Chromosomes

Chromosomes vary extensively between different organisms. The DNA molecule may be circular or linear, and can contain anything from tens of kilobase pairs to hundreds of megabase pairs. Typically eukaryotic cells (cells with nuclei) have large linear and prokaryotic cells (cells without defined nuclei) have smaller circular, although there are many exceptions to this rule. Furthermore, cells may contain more than one type.
In eukaryotes, nuclear are packaged by proteins into a condensed structure called chromatin. This allows the very long DNA molecules to fit into the cell nucleus. The structure of it and chromatin varies through the cell cycle. it may exist as either duplicated or unduplicated—unduplicated are single linear strands, while duplicated (copied during synthesis phase) contain two copies joined by a centromere. Compaction of the duplicated during mitosis and meiosis results in the classic four-arm structure.
In prokaryotes, a small circular DNA molecule may be called either a plasmid. These small circular genomes are also found in mitochondria and chloroplasts, reflecting their bacterial origins. The simplest are found in viruses: these DNA or RNA molecules are short linear or circular that often lack any structural proteins.

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