A feedback loop is a process in which the output of a system is fed back into the system as input, in order to regulate the behavior of the system. In biology, feedback loops are often involved in regulating biological processes such as cell growth and division (cell cycle).
There are two types of feedback loops: positive and negative. A positive feedback loop is one in which the output of the system increases the input, leading to an amplification of the original signal. In contrast, a negative feedback loop is one in which the output of the system decreases the input, leading to a dampening of the original signal.
The cell cycle is regulated by both positive and negative feedback loops. For example, when cells first receive signals that indicate that it's time to divide (known as mitogenic signals), this leads to an increase in cyclin levels inside the cells. Cyclin binds to and activates cdk enzymes, which then phosphorylate key proteins involved in cell division (such as those involved in chromosome replication). The phosphorylation of these proteins leads to further activation of cell division processes, culminating in cell division itself. This entire process forms a positive feedback loop: increased cyclin levels lead to increased cdk activity, which leads to more cell division; and more cell division leads to even higher cyclin levels.
However, if unchecked, this positive feedback loop would eventually lead to uncontrolled cell growth and tumor formation. Therefore, there must also be mechanisms in place that can halt or reverse this process when necessary. This is where negative feedback loops come into play. For example, once chromosomes have been replicated during S phase of the cell cycle, this triggers an increase in levels of p53 protein inside cells. p53 protein binds to and inhibits cdk enzymes (thereby reducing cyclin levels), leading to decreased rates of cell division; this provides a check on uncontrolled growth anddivision