Prokaryotes chromosomes

Eukaryotic chromosomes consist of one long, linear molecule of DNA bound to a com plex of proteins to form chromatin. Circular DNA is found in eukaryotic mitochondria, prokaryotic chromosomes, and extrachromasomal plasmids. 

At the moment, the overall pattern of DNA topology and DNA topoisomerases in archaebacteria is clearly more similar to the eubacterial one than to the eukaryotic one (Table 2). One can speculate that the similarities between DNA topology in eubacteria and archaebacteria are related to the presence of the same type of prokaryotic chromosomal organization in these two domains. An exception to this pattern is the presence of a type II DNA topoisomerase without gyrase activity in Sulfolobus. It remains to be determined whether the latter enzyme is indeed specifically related to the eukaryotic type II DNA topoisomerases or to E. coli DNA topoisomerase IV, and which kinds of type II DNA topoisomerases exist in other sulfothermophiles and methanogens. 

However, Prokaryotic chromosomes only have one origin of replication (Fig. 11.14)... 

Fig 11.14 Single bubble for DNA Replication in Prokaryotic Chromosomes Requirements for DNA Synthesis... 

Fig. 11.17 Topoisomerase 1 Allows Strain Relief Ahead of the Replication Fork. Prokaryotic chromosomes are Mobius strips. Topoisomerase 11 creates gates by nicking both strands of the dsDNA, allowing helices to cross one another (Fig 11.18). Tangles and linked loops can therefore be separated. It also forms part of the eukaryotic nuclear scaffold, for similar reasons.

Each prokaryotic chromosome consists of a supercoiled circular DNA molecule com-plexed to a protein core. Each eukaryotic chromosome consists of a single linear DNA molecule that is complexed with histones to form nucleohistone. 

PROKARYOTIC GENOMES Investigations of prokaryotic chromosomes, especially those of several strains of E. coli, have revealed the following ... 

In the prokaryotic chromosome there is a protein core to which the circular DNA molecule is attached. In addition, HU protein binds to the DNA and facilitates its bending and super-coiling. In eukaryotic chromosomes, DNA forms complexes with the histones to form nucleosomes. The polyamines are polycationic molecules that bind to negatively charged DNA so the latter molecule can overcome charge repulsions between adjacent coils during the compression process. 

The understanding of natural product biosynthetic systems has been extensively developed during the past 20-30 years. Details of both the biochemical mechanisms and the genetic organization of these systems have been investigated. Prokaryotes contain dedicated genes that direct the biosynthesis of natural products. With few exceptions, all the biosynthetic genes required to assemble one particular natural product lie adjacent to each other on prokaryotic chromosomes, that is, they are clustered. 

E. coli chromosome contains 4.6 x 10 base pairs while the 23 human chromosomes contain 3 x 10 base pairs (2) Prokaryotic chromosomes are usually circular while eukaryotic chromosomes are linear, (3) Eukaryotic chromosomal DNA is packaged into nucleosomes through specific interactions with histones and other proteins while prokaryotic DNA is not. 

DNA is replicated bidirectionally Prokaryotic DNA is a circular duplex. Circular DNA replication involving 9 stmctures is called 9 replication, which proceeds in bidirections. Prokaryotic chromosome contains an origin, Ori (a nucleotide sequence of 100-200 base pairs (bp)), where the two DNA replication forks extend in opposite directions. 

Termination and telomere The binding of the replication termination protein (Tus protein) to the terminus region (x locus) in prokaryotic chromosome impedes the progression of the replication fork and terminates DNA replication. In eukaryotes, the linear chromosomes terminate with telomeres by the action of telomerase. 

There are a number of structural differences between the prokaryotic and eukaryotic types of chromosomes (Ris and Chandler, 1963). For example, chromosomes of viruses, bacteria, and blue-green algae consist of a single DNA molecule which lacks associated histones. In all these prokaryotes the DNA is not separted from the cytoplasm by an enclosing envelope, and segregation of chromosomes at cell division does not occur by a mitotic type of arrangement. Prokaryote chromosomes usually occur singly in cells. All... 

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