DNA content is fairly constant in all cells of a species. Why ?
Answers
THE RELATIONSHIP OF NUCLEAR DNA CONTENT TO CELL SIZE IN HUMANS
Methodology
Our analysis for this work used published data from healthy human cell populations representing 19 different cell types, as designated in the original studies (data provided in Table 1). In the original studies, DNA content was estimated using the Feulgen staining method, and the size of cells or cell nuclei were directly measured. Feulgen staining (Feulgen and Rosenbeck 1942) has been the most widely used method for estimating DNA content for several decades, and is still generally considered a reliable method for making quantitative measurements of DNA content (Chieco and Derenzini 1999; Biesterfeld et al. 2011). Measurement errors are typically <5% using this method (Gregory 2005), which for our analyses is negligible given the orders of magnitude variation in DNA content among the cells we consider. The method works by staining DNA owing to the reaction of Schiff or pseudo-Schiff reagents with aldehydes, which are converted from deoxyribose in DNA after HCl hydrolysis (Chieco and Derenzini 1999). The light absorbance of the stained genetic material is then measured to quantify the relative DNA content of cells.
In cases in which the relative content of DNA was originally expressed in “arbitrary units” in the original studies, we converted these measures to pg DNA given that the Feulgen staining method results in a linear relationship between light absorbance and DNA content (Biesterfeld et al. 2011). In each case, a specific conversion factor was used for each cell type based on the observed relationship between light absorbance and DNA content in the particular study (see Table 1). This was done to avoid any bias associated with differences in DNA compaction level or uptake of stain across cell types. The conversions used for each cell type are given in Table 1.
Finally, in many studies considered here, only the population means of cell size and DNA content were reported. Thus, to remain consistent, for those studies that reported individual cell measurements, we calculated mean values and used these in the analyses. So, all points shown in the figures for humans represent means of DNA content and cell size from cell populations. In Table 1, we give all values for cell size and DNA content reported in the original sources, both population mean values and individual cell values. In the majority of cases, cell volumes were not directly reported, so we estimated these volumes from measures of nuclear size based on a published relationship between nuclear volume and cell volume across human cell types (see Table 1 for further details of methodology and data) (Swanson et al. 1991).
We evaluated the relationship between mean nuclear DNA content (pg) and cell volume (µ3) using a linear mixed model of the form log10 (DNA) = a + b log10 (cell volume) + C + D + E, where a and b are regression coefficients, C and D represent random effects of cell type (random slope and intercept, respectively), and E represents residual errors (Pinheiro and Bates 2000). This statistical model accounts for within-group autocorrelation, allowing us to include cells from multiple cell types in a single analysis. To determine whether DNA content was correlated with cell volume within cell types, we used ordinary least-squares regression. We verified normality of residuals using quantile–quantile plots.