Genetic information in bacteria is stored as a sequence of DNA bases (Figure 1). DNA exists as a complex of two double linear strands hydrogen bonded by complementary bases, adenine paired with thymine (A–T) and guanidine paired with cytosine (G–C) (Figure 2). Each of the four bases is bonded to phospho-2′-deoxyribose to form a nucleotide. The negatively charged phosphodiester backbone of DNA faces the solvent. The orientation of the two DNA strands is antiparallel: One strand is chemically oriented in a 5′→3′ direction, and its complementary strand runs 3′→5′. The complementarity of the bases enables one strand (template strand) to provide the information for copying or expression of information in the other strand (coding strand). The base pairs are stacked within the center of the DNA double helix, and they determine its genetic information. Each turn of the helix has one major groove and one minor groove.

Fig1. A schematic drawing of the Watson-Crick structure of DNA, showing helical sugar-phosphate backbones of the two strands held together by hydrogen bonding between the bases. (Reproduced with permission from Snyder L, Champness W: Molecular Genetics of Bacteria, 2nd ed. Washington, DC: ASM Press, 2003. © 2003 American Society for Microbiology.)

Fig2. Normal base pairing in DNA. Top: Adenine thymidine (A–T) pairing; bottom: guanine–cytosine (G–C) pair. Hydrogen bonds are indicated by dotted lines. Note that the G–C pairing shares three sets of hydrogen bonds, but the A–T pairing has only two. Consequently, a G–C interaction is stronger than an A–T interaction. dR, deoxyribose of the sugar-phosphate DNA backbone.

The length of a DNA molecule is usually expressed in thousands of base pairs, or kilobase pairs (kbp). Whereas a small virus may contain a single DNA molecule of less than 0.5 kbp, the single DNA genome that encodes E. coli is greater than 4000 kbp. In either case, each base pair is separated from the next by about 0.34 nm, or 3.4 × 10−7 mm, so that the total length of the E. coli chromosome is roughly 1 mm. Because the overall dimensions of the bacterial cell are roughly 1000 fold smaller than this length, it is evident that a substantial amount of folding, or supercoiling, contributes to the physical structure of the molecule in vivo.

Ribonucleic acid (RNA) most frequently occurs in single-stranded form. The uracil base (U) replaces thymine base (T) in DNA, so the complementary bases that determine the structure of RNA are A–U and C–G. The overall structure of single-stranded RNA (ssRNA) molecules is determined by pairing between bases within the intrastrand-forming loops, with the result that ssRNA molecules assume a com pact structure capable of expressing genetic information contained in DNA.

The most general function of RNA is communication of DNA gene sequences in the form of messenger RNA (mRNA) to ribosomes. These processes are referred to as transcription and translation. mRNA (referred to as +ssRNA) is transcribed as the RNA complement to the coding DNA strand. This mRNA is then translated by ribosomes. The ribosomes, which contain both ribosomal RNA (rRNA) and proteins, translate this message into the primary structure of proteins via aminoacyl-transfer RNAs (tRNAs). RNA molecules range in size from the small tRNAs, which contain fewer than 100 bases, to mRNAs, which may carry genetic messages extending to several thousand bases. Bacterial ribosomes contain three kinds of rRNA, with respective sizes of 120, 1540, and 2900 bases, and several proteins (Figure 3). Corresponding rRNA molecules in eukaryotic ribosomes are somewhat larger. The need for expression of an individual gene changes in response to physiologic demand and requirements for flexible gene expression are reflected in the rapid metabolic turnover of most mRNAs. On the other hand, tRNAs and rRNAs—which are associated with the universally required function of protein synthesis—tend to be stable and together account for more than 95% of the total RNA in a bacterial cell. A few RNA molecules have been shown to function as enzymes (ribozymes). For example, the 23S RNA in the 50S ribosomal subunit (see Figure 3) catalyzes the formation of the peptide bond during protein synthesis.

Fig3. The composition of a ribosome containing one copy each of the 16S, 23S, and 5S RNAs as well as many proteins. The proteins of the large 50S subunit are designated L1–L31. The proteins of the small 30S subunit are designated S1–S21. (Reproduced with permission from Snyder L, Champness W: Molecular Genetics of Bacteria, 2nd ed. Washington, DC: ASM Press, 2003. © 2003 American Society for Microbiology.)

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مواضيع ذات صلة

The Eukaryotic Genome

The Sugar Arabinose and Arabinose Metabolism

A Mechanism for Induction

The DNA-binding Domain of lac Repressor

Targeting of Mitochondrial Proteins

Intra-Golgi Transport and Protein Processing

Cotranslational Protein Translocation Into the Endoplasmic Reticulum

The Isolation and Structure of Operator

The Migration Retardation Assay and DNA Looping

Repressor Binds to DNA: The Operator is DNA

Detection and Purification of lac Repressor

The Difficulty of Detecting Wild-Type lac Repressor