DNA structure and replication

Where do we find DNA in eukaryotic cells?

Bound to proteins (histones) in chromosomes in the nucleus (DNA + all the proteins associated with it (mainly histones) = chromatin).

Plus in chloroplasts and mitochondria.

Where do we find DNA in prokaryotic cells?

As unbound circular DNA in the cytosol (this is QCAA’s phrasing, but this is technically known as the genophore)

Small rings of DNA called plasmids may also be present.

NB: The generalisation that ALL prokaryotes have circular genomes is incorrect. A growing number of prokaryotes have been discovered which have linear chromosomes, such as Borreliaburgdorferi, which causes Lyme disease; members of the Streptomyces family and Agrobacterium tumafaciens, which has both a circular and a linear chromosome.

State two of the differences between DNA found in eukaryotes and prokaryotes.

  • DNA is usually circular in prokaryotes and linear in eukaryotes.
  • The genome of prokaryotes is significantly smaller than eukaryotes.
  • Eukaryotes wrap their DNA around proteins called histones. Most prokaryotes do not have histones.
  • Extrachromosomal plasmids encode nonessential prokaryotic genes; these are not commonly present in eukaryotes.
  • Prokaryotic DNA is located in the cytosol, whereas DNA is found in chromosomes (the nucleus) in eukaryotes.