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  Section: Genetics » Physical Basis of Heredity » The Nucleus and the Chromosome
 
 
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Prokaryotic Nucleoids

 
     
 
Content
Physical Basis of Heredity 1.  The Nucleus and the Chromosome
The Nucleus 
Significance of nucleus : Hammerling's experiment
Number, shape and size of nucleus
Nucleus in prokaryotes and eukaryotes
Nuclear envelope
Nuclear pore complex and nucleocytoplasmic traffic
Nucleolus
Chromosomes
Number, size and shape of chromosomes
Morphology of chromosomes
Karyotypes
Euchromatin and heterochromatin
Constitutive and facultative heterochromatin
Single-stranded and multi-stranded hypotheses for chromosomes
Chemical composition of chromosomes
Infrastructure of chromosomes
Function of chromosomes
Special types of chromosomes 
Lampbrush chromosomes
Salivary gland chromosomes
B-Chromosomes
Prokaryotic Nucleoids


As indicated earlier, the light microscope could not show any nucleus in the cells of prokaryotes i.e. the bacteria and the blue green algae. These individuals are also designated as the akaryobionta as against the karyobionta having better differentiated nuclei. However, feulgen positive bodies are seen in bacteria and blue-green algae.

These bodies can be clearly observed with the help of electron microscope. It has also been demonstrated that these bodies consist of a network of fine threads (Fig. 6.19). Recent work by a number of workers demonstrated that the network of threads consists of a single chromosome in the form of a ring. The exact three dimensional arrangement by which 1100μ -1400μ long DNA chain, which forms 80% of the chromosome by mass (the remaining 20% being protein + RNA), is packed in a lμ long nucleoid, could also be established now.
 
A bacterial cell showing details of internal structure (redrawn from De Robertis et al., Cell Biology).
Fig. 6.19. A bacterial cell showing details of internal structure (redrawn from De Robertis et al., Cell Biology).

Atleast two proteins, which bind DNA, resemble histones of eukaryotes and organize the DNA into structures comparable to nucleosomes of eukaryotes. It has been shown that the chromosome of E. coli is organized in about 45 loops, which radiate out from a dense proteinaceous scaffold, which is assumed to anchor the DNA (Fig. 6.20). In each of the 45 loops, DNA is supercoiled and complexed with protein. The functional significance of looped domains is not clear but they do not represent units of transcription as in case of lampbrush chromosomes.
 
Diagrammatic representation of the organization of DNA and proteins in bacterial chromosome.
Fig. 6.20. Diagrammatic representation of the organization of DNA and proteins in bacterial chromosome.



















 
     
 
 
     




     
 
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