Question

Haemoglobin A is produced in red-blood cell (RBC) precursors but not in neurons. Focusing on regulation of transcription and using what we've learned of gene control, what factors would turn "off" gene expression in neurons, and turn gene expression "on" in RBC precursors?

NOTE: Address the cells as they are right now, you don't have to talk about tissue differentiation (fetal development). Think of it this way: you have two cell types, a gene is off in one of them and on in another, if you could look into the nucleus what would be different between the two cells that explains why the gene is expressed in one and not the other.

Answer 1

The cell specific or tissue specific gene expression is hallmark of eukaryote cellular organisation. The gene which is expressed in red blood cells precursor is not expressed in neurons, which enables them to perform specific functions.
Now, if we stain the nucleus of both the cells and look, we would find that some regions of Hemoglobin producing genes in neurons will be darkened or heterochromatin region. The same region is RBC precursor would stain light, called Euchromatin region. This is because, the histone protein near the gene neuron will be methylated and undergoes tight condensation. This inhibits binding of transcription enzymes and so the gene is not expressed. In RBC precursor cells, the histone protein near the Hemoglobin gene is not bound by methyl group, but may be by acetyl group which increased transcription efficiency. So henoglobin is produced in RBC precursor cell. This process is called chromatin remodeling, or histone modifications.