Fatigue can have causes that aren't due to underlying disease. Examples include lack of sleep, heavy exertion, jetlag, a large meal, or aging.
The animal’s anatomy is specifically evolved in and thus tied in with it physiology, thus affecting its interactions with the environment.
Interphase<span> begins with G</span>1<span> (G stands for gap) </span>phase<span>. </span>During<span> this </span>phase<span>, the cell makes a variety of proteins that are needed for </span>DNA<span> replication. </span>During S phase, which follows G1 phase<span>, all of the chromosomes are replicated. Following replication, each chromosome now consists of two sister chromatids </span>