The nucleus of each cell contains a number of chromosomes.
These are only visible when condensed into a cross shape during cell division.
If we zoom in on a chromosome, we can see that it consists of DNA arranged in the famous double helix structure.
The DNA itself is made up of four different nucleotide bases, which form two complementary pairs. This provides the basis for replication: in cell division, the DNA structure separates in two but each strand contains enough information to rebuild its complement.