2017. november 23., csütörtök

Vannak akik azt mondják, hogy az egysejtűektől a többsejtűekhez evolúciós ugrás kellett - pedig nem: sima átmenet !

At some later stage of evolution, unicellular organisms found it advantageous to cluster together, thereby acquiring greater motility, efficiency, or reproductive success than their free-living single-celled competitors. Further evolution of such clustered organisms led to permanent associations among individual cells and eventually to specialization within the colony – to cellular differentiation.

The advantages of cellular specialization led to the evolution of ever more complex and highly differentiated organisms, in which some cells carried out the sensory functions, others the digestive, photosynthetic, or reproductive functions. Many modern multicellular organisms contain hundreds of different cell types, each specialized for some function that supports the entire organism. Fundamental mechanisms that evolved early have been further refined and embellished through evolution. The simple mechanism responsible for the motion of myosin along actin filaments in slime molds has been conserved and elaborated in vertebrate muscle cells, which are literally filled with actin, myosin, and associated proteins that regulate muscle contraction. The same basic structure and mechanism that underlie the beating motion of cilia in Paramecium and flagella in Chlamydomonas are employed by the highly differentiated vertebrate sperm cell. Figure 2–25 illustrates the range of cellular specializations encountered in multicellular organisms.

Figure 2–26 Three types of junctions between cells.
(a) Tight junctions produce a seal between adjacent cells. (b) Desmosomes, typical of plant cells, weld adjacent cells together and are reinforced by various cytoskeletal elements. (c) Gap junctions allow ions and electric currents to flow between adjacent cells.

Lehninger-Nelson-Cox: Principles of Biochemistry, 49.o.

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Szószedet: bioinformatika (language of biochemistry)

We begin in each case by considering the covalent structure of the simple subunits (amino acids, fatty acids, monosaccharides, and nucleoti...