Biochemical Foundations of Human Vital Activity - Volkov N.I., Nesen E.N. 2000

Biochemical Foundations of Human Vital Activity
Biochemistry of Nucleic Acids
Structure, Properties, and Biological Role of RNA

Ribonucleic Acids are polynucleotide chains consisting of about 6,000 NUCLEOTIDES. They have a relatively small molecular weight (up to two million). The carbohydrate component of RNA is ribose. The nitrogenous bases that make up RNA are adenine, guanine, cytosine, and uracil. Unlike DNA, human RNA consists of a single polynucleotide chain with individual helical regions (Fig. 83). Double-stranded RNA molecules are found only in certain Viruses.

The Introduction/19.html">Primary Structure of RNA, much like that of DNA, is formed by a specific sequence of nucleotides alternating within the polynucleotide chain. They are linked via phosphoric acid residues (Fig. 84). Different types of RNA vary in nucleotide composition, quantity, and sequential arrangement.

The Secondary structure of RNA depends on the specific type of RNA and the functional state of The Cell. RNA molecules can adopt a more compact spatial arrangement, as base pairing leads to The formation of additional folds, which determines the Tertiary Structure of RNA. According to Holley's model, the secondary METABOLISM/34.html">Structure of Transfer RNA resembles a cloverleaf.

Cells always contain three Major Types of RNA, which differ in localization, molecular weight, nucleotide composition, structure, and biological Functions. These include Transfer RNA, Messenger RNA, and ribosomal RNA.

Transfer RNA (tRNA) accounts for 10–20% of the total cellular RNA, consists of 75–90 nucleotides, and has a molecular weight ranging from 23,000 to 30,000. Located in the Cell Cytoplasm, tRNA transports Amino Acids to the Ribosomes, where Protein Synthesis takes place. Approximately 60 different types of tRNA have been identified in cells. Each of the 20 amino acids corresponds to several distinct tRNAs.

Due to its unique cloverleaf-like structure, the tRNA molecule interacts not only with Amino Acids and enzyme Proteins, but also with mRNA on the ribosomes (Fig. 85). One end of the tRNA molecule features the CCA-OH triplet—identical across all tRNAs—which serves as the attachment site for amino acids, while the opposite end contains an anticodon region complementary to a triplet (codon) on the mRNA molecule. Using the anticodon, tRNA "recognizes" its proper placement for incorporating The amino acid into the synthesized protein chain.

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Fig. 83 Model of an RNA molecule chain

Messenger, or mRNA, accounts for 3–5% of total cellular RNA. An mRNA molecule contains up to 6,000 nucleotide residues and has a molecular weight ranging from 500,000 to 2 million. It is synthesized very rapidly (1 molecule in 25 s) and degrades quickly (within 3–5 minutes).

mRNA is synthesized on a DNA template strand—a Gene—and carries Genetic information regarding the Amino Acid Sequence of proteins from the nuclear DNA to the sites of protein synthesis, the ribosomes.

Fig. 84 Primary structure of RNA

Fig. 85 Diagram of tRNA molecule structure

Cells contain diverse forms of mRNA that direct the synthesis of thousands of different proteins, the structures of which are encoded in the specific sequence of the mRNA.

Ribosomal, or structural RNA (rRNA), makes up to 80% of total cellular RNA and has a Molecular Weight of 0.5–2 million. It resides in the ribosomes, where protein synthesis occurs, and combines with specific proteins to form the ribosomal structure while also activating the protein synthesis process.

The cellular content of RNA is 5 to 10 times greater than that of DNA. RNA is found in almost all cellular structures. The highest concentration (60–80%) is localized in ribosomes, and the lowest in the cytoplasm.