A linear chromosome is a
chromosome
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins ar ...
which is linear in shape, and contains terminal ends. In most
eukaryotic cells
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...
, DNA is arranged in multiple linear chromosomes. In contrast, most
prokaryotic cells
A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Conne ...
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
''Borrelia burgdorferi'' is a bacterial species of the spirochete class in the genus '' Borrelia'', and is one of the causative agents of Lyme disease in humans. Along with a few similar genospecies, some of which also cause Lyme disease, it ...
''.
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
Any cause that reduces or increases reproductive success in a portion of a population potentially exerts evolutionary pressure, selective pressure or selection pressure, driving natural selection. It is a quantitative description of the amount of ...
in favor of linear chromosomes relates to the size of an organism's
genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ...
: 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
A telomere (; ) is a region of repetitive nucleotide sequences associated with specialized proteins at the ends of linear chromosomes. Although there are different architectures, telomeres, in a broad sense, are a widespread genetic feature mos ...
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
Senescence () or biological aging is the gradual deterioration of functional characteristics in living organisms. The word ''senescence'' can refer to either cellular senescence or to senescence of the whole organism. Organismal senescence inv ...
. 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
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal bl ...
.
References
{{Self-replicating organic structures
Chromosomes