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  Section: Genetics » Physical Basis of Heredity » The Nucleus and the Chromosome
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Number, size and shape of chromosomes

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
Number, size and shape of chromosomes
Morphology of chromosomes
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
Prokaryotic Nucleoids

Chromosome number varies from 2n = 4 (n = 2) in Haplopappus gracilis (Compositae) to 2n = > 1200 in some pteridophytes. In Aulacantha a radiolarian, a diploid number of approximately 1600 chromosomes is found (consult Swanson, Cytology and Cytogenetics). While V normally signifies the gametic or haploid chromosome number, '2n' is the somatic or diploid chromosome number in an individual. In polyploid individuals, however, it becomes necessary to establish an ancestral primitive number, which is represented as 'x' and is called the base number. For instance, in common wheat {Triticum aestivum), 2n =42; n = 21 and x = 7, showing that common wheat is a hexaploid (2n = 6x). A chromosome is normally measured at mitotic metaphase and may be as short as 0.25μ (μ = .001 mm) in fungi and birds, or as long as 30μ, in some plants like Trillium. As a rule, most metaphase chromosomes fall within a range of 3μ in fruitfly (Drosophila), 5μ in man and 8-12μ in maize.
Different positions of centromeres controlling shapes of chromosomes at anaphase (redrawn from De Robertis et al. Cell Biology, 6th Edn.)
Fig. 6.8. Different positions of centromeres controlling shapes of chromosomes at anaphase (redrawn from De Robertis et al. Cell Biology, 6th Edn.)

Chromosome shape is usually observed at anaphase, when the position of primary constriction or what is also called centromere determines chromosome shape. This constriction or centromere can be terminal, sub-terminal or median in position. The terminal centromere will give rod shaped chromosome, a sub-terminal one would give J-shaped and a median centromere would give a V-shaped chromosome (Fig. 6.8).

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