Why is the nucleus a circle?

Answer 1

Some clarification in your question would be desirable, for you could be asking about the nucleus of an atom or the nucleus of a Eukaryotic cell.

The simple answer for both is to relate it to a soap bubble (free floating). They are a circle (sphere) for the same reason that a bubble ultimately forms a sphere. The sphere is the shape with the largest volume to surface area ratio, i.e. the shape with the least amount of expended energy to hold (in that shape). The more complicated answer is that a cellular nucleus is not always a perfect sphere, it is maleable, but just like the soap bubble, given no outside influence, it will rebound into a more or less spherical shape. An atomic nucleus is represented as a sphere for roughly the same reason, i.e. conservation of energy. Restraints on the spherical nature would be the type of atom, and whether or not the protons and neutrons are balanced in number. Nevertheless, it will hold a more or less spherical shape.

Answer 2

The nucleus appears as a circle when viewed under a microscope due to its spherical shape and the way it is stained to make it visible. The nucleus is not always perfectly circular and can have irregular shapes depending on the cell type and its function.