 The BIR pathway can also help to maintain the length of telomeres ( regions of DNA at the end of eukaryotic chromosomes ) in the absence of ( or in cooperation with ) telomerase . Without working copies of the telomerase enzyme , telomeres typically shorten with each cycle of mitosis , which eventually blocks cell division and leads to senescence . In budding yeast cells where telomerase has been inactivated through mutations , two types of " survivor " cells have been observed to avoid senescence longer than expected by elongating their telomeres through BIR pathways . 
