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So school has taught us that eukaryotes have DNA that is linear and inside of a membrane - called the nucleus. And that prokaryotes have circular DNA that is free floating inside of the cell.
We also know the pros and cons of both linear and circular DNA.
Strictly speaking, all nucleated (eukaryotic) cells contain linear DNA. In addition to the nucleus, the mitochondria (which break down food molecules and create chemical energy) and chloroplasts (which facilitate photosynthesis) also have small stores of their own DNA and this DNA is circular. This adds weight to the hypothesis that these organelles were once prokaryotic cells that came to exist in symbiosis with early eukaryotes.
There is also cytoplasmic DNA free-floating in eukaryotic cells. These are also circular and normally contain short copies of chromosomal DNA sequences. Their function is not well understood.
In prokaryotes, some species of spirochaetes and bacteria have been found with linear chromosomes. In addition, prokayrotes contain extra-chromosomal DNA called plasmids. Bacteria are able to pass plasmids from one to another and express the genes on them: this is an important source of the spread of antibiotic resistance genes through bacterial populations. Plasmids are also normally circular, but linear structures also exist.
References:
Cohen, Sarit; Andreas Houben; Daniel Segal (2008). "Extrachromosomal circular DNA derived from tandemly repeated genomic sequences in plants". The Plant Journal 53 (6): 1027–1034.
Hinnebusch J; Tilly K (1993). "Linear plasmids and chromosomes in bacteria". Molecular Microbiology. 10(5):917-22
Finbarr Hayes (2003). "Chapter 1 - The Function and Organization of Plasmids".
Yes there are prokaryotes with linear chromosomes (especially archaea). Eukayotic chromosomes are almost always linear but this, study reports that yeasts with a loss of function mutation in the ATM homolog genes TEL1 and rad3, form circular chromosomes as a consequence of lack of telomeres. Upon prolonged culture these yeasts were found to undergo mitosis but not meiosis.
