Question

A gene on human chromosome 15 is expressed throughout the body. However, in the brain, only the maternal copy of the gene is expressed, whereas the paternal copy of the gene is silent and not transcribed. What accounts for this pattern of expression this gene displays in the brain?
a) In the brain, the maternal copy of the gene is methylated whereas the paternal copy is un-methylated.
b) The copies of this gene in brain cells are coordinately controlled with the copies of the gene in body cells.
c) Through genomic imprinting, methylation regulates expression of the paternal copy of the gene in the brain.

Answer 1

Answer:

C) Through genomic imprinting, methylation regulates expression of the paternal copy of the gene in the brain.

Explanation:

The pattern of gene expression wherein either paternal or maternal gene is expressed in specific cells while the other one is prevented from expression is known as genomic imprinting.

In the given example, the maternal copy of the gene on chromosome 15 is expressed in brain cells while its paternal copy is not expressed in these cells. Hence, the pattern of expression of this gene is regulated through genome imprinting. One of the mechanism is methylation of cytidine residues of CpG islands of the DNA that are more frequently present within promoters of the genes.

When the cytidine residues of these sequences are methylated into 5-methylcytidine, the transcription factors do not bind to these promoters preventing the expression of these genes.

Hence, methylation of cytidine residue in CpG islands of the promoters of the gene present on chromosome 15 could have silenced its expression in brain cells.