Nuclear pores and chromatin. Coloured high resolution scanning electron micrograph of nuclear pores and chromatin fibres in an onion root tip cell. Nuclear pores (black circles) are holes in the double membrane (purple) that surrounds the cell nucleus. The pores allow interaction between the nucleus and the cell cytoplasm (not seen). Contained within the nucleus are the chromatin fibres (yellow and orange). Chromatin is a diffuse network which contains the chromosomes. The chromosomes contain the genetic instructions for an organism but only become visible when the cell divides.