How can a strand of DNA stored inside a nucleus?

Answer 1

This is a very interesting question that properly deserves a very scientific answer but I guess you want the simplified version. Basically, the way a strand or strands of DNA can fit inside nuclei has to do with something called "chromosome condensation". In non-scientific speak this means that the DNA is squashed together to make a much smaller and tighter package. As you have rightly guessed, nuclei of cells aren't particularly big, and it's even more amazing that not withstanding its size, DNA does actually get transferred between the inside and outside of the nucleus otherwise cell replication simply couldn't occur. Transit (movement) from the inside to the outside of the nucleus is facilitated through nuclear pores. Free DNA is comparatively very bulky and is at great risk of chemical attack from substances called enzymes. Since DNA contains the "blueprint" for the synthesis of all cellular components, it's therefore critical that these molecules are not damaged otherwise the messages and information that they carry could be disrupted. After cells divide, the DNA is therefore condensed (made smaller) into structures called chromosomes. These consist of the DNA strands and a substance called chromatin, the major component of which are proteins called histones. The chromatin assists with the process of folding up the free DNA into a smaller package.

Answer 2

The DNA molecule is tightly coiled around histone proteins to form chromatin. This chromatin is further packaged and organized into chromosomes. The chromosomes are contained within the nucleus of a cell, which is separated from the rest of the cell by a nuclear membrane. This organization helps protect and regulate access to the genetic material stored in DNA.