Symmetry breaking in biology is the process by which uniformity is broken, or the number of points to view invariance are reduced, to generate a more structured and improbable state. That is to say, symmetry breaking is the event where symmetry along a particular axis is lost to establish a polarity. Polarity is a measure for a biological system to distinguish poles along an axis. This measure is important because it is the first step to building complexity. For example, during organismal development, one of the first steps for the embryo is to distinguish its dorsal-ventral axis. The symmetry-breaking event that occurs here will determine which end of this axis will be the ventral side, and which end will be the dorsal side. Once this distinction is made, then all the structures that are
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| - Symmetry breaking in biology is the process by which uniformity is broken, or the number of points to view invariance are reduced, to generate a more structured and improbable state. That is to say, symmetry breaking is the event where symmetry along a particular axis is lost to establish a polarity. Polarity is a measure for a biological system to distinguish poles along an axis. This measure is important because it is the first step to building complexity. For example, during organismal development, one of the first steps for the embryo is to distinguish its dorsal-ventral axis. The symmetry-breaking event that occurs here will determine which end of this axis will be the ventral side, and which end will be the dorsal side. Once this distinction is made, then all the structures that are (en)
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| - Symmetry breaking in biology is the process by which uniformity is broken, or the number of points to view invariance are reduced, to generate a more structured and improbable state. That is to say, symmetry breaking is the event where symmetry along a particular axis is lost to establish a polarity. Polarity is a measure for a biological system to distinguish poles along an axis. This measure is important because it is the first step to building complexity. For example, during organismal development, one of the first steps for the embryo is to distinguish its dorsal-ventral axis. The symmetry-breaking event that occurs here will determine which end of this axis will be the ventral side, and which end will be the dorsal side. Once this distinction is made, then all the structures that are located along this axis can develop at the proper location. As an example, during human development, the embryo needs to establish where is ‘back’ and where is ‘front’ before complex structures, such as the spine and lungs, can develop in the right location (where the lungs is placed ‘in front’ of the spine). This relationship between symmetry breaking and complexity was articulated by P.W. Anderson. He speculated that increasing levels of broken symmetry in many-body systems correlates with increasing complexity and functional specialization. In a biological perspective, the more complex an organism is, the higher number of symmetry-breaking events can be found. Without symmetry breaking, building complexity in organisms would be very difficult. The importance of symmetry breaking in biology is also reflected in the fact that it's found at all scales. Symmetry breaking can be found at the macromolecular level, at the subcellular level and even at the tissues and organ level. It's also interesting to note that most asymmetry on a higher scale is a reflection of symmetry breaking on a lower scale. Cells first need to establish a polarity through a symmetry-breaking event before tissues and organs themselves can be polar. For example, one model proposes that left-right body axis asymmetry in vertebrates is determined by asymmetry of cilia rotation during early development, which will produces a constant, unidirectional flow. However, there is also evidence that earlier asymmetries in serotonin distribution and ion-channel mRNA and protein localization occur in zebrafish, chicken and Xenopus development, and similar to observations of intrinsic chirality generated by the cytoskeleton leading to organ and whole organism asymmetries in Arabidopsis this itself seems to be controlled from the macromolecular level by the cytoskeleton. There are several examples of symmetry breaking that are currently being studied. One of the most studied examples is the cortical rotation during Xenopus development, where this rotation acts as the symmetry-breaking event that determines the dorsal-ventral axis of the developing embryo. This example is discussed in more detail below.Another example that involves symmetry breaking is the establishment of dendrites and axon during neuron development, and the PAR protein network in C. elegans. It is thought that a protein called shootin-1 determines which outgrowth in neurons eventually becomes the axon, at it does this by breaking symmetry and accumulating in only one outgrowth. The PAR protein network works under similar mechanisms, where the certain PAR proteins, which are initially homogenous throughout the cell, break their symmetry and are segregated to different ends of the zygote to establish a polarity during development. (en)
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