Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

The Setting
The Simplest Living Organisms
The Genome

All genetic "instructions" issued to The Cell originate from DNA. Both DNA and protein molecules are structured as chains—made of NUCLEOTIDES in the former case and Amino Acids in the latter. DNA molecules are typically double-stranded, meaning they consist of two polynucleotide chains forming a double helix, with complementary bases on opposite strands pairing up to form nucleotide pairs (Fig. 2-21). It is now well established that the vast majority of genetic messages in DNA are sequences of code "words," or codons. Each codon consists of three nucleotides (or three nucleotide pairs in double-stranded DNA) and corresponds to one of the 20 amino acids that make up Proteins. The sequence of codons in DNA dictates the precise order in which amino acids must be linked during the synthesis of each of the numerous cellular proteins.

Let us assume that an "average" protein molecule is a folded chain consisting of ~300 amino acids. Consequently, the segment of the DNA molecule (a single Gene) encoding the synthesis of this protein must comprise about 900 nucleotide pairs. Factoring in some additional nucleotides that serve as spacers between genes, we find that the average number of nucleotide pairs comprising a single gene is approximately 1,000.

The genome refers to the total amount of DNA carrying the entire sum of Genetic information for a given Organism. In Bacteria, the genome is represented by a single chromosome consisting of a single double-stranded DNA molecule. In Mycoplasma arthritidis, the DNA mass is 0,5∙10⁹ daltons (mol. wt. = 0,5∙10⁹). The DNA content in E. coli is approximately 5 times greater (mol. wt. = 2,5∙10⁹).

The average Molecular Weight of a nucleotide pair (in the form of its sodium salt) is 664. Consequently, the DNA of M. arthritidis contains nucleotide pairs, which is sufficient to encode 800 different medium-sized proteins. E. coli DNA consists of 3,8∙10⁶ nucleotide pairs, allowing it to encode ∼ 4,000 different proteins. Because some DNA genes serve other Functions, the actual number of protein-coding genes is somewhat lower. The length of bacterial Chromosomes is remarkable. The distance between adjacent nucleotide pairs is 0.34 nm; therefore, the total length of the DNA molecule in the E. coli chromosome is 1.3 mm, which is 600 times greater than the length of the cell housing it. This clearly indicates that the DNA molecule is tightly packed within the cell, appearing under an Electron microscope as a dense "core" occupying only about 1/5 of the cell volume (Fig. 1-2, B, D). Each bacterial core contains a complete set of genetic "templates" and functions more or less independently. These cores are haploid, meaning they possess a single set of genes¹.

¹ Prokaryotes are typically characterized by haploid nuclei, although "partially diploid" Cells containing a double set of individual genes can form during bacterial sexual conjugation as well as under certain experimental conditions.