Question

What is the function of nucleolus and chromatin ?

Answer 1

The nucleus of many eukaryotic cells contains a structure called a nucleolus. As the nucleus is the "brain" of the cell, the nucleolus could loosely be thought of as the brain of the nucleus. The nucleolus takes up around 25% of the volume of the nucleus.

This structure is made up of proteins and ribonucleic acids (RNA). Its main function is to rewrite ribosomal RNA (rRNA) and combine it with proteins. This results in the formation of incomplete ribosomes. There is an uninterrupted chain between the nucleoplasm and the interior parts of the nucleolus, whichoccurs through a system of nucleolarpassages. These passages allow macromolecules with a molecular weight up to 2,000 kDato be easily circulated throughout the nucleolus.

Because of its close relationship to the chromosomal matter of the cell and its important role in producing ribosomes, the nucleolus is thought to be the cause of a variety of different human diseases. 
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Chromatin is comprised of histones and DNA: 147 base pairs of DNA wraps around the 8 core histones to form the basic chromatin unit, the nucleosome.

The function of chromatin is to efficiently package DNA into a small volume to fit into the nucleus of a cell and protect the DNA structure and sequence. Packaging DNA into chromatin allows for mitosis and meiosis, prevents chromosome breakage and controls gene expression and DNA replication.

1. Chromatin formation. DNA wraps around the histone proteins to form nucleosomes; these in turn couple to become the chromatin fiber. 1) Unpackaged DNA.
2) DNA wrapped around histone octamers to form nucleosomes. 3) Nucleosomes compacted into a chromatin fiber.

Heterochromatin is a tightly packed form of chromatin that can silence gene transcription. Heterochromatin constitutes telomeres, pericentric regions and areas rich in repetitive sequences. Euchromatin is less condensed and contains most actively transcribed genes.

Certain proteins – including histones, chromatin interacting proteins such astranscription factors and the DNA repair machinery – play a role in shaping chromatin structure.

Chromatin remodeling complexes can change chromatin architecture by modulating the interaction between nucleosomes and DNA, often by adding post-translational modifications to histones.