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The term plasmid was coined in 1952 by the American molecular biologist Joshua Lederberg to refer to "any extrachromosomal hereditary determinant." [14] [15] The term's early usage included any bacterial genetic material that exists extrachromosomally for at least part of its replication cycle, but because that description includes bacterial viruses, the notion of plasmid was refined over time ...
Eukaryotic cells are some 10,000 times larger than prokaryotic cells by volume, have their DNA organised in a nucleus, and contain membrane-bound organelles. [ 49 ] The division between prokaryotes and eukaryotes has been considered the most important distinction or difference among organisms.
The second chromosome has also been called a chromid, in that they have similar codon usage to the chromosome, are essential to life like the main chromosome, but has plasmid-type elements such as the origin of replication. Many other sequenced Paracoccus also have chromids. [25] Vibrio have two circular chromosomes.
These types of linear plasmids represent the largest class of extrachromosomal DNA as they are not only present in certain bacterial cells, but all linear extrachromosomal DNA molecules found in eukaryotic cells also take on this invertron structure with a protein attached to the 5’ end. [16] [17]
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.
The nucleoid (meaning nucleus-like) is an irregularly shaped region within the prokaryotic cell that contains all or most of the genetic material. [1] [2] [3] The chromosome of a typical prokaryote is circular, and its length is very large compared to the cell dimensions, so it needs to be compacted in order to fit.
Most bacteria have not been characterised and there are many species that cannot be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology. Like all animals, humans carry vast numbers (approximately 10 13 to 10 14) of bacteria. [3] Most are in the gut, though there are many on the skin.
In most bacteria the most numerous intracellular structure is the ribosome, the site of protein synthesis in all living organisms. All prokaryotes have 70S (where S=Svedberg units) ribosomes while eukaryotes contain larger 80S ribosomes in their cytosol. The 70S ribosome is made up of a 50S and 30S subunits.