The Nucleus in the Cell

The nucleus contains the cell's genetic information - in other words, the DNA. The DNA in eukaryotes is found in several pieces.

Each piece is associated with various proteins and is called a chromosome. So when we talk about chromosomes, we're really just talking about the pieces of DNA in the nucleus of a cell.

Notice that we specified eukaryotes. Prokaryotes have DNA too, even though they don't have a nucleus. Their DNA is found as one large circular chromosome floating around in the cytoplasm. The DNA is still a double helix; the double helix is just joined at both ends to make a circle.

Even though all chromosomes contain DNA, it is not true that all chromosomes are identical. The nucleotide base sequences in two different pieces of DNA - two different chromosomes - can be (and are) different.