A linearchromosome is a chromosome which is linear in shape, and containsterminal ends. In most eukaryotic cells, DNA is arranged in multiple linear chromosomes. In contrast, most prokaryotic cells generally contain a singular circular chromosome. However, linear chromosomes are not limited to eukaryotic organisms; some prokaryotic organisms have linear chromosomes as well, such as Borrelia burgdorferi. In fact, experiments in which the circular chromosomes of prokaryotic organisms have been linearized have demonstrated that some prokaryotes can maintain viability even with linear chromosomes. In general, the factors which led to the evolution of linear chromosomes in eukaryotes are not well understood. One potential selective pressure in favor of linear chromosomes relates to the size of an organism's genome: linear chromosomes may make transcription and replication of large genomes easier. In an organism with a very large genome, circular chromosomes could potentially cause problems relating to torsional strain. Linear chromosomes are also in some ways disadvantageous or problematic, one of the biggest potential issues being the end replication problem. This is a phenomenon which occurs due to the directionality of DNA replication enzymes, resulting in the gradual loss of genetic material at the ends of linear chromosomes after each subsequent cycle of cell and DNA replication. In order to mitigate the negative effects of this gradual loss of genetic material, eukaryotes have evolved repetitive, non-coding terminal DNA sequences known as telomeres on the ends of chromosomes. These repetitive, non-coding sequences are lost instead of important coding DNA, and they are replenished using enzymes known as telomerases. However, telomeres do not fully prevent the loss of coding DNA at the terminal ends of linear chromosomes. In fact, the eventual loss of coding DNA in cellular lines within an organism is thought to play a role in senescence. Furthermore, evidence suggests that telomeres can be unstable and can be prone to mutations which lead to tumor development. Mutations which lead to the constitutive activity of telomerase can result in the loss of cellular mortality in tumor cell lines, which is associated with the development of cancer.
