How does the location and amount of genetic material differ across cell types?

In prokaryotic cells, the main DNA lies in a nucleoid, a region with no surrounding nuclear membrane. In eukaryotic cells, most DNA is packaged into chromosomes inside a nucleus. Mitochondria also have genomes, as do plastids such as chloroplasts in photosynthetic plants and algae. Viruses are not cells; their particles contain a DNA or RNA genome.

"Typical" matters. Some bacteria have linear or multiple chromosomes, and not every bacterium carries plasmids. Bacterial DNA also interacts with DNA-binding proteins; describing it as completely protein-free is misleading. Many archaea have histones. Eukaryotic plasmids exist too, including well-studied examples in yeast.

Keep genome size, ploidy, chromosome number and DNA content separate. Genome size is the amount of sequence in a genomic set; ploidy is the number of chromosome sets. During S phase, when DNA is copied, a cell can double its DNA content without immediately doubling its chromosome count. This is because the copied sister chromatids remain joined until they separate.

For example, a typical human diploid cell has 46 chromosomes before replication. After S phase it still has 46 chromosomes but twice as much nuclear DNA, arranged in 92 chromatids. A mature gamete normally has one chromosome set. These comparisons concern nuclear DNA; they do not count every mitochondrial genome molecule.

Amounts also vary among species. A larger genome does not automatically imply a more complex organism or more genes, because genomes differ in repetitive and non-coding sequence.

Finally, a mature mammalian red blood cell is not prokaryotic just because it has lost its nucleus during specialisation. It develops from a eukaryotic cell. Classification depends on the organism's cell organisation and ancestry, not one feature taken on its own.

Try the genetic-material and cell-structure question in the Biology Unity and Diversity questionbank. Use the DNA's location to support your cell identification, then compare typical prokaryotic and eukaryotic DNA without turning common features into rules with no exceptions.