The basic of Cell Reproduction.

            One of the most defining characteristics of the living condition is the ability to reproduce. All living cells and organisms reproduce, producing offspring like themselves, and pass on the hereditary information contained in their DNA molecules. The processes of reproduction, while varied and complex, depends upon the ability of individual cells to replicate. All cells arise from preexisting cells by some mechanism of cell reproduction or division.

            Cell reproduction is often divided into two major types: asexual and sexual cell reproduction. Typically in asexual reproduction, a single cell gives rise to a genetic duplicate of the parental cell, without any genetic contribution from another individual, while sexual cell reproduction involves the genetic recombination of two cells.

            In procaryotes, asexual cell division often proceeds by a process of fission. Reproduction occurs when a parental cell replicates its bacterial DNA producing a complete and faithful copy of its chromosome.  Growth of the bacterial cell to an appropriate size seems to induce division by binary fission. New plasma membranes and wall material are laid down constricting the cell into two pieces. The splitting of the cytoplasmic domains and the two DNA molecules into nearly equal halves results in two daughter cells. 

Budding, which is another method of asexual reproduction, occurs in most yeast, hydra, and in some filamentous fungi. In this process, a small cytoplasmic swelling protrudes (a bud) develops on the surface of either the yeast cell or the hypha, with its cytoplasm being continuous with that of the parent cell. The parental cell nucleus then divides and one of the daughter nuclei migrates into the bud. Continuous synthesis of cytoplasm and repeated nuclear divisions results in many buds over the cells surface. Buds are pinched off and behave as spores, germinating and forming a new hypha, all genetically identical to the parent.

In higher organisms (most eucaryotes) asexual cell reproduction involves an elaborate duplication of the chromosomes followed by their separation in a nuclear division called mitosis.  Mitosis is often followed by cytokinesis, a division of the cytoplasm. In the hard-walled cells of higher plants, a medial cell plate forms and divides the parental cell into two compartments. In animal cells, which do not have a hard cell wall, a membrane furrow, made of a microfilament contractile ring, that constricts as a camera diaphragm, pinching the cell in two daughter cells.

Cells that reproduce sexually are characterized by meiosis, the nuclear division process by which sex cells ( gametes) are formed.  Every chromosome of a somatic cell occurs in a pair ( diploid). During meiosis these diploid pairs of chromosomes duplicate and are separated so that each meiotic sex cell has only one chromosome (haploid) of each pair. Two successive meiotic divisions result in the production of haploid sperm and egg cells, each with one-half of the amount of the parental DNA.

   The wonder of cell and molecular biology is how a single progenitor cell (the fertilized zygote) can give rise to the multivariate cellular makeup of the adult organism. 

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