  Among the newly discovered features of multiple stellar populations in
Globular Clusters, the cluster NGC 1851 harbours a double subgiant branch, that
can be explained in terms of two stellar generations, only slightly differing
in age, the younger one having an increased total C+N+O abundance. Thanks to
this difference in the chemistry, a fit can be made to the subgiant branches,
roughly consistent with the C+N+O abundance variations already discovered two
decades ago, and confirmed by recent spectroscopic data. We compute theoretical
isochrones for the main sequence turnoff, by adopting four chemical mixtures
for the opacities and nuclear reaction rates. The standard mixture has
Z=10$^{-3}$ and [$\alpha$/Fe]=0.4, the others have C+N+O respectively equal to
2, 3 and 5 times the standard mixture, according to the element abundance
distribution described in the text. We compare tracks and isochrones, and show
how the results depend on the total CNO abundance. We notice that different
initial CNO abundances between two clusters, otherwise similar in metallicity
and age, may lead to differences in the turnoff morphology that can be easily
attributed to an age difference. We simulate the main sequence and subgiant
branch data for NGC 1851 and show that an increase of C+N+O by a factor $\sim$3
best reproduces the shift between the subgiant branches. We compare the main
sequence width in the color m$_{F336W}$-m$_{F814W}$ with models, and find that
the maximum helium abundance compatible with the data is Y$\simeq$0.29. We
consider the result in the framework of the formation of the second stellar
generation in globular clusters, for the bulk of which we estimate a helium
abundance of Y$\simlt 0.26$.
