The DNA and RNA molecules assume various geometric shapes or topologies that are independent of base pair interactions. Eukaryotic nucleic acids take on linear forms, in contrast to the circular forms of mitochondrial and bacterial chromosomal DNA. Viral genomes occur as different forms, ranging from segmented linear to circular. Although the conformation of RNA molecules can be complex via intramolecular base pairing, the topology of messenger RNA (mRNA) molecules is primarily linear. An organism's genomic topology influences the biochemical mechanisms used during replication and the number of replication cycles a given chromosome can undertake. In contrast to circular genomes, linear genomes limit the total number of possible replication cycles due to progressive shortening of the linear chromosome.
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