Biological World

Chromatin Fiber Genetic information is written in the sequence of nucleotides, or bases, in the DNA double helix of chromosomes, whose lengths in human cells achieve a total of approximately two meters. Chromosomes are coated with histone proteins, which assemble the DNA into compact chromatin fibers that enable condensation of this extraordinary length into the tiny volume of the nucleus. In addition to allowing DNA to be packaged into the compact structure of chromosomes, chromatin is involved in the regulation of gene expression and other genomic functions such as DNA repair and replication, and organization of nuclear architecture. The intimate relationship between the bases of DNA and the histone proteins forming a complex constitutes the chromatin molecule. A significant part of the chromatin fiber is made of DNA and nucleosome structures, which are built of histone proteins, around which the DNA is coiled. In addition to that, the fiber contains significant quantities of non-his...

Ultra Structure of Nucleolus The nucleolus is a tri-molecular nuclear body that participates in rRNA synthesis and maturation, ribosome subunit assembly and export, cell cycle regulation, stress sensing, and response. The nucleolus consists in general terms of transcriptionally active and inactive compartments, which have been termed Fibrillar Centers and Dense Fibrillar Components, the Granular Component is primarily a packaging area and the Nucleolar Matrix is a supporting structure. The nucleolus of different eukaryotes varies greatly in complexity and structure but the major function is conserved and all nucleoli possess 30-60 tandem arrays of rDNA genes which get activated during nucleolus formation. The typical effect of inhibition of RNA polymerases that transcribe rRNA is disassembly of the nucleoli. The regions of nucleolar rDNA genes that get transcribed are seen as amorphous Fibrillar Centers surrounded by the Dense Fibrillar Components where nascent transcripts are modified...

  Ultra Structure of the Nuclear Envelope 1. Introduction to the Nuclear Envelope The elegant structure of the eukaryotic nucleus, which is disconnected from the cytoplasm by the nuclear envelope, was recognized a long time ago. The model formed the basis of thoughtful investigations into the dynamic functional reorganization of the nuclear envelope and its components during varying states of the cell cycle. The nuclear envelope consists of an inner and an outer membrane, nuclear pore complexes, and the underlying nuclear lamina, a filamentous scaffold structure formed by lamins. The inner membrane is linked to the lamina and chromatin by its integral membrane proteins, such as lamin B receptor, emerin, and various isoforms of lamina-associated polypeptides 1 and 2. The outer membrane is continuous with the endoplasmic reticulum, and associating protein constituents include ribosome-binding protein, the transmembrane 'stretchability sensor' protein, and protein targeting factor...