<P> Since each cell, regardless of cell type, possesses the same genome, determination of cell type must occur at the level of gene expression . While the regulation of gene expression can occur through cis - and trans - regulatory elements including a gene's promoter and enhancers, the problem arises as to how this expression pattern is maintained over numerous generations of cell division . As it turns out, epigenetic processes play a crucial role in regulating the decision to adopt a stem, progenitor, or mature cell fate . This section will focus primarily on mammalian stem cells . </P> <P> In systems biology and mathematical modeling of gene regulatory networks, cell - fate determination is predicted to exhibit certain dynamics, such as attractor - convergence (the attractor can be an equilibrium point, limit cycle or strange attractor) or oscillatory . </P> <P> The first question that can be asked is the extent and complexity of the role of epigenetic processes in the determination of cell fate . A clear answer to this question can be seen in the 2011 paper by Lister R, et al. on aberrant epigenomic programming in human induced pluripotent stem cells . As induced pluripotent stem cells (iPSCs) are thought to mimic embryonic stem cells in their pluripotent properties, few epigenetic differences should exist between them . To test this prediction, the authors conducted whole - genome profiling of DNA methylation patterns in several human embryonic stem cell (ESC), iPSC, and progenitor cell lines . </P> <P> Female adipose cells, lung fibroblasts, and foreskin fibroblasts were reprogrammed into induced pluripotent state with the OCT4, SOX2, KLF4, and MYC genes . Patterns of DNA methylation in ESCs, iPSCs, somatic cells were compared . Lister R, et al. observed significant resemblance in methylation levels between embryonic and induced pluripotent cells . Around 80% of CG dinucleotides in ESCs and iPSCs were methylated, the same was true of only 60% of CG dinucleotides in somatic cells . In addition, somatic cells possessed minimal levels of cytosine methylation in non-CG dinucleotides, while induced pluripotent cells possessed similar levels of methylation as embryonic stem cells, between 0.5 and 1.5% . Thus, consistent with their respective transcriptional activities, DNA methylation patterns, at least on the genomic level, are similar between ESCs and iPSCs . </P>

Which of the following refers to the differentiation of cells along specialized lines of function