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  Section: Genetics » Physical Basis of Heredity » The Nucleus and the Chromosome
 
 
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Single-stranded and multi-stranded hypotheses for chromosomes

 
     
 
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

When chromosomes are compared in related species which differ widely in DNA content, such differences may be attributed to one of the two causes : (i) lateral multiplication of chromonemata leading to multiple strandedness or (ii) tandem duplication, where length-wise duplication instead of lateral multiplication is responsible for difference in DNA content. This later condition will retain the single stranded feature of chromosomes. Although multiple strandedness has been demonstrated in several cases of plants like Vicia faba and animals like dipteran salivary gland chromosomes, there are evidences against such a hypothesis to become a generalization. In Vicia faba chromosomes, sub-chromonemata were actually observed, while in related V. sativa they could not be observed. In other genera also like in Allium and Lolium, it has been shown that increase in DNA content is mainly brought about by tandem duplication rather than by lateral multiplication leading to multiple stranded feature. Therefore, the two hypotheses assuming single stranded and multiple stranded nature of chromosomes are complementary and not exclusive to each other.
 
     
 
 
     




     
 
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