Translation:The nucleus is an important section in cells.
I agree. At least in the US, it is much more idiomatic to use the singular for a topic of study. Or to remove the definite article if using the plural.
Las is required in Spanish when it's either a specific group--"Las células en esta cultura..."--or the group definition (all individuals defined by the noun)--"Las células tienen núcleos." Las is not required in Spanish if you're speaking of some cells, generically--"Tengo células en mi cuerpo."
In English the article is omitted when you're talking about all cells everywhere or generic cells. It is only used for a specific group of cells--"The cells in this culture..."
Does this help?
No, this organelle is called nucleus in English. Take a look. https://en.wikipedia.org/wiki/Cell_nucleus?wprov=sfla1