SBA — Single Best Answer · Review
Question 585
Genetics → General
Classification
Question
SSBAOptions
Explanation
An enhancer modifies gene activity on the same chromosome, fitting the selective reduction of the A transcript after removal of its regulatory element. A transacting protein can modify genes on both homologous chromosomes, fitting the reduction of both transcripts when the protein is neutralised. Restoring that protein recovers the shared regulatory contribution, but it does not replace the missing DNA element on A; the residual A-specific reduction therefore remains. The preserved promoter binding distinguishes the separate DNA element from the transcription-initiation site.
Option Validity
A) The DNA perturbation has an A-specific effect, whereas protein neutralisation affects both homologues; the proposed ranges are reversed.
B) The promoters remain intact and able to bind RNA polymerase. The separate protein also affects the A transcript, so its action is not confined to B.
C) An A-only protein effect cannot explain why neutralising the protein reduces both transcripts or why restoring it recovers B.
E) DNA-element removal leaves B unchanged, and restoring the protein does not remove the residual A-specific effect. The observations require distinct local and shared regulatory contributions.
Further Reading
Saxena R. Textbook for MRCOG-1: Basic Sciences in Obstetrics and Gynaecology. Jaypee Brothers Medical Publishers; 2016. Chapter 8, Genetics: Transcription, regulatory elements. Printed page 255.