-translational modifications on histone proteins (like methylation or acetylation) that regulate gene expression epigenetically by altering chromatin structure. While involved in X inactivation (e.g., H3K27me3 marks), it is not the overarching mechanism for equal X-gene expression between sexes.​

(C) RNA silencing
RNA silencing (or RNA interference) is a posttranscriptional process where small non-coding RNAs target and degrade specific mRNAs or inhibit translation, often as an antiviral defense or gene regulation tool. It does not achieve chromosome-wide dosage compensation in mammals.​

(D) Heterochromatin formation
Heterochromatin formation compacts chromatin into a transcriptionally inactive state, which occurs on the inactive X chromosome via repressive marks and proteins like HP1. However, this is a downstream effect of dosage compensation rather than the process ensuring equal expression.​

In mammals, dosage compensation in mammals X chromosome balances gene expression between females with two X chromosomes and males with one X and one Y. This critical process, essential for CSIR NET Life Sciences preparation, prevents overexpression of X-linked genes in females through a mechanism called X-chromosome inactivation.​

Mechanism of Dosage Compensation

Dosage compensation achieves parity by silencing one X chromosome in each female cell via the Xist long non-coding RNA. Xist coats the future inactive X (Xi), recruiting silencing factors that trigger epigenetic changes like DNA methylation and histone modifications, forming the compact Barr body. Males maintain expression from their single active X, ensuring equivalent dosage.​

Why Other Options Fail

• Histone code: Regulates general gene expression via histone marks but lacks specificity for sex-specific X balancing.​

• RNA silencing: Targets individual mRNAs, not entire chromosomes.​

• Heterochromatin formation: Results from inactivation but does not initiate dosage equalization.​