Clonal exclusion

Fig. 6.5(b) Clonal exclusion (five OR cells) selective expression of one gene per cell out of five alleles (from Lancet et al., 1993). 

Ultimately a plasmid is defined by its mode of DNA repHcation. DNA repHcation is initiated at a single, characteristic sequence, termed the origin. The origin sequence determines the copy number of the plasmid relative to the host chromosome and the host enzymes that are involved in plasmid repHcation. Two different plasmids that contain the same origin sequence are termed incompatible. This term does not refer to the active exclusion of one plasmid by another from the ceU but rather to a stochastic process by which the two plasmids are partitioned differentially into progeny ceUs. A ceU which contains two plasmids of the same incompatibiHty group segregates two clonal populations, each of which has one of the two plasmids in it. 

FIG. 1. Shoot apical meristem structure. The shoot apical meristem consists of three clonally distinct cell layers (LI, L2, L3). LI and L2 represent tunica layers, L3 represents the corpus. Cell divisions in LI and L2 are exclusively anticlinal, cell divisions in the L3 occur in all planes. The central zone harbours the stem cells and is surrounded by the peripheral zone where organ primordia are initiated. 

The second half of the chapter considers restrictions on and elaborations of the process of gene rearrangement. Allelic exclusion—the expression of an allele from only one chromosome of a pair—assures that each antibody-forming cell will produce antibodies of only a single specificity, as postulated by the clonal selection hypothesis. The preimmune repertoire is modified after exposure to antigen an... 

A key feature of the clonal selection theory is that each antibody-forming cell or clone is committed to the expression of one antibody. The diversity in the response is a consequence of the great variety of distinct clones. Initially, Burnet (1957) assumed that antibody diversification occurs exclusively early in life, by a random... 

Numerous Hevea varieties, referred to as clones in professional jargon, can be found in estates. Subramaniam was the first to study NR from different clone by size-exclusion chromatography (SEC) and to know about the native molar mass distribution (MMDq). MMDo is the molar mass distribution of polyisoprene leaving the tree, without any treatments (drying, shearing) that could modify it. As we see later, MMDq is a very important clonal parameter for prediction of some properties of raw commercial NR. 

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