Covers the structural parts of the interphase nucleus: the nuclear envelope, chromatin, nucleolus and nuclear matrix. It sets out heterochromatin and euchromatin, including constitutive and facultative heterochromatin and the Barr body example, then the three levels of chromatin folding from nucleosomes to loop domains. It ends with the three morphological regions of the nucleolus and the nucleoplasm's role.
Topic 14 of 59 in the MU Pleven syllabus for Cytology (Medicine, Year 1). The free AceMed note covers it in 8 sections, about 609 words (4 min of reading), with 16 flashcards.
Nuclear pores – pathway between nucleus and cytoplasm
Chromatin
Double stranded DNA in the mitotic eukaryotic cells is organized in Chromosomes. Chromosomes are made of chromatin which is a complex of DNA, histones and non-histone proteins.
Chromatin of the interphase nucleus is classified into heterochromatin and euchromatin.
Heterochromatin
Staining: Dark with hematoxylin and basic dyes (basophilic and acidic).
Structure & Activity: Highly condensed and transcriptionally inactive.
Occurrence: Only present in eukaryotes.
Locations in the nucleus:
Marginal chromatin: Found at the periphery of the nucleus.
Karyosomes: Discrete bodies of chromatin, irregular in size and shape, dispersed around the nucleus.
Nucleolar-associated chromatin: Found in association with the nucleolus.
a. Constitutive heterochromatin:
Inactive, condensed.
Present in all cells and at all stages.
Highly repetitive; includes centromeres and telomeres.
plays a role in chromosome structure.
b. Facultative heterochromatin:
Inactive, condensed.
Not repetitive.
Shows inconsistent chromosomal localization between cell types.
Example: Barr body — used to identify the sex of a fetus.
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