Evolution  builds  towards  perfection 

Evolution constantly drives towards the optimal solution and is one of the most essential principles in nature. This truth is illustrated beautifully by the structure and function of cellular architecture. The information of the blueprint is encoded in the structure itself, the sequence of the DNA, whereas cellular structure and function is regulated by proteins – molecular machines that themselves are built by the transcription and translation of this DNA sequence.
The main structural component of the cell is the cytoskeleton – a complex polymer network of actin filaments, microtubules and intermediate filaments that works in an intriguing interplay with molecular motors and accessory proteins.
An electron micrograph of the cytoskeleton of fibroblast cells is shown in Figure 1. The nuclei and the filament network are clearly visible. Actin is concentrated in the cortex and bundles of actin filaments, referred to as stress fibers, which span across the entire cell (see also fig. (2)). Actin filaments are semiflexible and about 7 nm in diameter. Their mechanical properties can change by orders of magnitude depending on the type of cytoskeletal arrangement – from single entangled filaments with spaghetti like consistency, to crosslinked networks or tightly
coupled bundles exceed-ing the strength of steel. Polymerization of actin filaments and their interaction with molecular motors of the myosin family are the main force generation mechanisms of cell motility. An in vitro structure formed by actin polymerization and myosin motors is shown in the large image on the left page. Evolutionarily speaking, actin is a very successful molecule as it is one of the most abundant proteins in all eukaryotic cells and genetically most highly conserved across different species.Microtubules are hollow cylinders with a diameter of 25 nm made up of tubulin dimers and are much stiffer than actin or intermediate filaments. They grow from the microtubule organizing center which is located close to the cell nucleus. The microtubule network provides robust highway tracks for intracellular transport by kinesins and dyneins, motor proteins that move along the microtubules in opposite directions. In the case of neuronal cells, these tracks enable fast transport of material along the axon and can reach lengths of up to meters. The microtubule cytoskeleton of a neuronal cell extending its neurites is shown in fig. (3). Another striking structure formed by microtubules is the mitotic spindle. It controls cell division as it centers the

chromosomes before it pulls them apart separating the genetic information into equal halves for each daughter cell. Intermediate filaments have a diameter in between actin filaments and microtubules and are built up from families of related proteins like keratins or vimentins. They are important for cellular structure and tissue organization but vary depending on cell type and function.
The cytoskeleton is a highly dynamic structure that is constantly being reorganized and optimally adapting to its function – like rebuilding your house every day to accommo- date for the weather. This enables the cell to quickly change its morphological shape, actively probe its environment and dynamically react to chemical stimuli or mechanical forces.