Question

Briefly explain, under what general circumstances you think that each of the three signaling concepts listed below is important. (Think of the general types of external signal to the types of internal responses. For example, signal amplification would be very important when a very low concentration of external signal is bound by a single receptor on a cell and the cell is expected to respond. A single cytoplasmic response messenger would probably not be enough to trigger a transcriptional response. Also, there may be multiple transcriptional elements that need to be activated and a cytoplasmic response would therefore need to be further amplified. Pick another situation where signals would need to be amplified in order to answer a.)

a_Signal amplification

b_Signal redundancy (more than one way to activate a gene)

c_Signal reinforcement (the need for co-activation by two or more signals)

Answer 1

a. Signal amplification: A signal may reach a cell in the form of a single hormone molecule. Inside the cell signal should be amplified so that response is carried out multiple times instead of having by single molecule.Any molecule that catalyzes a reaction can do so multiple times producing more than one product molecule. So each step in signalling chain has potential to amplify the signal. Example if the membrane receptor can produce 10 second messengers and each second messenger can generate the transcription of 10 mRNA chains then the signal has been amplified one thousand fold.Most cell surface receptors stimulate intracellular target enzymes, which may be either directly linked or indirectly coupled to receptors by G proteins. These intracellular enzymes serve as downstream signaling elements that propagate and amplify the signal initiated by ligand binding and transmit a signal to various intracellular targets. targets of such signaling pathways frequently include transcriptionfactors that function to regulate gene expression. Intracellular signaling pathways thus connect the cell surface to the nucleus, leading to changes in gene expression in response to extracellular stimuli.

b. Signal redundancy: The cellular system is a spatiotemporal nonlinear dynamic system that has an underlying architecture guided by universal principles. One such principle is that this system includes redundant elements that can compensate for one another. Redundancy has been widely addressed in cellular research . In its global description, redundancy describes a scenario in which different elements may potentially act in the same biologic or dynamic manner, where the inhibition of one of these elements has no significant effect on the global biologic outcome or on the system's dynamic behavior.The elements are redundant with respect to a given function, and not necessarily concerning all of their activities. The redundant elements can compensate for one another either because they function simultaneously or act as a backup mechanism that is activated only given a stress signal. To maintain a regulated cellular system that functions consistently despite routine noisy signals, the cellular system must intrinsically include redundancy mechanisms at many levels.One example is when two or more transcription factors (TF) share targeted genes, such as E2F1-3. The E2F family regulates a wide range of cellular processes, including DNA replication, mitosis, the function of DNA damage checkpoints, DNA repair, differentiation, and autophagy. It was shown that only the combined loss of the E2F1-3 TFs eliminates the ability of mouse embryonic fibroblasts to enter S phase.. A second well-known level concerns biologic function, where different elements participate in the same pathway, with shared effects on the pathway. Experiments in a mouse germline that perturbed all cyclins and cyclin-dependent kinases (Cdk) that govern the G1 phase of the cell cycle have revealed that much of fetal development occurs normally in their absence. Thus, most of these elements can be compensated for by others, such as when Cdk2 compensates for Cdk4, 6, and 1, due to their redundant functions in cell-cycle feedback loops.

c. Signal reinforcement: Reinforcement signals are generally thought to carry only general information about the overall performance. Specific information to individual synapses as to their relative responsibility in the task would be very difficult to determine. Biological mechanisms for assigning responsibility to each individual synapse are highly unlikely.A reinforcement signal can be generated by a cognitive system only when a nominal target outcome is known.