Search results
Results from the WOW.Com Content Network
A circular chromosome is a chromosome in bacteria, archaea, mitochondria, and chloroplasts, in the form of a molecule of circular DNA, unlike the linear chromosome of most eukaryotes. Most prokaryote chromosomes contain a circular DNA molecule. This has the major advantage of having no free ends to the DNA.
Bacterial genomes divided between a main chromosome and one or more chromids (and / or megaplasmids) are said to be divided or multipartite genomes. The vast majority of chromid-encoding bacteria only have a single chromid, although 9% have more than one (compared with 12% of megaplasmid-encoding bacteria containing multiple megaplasmids).
Usually, a single bacterial chromosome is present, although some species with multiple chromosomes have been described. [5] Along with chromosomal DNA, most bacteria also contain small independent pieces of DNA called plasmids that often encode advantageous traits but are not essential to their bacterial host.
There are many exceptions to this; for example, some Streptomyces and Borrelia species contain a single linear chromosome, [129] [130] while some bacteria including species of Vibrio contain more than one chromosome. [131] [132] Some bacteria contain plasmids, small extra-chromosomal molecules of DNA that may contain genes for various useful ...
The genomes of most eukaryotic mitochondria and plastids are in a single circular chromosome, in line with their bacterial ancestor. However, a good number of eukaryotic species do harbor linear Mitochondrial DNA (mtDNA), some even broken into multiple molecules, across a wide variety of taxa: animals (mammals, medusozoans, sponges), fungi (especially yeast), plants, and Alveolatas.
Prokaryotic chromosomes have less sequence-based structure than eukaryotes. Bacteria typically have a one-point (the origin of replication) from which replication starts, whereas some archaea contain multiple replication origins. [26] The genes in prokaryotes are often organized in operons and do not usually contain introns, unlike eukaryotes.
The list of organisms by chromosome count describes ploidy or numbers of chromosomes in the cells of various plants, animals, protists, and other living organisms. This number, along with the visual appearance of the chromosome, is known as the karyotype , [ 1 ] [ 2 ] [ 3 ] and can be found by looking at the chromosomes through a microscope .
Organisms in which a particular chromosome, or chromosome segment, is under- or over-represented are said to be aneuploid (from the Greek words meaning "not", "good", and "fold"). Aneuploidy refers to a numerical change in part of the chromosome set, whereas polyploidy refers to a numerical change in the whole set of chromosomes. [44]