Usually, your body's mechanism will kill cells that have damaged DNA (if the damage is beyond repair), but if the cells somehow avoid the apoptosis cues, they could turn precancerous and not listen to our body's system.
When cells have DNA damage but fail to undergo apoptosis, they may be on the road to cancer.
The inner layers are the Core, Radiative Zone and Convection Zone. The outer layers are the Photosphere, the Chromosphere, the Transition Region and the Corona. IRIS will focus its investigation on the Chromosphere and Transition Region.