some specialized cells such as neurons and red blood cells lose their ability to replicate when they mature. which phase of the cell cycle do you suspect is non-functional for these cells? why?

G1

Explanation:

Some specialized cells such as red blood cells, neurons lose their ability to replicate in the G1 phase of cell cycle. G1 phase is indeed the terminal phase of the cell division as now cell do not divide any more. These cells do not have the necessary nucleus for further cellular replication in the G1 phase. They become mature need not divide anymore.