Reality → Life → Cell → Division
The cell cycle consists of a relatively long preparation phase and a short period of cell division, followed by a variable period of resting [1] . Chromosomes become visible only during the period of cell division, after the replicated long DNA strands have coiled into extremely dense packages. Division of the nucleus (mitosis, or karyokinesis), followed by division of the remainder of the cell (cytokinesis), is a marvelously orchestrated and controlled process that sorts the chromosomes, pulls them apart, and breaks down and reassembles the nuclear membrane [2] . A special, twofold division (meiosis) produces germ cells with half the number of chromosomes and shuffled parental genes [3] . The fertilized egg cell splits up through a sequence of a few rapid mitotic divisions into a number of equal, totipotent cells [4] . For mammals, the first cellular differentiation starts with the forming of the blastocyst, which contains the pluripotent embryonal stem cells [5] . Another major early developmental step follows at gastrulation, when ectodermal, endodermal, and mesodermal cell types begin to differentiate and eventually develop into the many fully differentiated cells of the adult's tissues and organs [6] . Even highly specialized cells of the adult organism still contain the same genome as the original egg cell; differentiation is mainly explained by inhibiting/activating effects of special proteins (transcription factors) that bind to specific DNA sequences and thus regulate gene expression.
The cell cycle can be divided into three major phases: the resting phase, the interphase, and the mitotic phase. In the resting phase (which may be the final and only phase for a mature specialized cell that no longer divides), a cell executes its normal biological functions. Activity and the cell's volume increase during the interphase, when proteins are synthesized and DNA is replicated in preparation for the cell's division. The following short (1-2 hours) mitotic phase can be subdivided into mitosis (division of the nucleus) and cytokinesis (completion of the division of the remaining cell).
Mitosis (see animation) begins with the very dense, highly sophisticated packing of chromosomes in the nucleus (see Chromatin). The mitotic spindle begins to develop as microtubules disassemble at the nuclear lamina and reassemble at the centrosomes). The nuclear membrane breaks up into small vesicles, chromosomes move to the cell equator, mysteriously align and connect with their kinetochores to the spindle's microtubules. Sister chromatids are being pulled apart by shortening spindle fibers that converge in centrosome-controlled poles moving in opposite direction to the cell's periphery. The nuclear membrane reassembles around the new set of chromosomes at each pole, and the spindle begins to disassemble. At the cell's equator, a contractile ring of myosin and actin filaments splits the cell into two daughter cells. Cyclin-dependent kinases are pivotal for regulating the timely and faultless succession of all process steps.
Meiosis (see animation) produces four daughter cells, each with half the number of chromosomes of the parent cell. Before the first meiotic division, homologous chromosomes align next to each other and exchange matching sections through crossover. The immediately following first division separates the shuffled, non-identical homologous chromosomes. The identical chromatids of a chromosome are then separated in the second, mitosis-like division (see comparison of mitosis and meiosis).
For most mammals, including humans, the potential of the egg cell to grow into a full organism or any of its parts is maintained up to the first four cell divisions, yielding 16 totipotent cells. These very early embryonic stem cells are produced without any cell growth during the interphases. Through the successive four divisions the initial plasma/DNA ratio of the egg cell drops to 1/16 (that ratio is believed to trigger the next step in embryonal development).
The mammalian blastocyst is formed from the initial compact ball of cells by the buildup of fluid in the center, which pushes the cells outwards to form a blastula. A differentiation occurs: some cells bind tightly and build the outer shell (these cells develop later into the placenta); and some cells bind softly and accumulate at one pole of the blastula's interior (these cells are the pluripotent embryonic stem cells that will build the embryo and all tissues and organs of the adult organism).
Gastrulation creates the three fundamental germ layers (ectoderm, endoderm, mesoderm), which give rise to four basic types of animal tissues (connective, muscle, nervous, epithelial) and eventually to the development of all organs. Adult stem cells exist in various tissues and organs of the adult organism. When needed, they differentiate at the right time and location to replace dying or damaged cells of the adult body. Like all stem cells, adult stem cells have the twofold capacity of (a) unlimited self-renewal as a stem cell, and (b) differentiation into final cell types (see asymmetric cell division).