Textbook - BIOLOGICAL CHEMISTRY - Gubsky Yu.I. - 2000

Chapter I. BIOMOLECULES AND CELLULAR STRUCTURES

CHAPTER 3. NUCLEIC ACIDS. NUCLEOTIDES

3.5. MOLECULAR ORGANIZATION OF NUCLEAR CHROMATIN AND RIBOSOMES

In terms of their molecular Organization, the Chromatin of the Cell Nucleus (or nucleoid) and the Ribosomes of eukaryotes and prokaryotes are complex nucleoprotein structures.

Class="center">Nuclear Chromatin

Eukaryotic nuclear chromatin appears under the Electron microscope as fibers resembling strings of beads. The individual spherical beads of these chromatin fibers are called nucleosomes.

Inside the nucleosome lies a protein core (derived from the English word "core"), around The surface of which a segment of double-stranded DNA is wound (Fig. 3.9).

Fig. 3.9. Diagram of the Molecular organization of nucleosomes in nuclear chromatin.

The nucleosomal core consists of eight histone molecules (the histone octamer): two molecules each of Histones H2A, H2B, H3, and H4 (Table 3.4).

Table 3.4. Histones of Nuclear Chromatin

The histone octamer within the nucleosome forms a spherical Structure wrapped with double-stranded DNA in a superhelical conformation. The nucleosomal DNA makes about 1.75 turns around the core (approximately 140 nucleotide pairs) and continues to the next nucleosome. Histone H1 is associated with the DNA segment connecting adjacent nucleosomes (linker DNA, which is 50 nucleotide pairs long).

Histones, as core components of chromatin, are basic Proteins containing significant amounts of the diaminomonocarboxylic Amino Acids lysine and arginine. Their positive charge enables them to form salt-like complexes with the phosphate groups of DNA. The biological function of histones is to regulate the reading of Genetic information from DNA molecules.

Along with histones (basic proteins), nuclear chromatin contains a certain amount of acidic non-histone proteins (NHPs) spanning several dozen fractions, as well as minor percentages of Lipids and Metal Ions, particularly Mg²⁺, Ca²⁺, and Fe²⁺. The biochemical Functions of these minor chromatin components are not yet fully understood.

Ribosomes

Ribosomes are sub-cellular particles whose biochemical composition is represented by ribonucleoprotein structures. They act as "molecular machines" where the fundamental stages of METABOLISM/35.html">Protein Biosynthesis—Translation—take place. In Human and Animal Cells, most ribosomes are bound to the membranes of The Endoplasmic reticulum.

Ribosomes of prokaryotic and Eukaryotic cells share a similar molecular organization, consisting of two subunits—a smaller one and a larger one—which differ in size and in their sets of specific rRNAs and ribosomal proteins (Fig. 3.10).

Fig. 3.10. Diagram of the Structure of Prokaryotic and eukaryotic ribosomes.

The involvement of individual ribosomal subunits in the biochemical reactions of translation will be discussed in Chapter 21.