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Index >> Bacterial Structure >> Nucleiod - Nucleus

Bacterial Structure
Part 1
Structure of Fimbriae or Pili
Functions of Fimbriae or Pili
Parts of Flagella
Bacterial Movement
Spirochaetal Movement
Gliding Movement
Nucleoid (Nucleus)
Plasmids
Classes of Plasmids
Gene Pickup and Sex Factors
R Plasmids
Plasmids Conferring Pathogenecity to mammals
Col Plasmids
Degradative Plasmids
Mercury Resistance Plasmids
Tumor Inducing Plasmid in Agrobacterium Tumifaciens
Cryptic Plasmids
Staphylococcal Plasmids

Nucleiod - Nucleus

	Nucleiod - Nucleus - The nucleus of eukaryotes is bounded by a double-walled nuclear envelope. The DNA, of bacteria is however, not enclosed within a nuclear envelope, and is hence known as the nucleoid or chromatin body. The nucleoid consists of a single, large DNA double helix which has been shown to be circular in several bacteria. Two or more nucleoids may be seen in most bacteria, since nuclear division proceeds cell division. The bacterial DNA is usually called the chromosome.
It, however, differs from the chromosomes of eukaryotes in not being associated with basic proteins such as histones. Small amounts of basic proteins have been found associated with isolated DNA. These might exclude segments of the chromosome from transcription, and thus function as repressor proteins. Thus the absence of DNA-his tone association in prokaryotes may not be such a fundamental character of differentiation from eukaryotes as it has been made out to be. The bacterial chromosome does not appear to be just a large molecule of DNA folded in a random manner.
Replication and segregation would not be possible in such a structure. Isolation of E. coli nuclei indicates that the chromosome has a higher level of organization. According to one model the DNA appears to be folded into a number. The loops appear to be held in position by a core of RNA which binds to the DNA and determines the positions of the folds, and protein. The RNA is newly transcribed single stranded RNA, and the protein is largely RNA polymerase. Nicking with DNAs relaxes the super coiling in one loop without affecting the other loops. Cutting of the RNA molecules in adjacent loops causes the two loops to form one large loop without loss of supercoiling.
When bacterial DNA is isolated, a small amount of membrane material generally remains attached to it. This indicates that the DNA is attached to the plasma membrane. The enzymes required for replication are probably associated at the contact point. The plasma membrane appears to be involved in the separation of the replicated DNA molecules during cell division. Organisms having mesosomes (e. g. Bacillus subtilis) usually show points of contact between the DNA and the mesosomal membranes: As the cell and the plasma membranes grow, the mesosomes are separated and drag the DNA molecules along with them.

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