BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 9. NUCLEIC ACIDS

9.1. General Characteristics of Nucleic Acids

Nucleic Acids (polynucleotides) are linear or cyclic Biopolymers composed of A large number of NUCLEOTIDES linked together by phosphodiester bonds. There are two known Types of Nucleic acids—deoxyribonucleic acid (DNA) and ribonucleic acid (RNA)—which differ in their cellular localization, functional role in cellular METABOLISM, and the Chemical Structure of their constituent purine and pyrimidine nitrogenous bases and pentoses.

Nucleic acids are found in all types of organisms. Their content in Cells depends on the functional state of The Cell. For instance, in most cells, The amount of DNA ranges from 1-10% (of dry cell mass), in Muscles it is about 0.2%, while in spermatozoa this value reaches 60%. The RNA content in any given cell exceeds the DNA content by 5-10 times. The RNA/DNA ratio in Tissues with an active protein-synthesizing system ranges from 4-10, whereas in tissues with a moderate rate of Protein Synthesis, it is 0.3-2.5.

The bulk of cellular DNA is localized in The Nucleus (97-99%), with only about 1-3% being extranuclear DNA. Unlike DNA, RNA is distributed more evenly throughout the cell, and the functioning of these molecules is more dynamic and diverse. In Eukaryotic cells, about 11% of all RNA is found in the nucleus, nearly 15% in the Mitochondria, 24% in the Cytoplasm, and 50% in the Ribosomes.

The Molecular Weight of DNA depends on The complexity of the Organism. In Bacteria, DNA appears as a single large molecule with a mass of 2·10⁹ Da. In animals and humans, the molecular weight can reach 10¹¹ Da (Table 9.1).

RNAs have a significantly lower molecular weight than DNAs. Specifically, for mRNA it is 4·10⁴-1.2·10⁶, for rRNA it ranges from 4·10⁴ to 1.5-1.75·10⁶, and for tRNA it is 2.5·10⁴.

Class="center">Table 9.1

Size of DNA molecules from various sources