Many of the phenotypes of DiGeorge syndrome have been traced back to one of the genes that is in the deleted region, TBX1. Some of the evidence for the important role of TBX1 in DiGeorge syndrome came from individuals who have some of the symptoms of DiGeorge syndrome without having the typical deletion. Closer examination showed that some of these individuals had SNV in the TBX1 gene. Which of the following SNVS would be most likely to create a similar phenotype to the deletion?
A. A variant in the promoter region of TBX1 that increases its expression
B. A variant in the intronic region of TBX1 that does not affect splicing. X
C. A nonsense variant near the 5' end of the TBX1 gene.
D. A silent variant near the 5' end of the TBX1 gene.

Answer:

Explanation:

Step 1: Generating a Proton Motive Force

The hydrogen carriers (NADH and FADH2) are oxidised and release high energy electrons and protons

The electrons are transferred to the electron transport chain, which consists of several transmembrane carrier proteins

As electrons pass through the chain, they lose energy – which is used by the chain to pump protons (H+ ions) from the matrix

The accumulation of H+ ions within the intermembrane space creates an electrochemical gradient (or a proton motive force)

Step Two: ATP Synthesis via Chemiosmosis

The proton motive force will cause H+ ions to move down their electrochemical gradient and diffuse back into matrix

This diffusion of protons is called chemiosmosis and is facilitated by the transmembrane enzyme ATP synthase

As the H+ ions move through ATP synthase they trigger the molecular rotation of the enzyme, synthesising ATP

Step Three: Reduction of Oxygen

In order for the electron transport chain to continue functioning, the de-energised electrons must be removed

Oxygen acts as the final electron acceptor, removing the de-energised electrons to prevent the chain from becoming blocked

Oxygen also binds with free protons in the matrix to form water – removing matrix protons maintains the hydrogen gradient

In the absence of oxygen, hydrogen carriers cannot transfer energised electrons to the chain and ATP production is halted