Principles of Biochemistry Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transfer
DNA: Structure of Chromosomes and Genes
DNA and RNA perform different functions

To begin with, for easier orientation, let us briefly review the nature, function, and cellular localization of the Major Classes of Nucleic Acids. DNA consists of extremely long polymer chains made up of many thousands of monomeric units—deoxyribonucleotides of four different types—joined together to form species-specific sequences characteristic of each Organism. DNA molecules are typically double-stranded. The chromosome of a Introduction/4.html">Prokaryotic Cell is a single, very long double-stranded DNA molecule packed into a compact nuclear Structure known as the nucleoid. Recall that in prokaryotes the genetic material is not bounded by a membrane (Section 2.4).

Eukaryotic Cells contain A large number of DNA molecules, each of which is generally much longer than the single prokaryotic DNA molecule.

In eukaryotes, DNA molecules are complexed with Proteins and organized into Chromatin fibers within a Nucleus enclosed by a complex double-membrane system. The function of DNA is to store the Genetic information required to encode the structures of all proteins and RNAs for each species, to regulate the spatial and temporal Biosynthesis of cellular and tissue components, to govern the organism's activity throughout its life cycle, and to ensure its individual identity.

Viruses (Section 2.22) also contain Selection/9.html">NUCLEIC ACIDS AS their genetic material—DNA in some, RNA in others (Table 27-1). Viral nucleic acids, which are small compared to bacterial DNA, encode specific proteins found in the Viral Particles as well as several Enzymes required for viral Replication within the host cell.

Class="center">Table 27-1. Some of the most notable representatives of viruses¹⁾

¹⁾ The host for all of the bacteriophages listed below is E. coli.

Ribonucleic acid molecules are long polymer chains composed of interconnected monomeric units called ribonucleotides. In terms of size, RNA molecules are much shorter than DNA; however, their total Abundance in most cells greatly exceeds that of DNA. Both prokaryotic and eukaryotic cells contain three principal classes of RNA: Messenger RNA (mRNA), ribosomal RNA (rRNA), and Transfer RNA (tRNA). Unlike DNA, every RNA is a single-stranded polyribonucleotide characterized by a strictly defined molecular weight, a specific nucleotide sequence, and a distinct biological function (Table 27-2). Messenger RNAs (mRNAs) serve as templates utilized by Ribosomes during the Translation of genetic information into the Amino acid sequences of proteins. The nucleotide sequence of an mRNA is complementary to the genetic message contained within a specific region of the coding DNA strand. A single Eukaryotic Cell may contain over 10⁴ different mRNA molecules, each encoding one or more distinct polypeptide chains.

Like mRNAs, tRNAs are single-stranded polyribonucleotides that fold into compact, highly specific three-dimensional structures. tRNA molecules consist of 70-95 ribonucleotides, with molecular weights ranging from 23,000 to 30,000. Each of the 20 Amino Acids found in proteins corresponds to one or more tRNAs, which bind The amino acid, deliver it to the ribosomes, and act as an "adapter" during the Translation of the genetic text encoded in the mRNA into the Amino Acid Sequence of proteins. Each tRNA contains a specific trinucleotide (triplet) called its anticodon, which is complementary to the trinucleotide sequence (triplet)—the mRNA codon—encoding the amino acid specific to that tRNA.

Ribosomal RNAs (rRNAs) are the major components of ribosomes, accounting for up to 65% of their mass. Prokaryotic ribosomes contain Three types of rRNA (Table 27-2), whereas eukaryotic ribosomes, as we will see in Chapter 29, contain four types of rRNA. rRNAs play a crucial role in the structure and biosynthetic function of ribosomes.

In eukaryotic cells, There are two additional classes of RNA: Heterogeneous nuclear RNAs (hnRNAs), which are nuclear precursors to mRNA, and Small nuclear RNAs (snRNAs), which are involved in RNA Processing.

Table 27-2. Properties of various RNAs of E. coli

¹⁾ The symbol S stands for the Svedberg unit, a measure of the Sedimentation Rate of a macromolecule during centrifugation, which depends on the size and shape of the molecule. The unit is named after Svedberg, the Swedish physical chemist who invented the ultracentrifuge.

Let us now examine the Structure of Nucleic Acids in greater detail, beginning with their building blocks—NUCLEOTIDES.