Ubiquitin; conjugated to proteins, marking them for selective degradation via the Ub-26S proteasome system; essential for the cellular stress response; encoded as a polyubiquitin precursor with 5 head-to-tail repeats; repeat number is evolutionarily unstable within and between species; 5th repeat has a C-terminal extension and is required for proteostasis, mitophagy, and longevity; regulates SC formation and meiotic recombination; protein abundance increases in response to DNA replication stress