Introduction:
In both bacteria and eukaryotes, the percentage dry weight of RNA is higher than DNA though if the DNA is stretched in one cell all the way out, it would be about 2m long and all the DNA in all cells put together would be about twice the diameter of the Solar System.
DNA and RNA Composition in Cells:
In both bacteria and eukaryotes, the percentage dry weight of RNA is higher than DNA though if the DNA is stretched in one cell all the way out, it would be about 2m long and all the DNA in all cells put together would be about twice the diameter of the Solar System. Bacterial genomes vary in size from about 0.4 × 109 to 8.6 × 109 daltons (Da), some of the smallest being obligate parasites (Mycoplasma) and the largest belonging to bacteria capable of complex differentiation such as Myxococcus. The maximum amount of information that the genome can encode depends on its DNA content. The majority of bacteria have haploid genomes, which are made up of a single chromosome made of circular, double-stranded DNA molecules.
Bacterial Genome Structure and Variation:
Agrobacterium tumefaciens, a Gram-negative bacterium, has both a linear and a circular chromosome, although the majority of bacteria have circular chromosomes. Gram-positive bacteria, such as Borrelia and Streptomyces, have linear chromosomes.
Escherichia coli Genome and Replication:
The single chromosome of Escherichia coli, a typical gut bacterium, is about 4,500 kilobase pairs (kbp) in length, with a molecular mass of around 3 × 10⁹ Daltons, making up roughly 2–3% of the cell’s dry weight. Despite its actual contour length of approximately 1.35 mm—several hundred times longer than the bacterial cell—this DNA is tightly supercoiled and compacted within the nucleoid region.
Replication of the E. coli chromosome takes about 40 minutes, which is nearly twice the time required for cell division. In fast-growing bacterial populations, a new replication cycle often begins before the previous one concludes, ensuring that DNA duplication keeps pace with cell proliferation. As a result, rapidly dividing bacteria may have multiple overlapping replication cycles, leading to up to eight chromosomes being formed from four ongoing replication events at the time of cell division. This overlapping replication strategy ensures efficient genome duplication. The entire process of bacterial chromosome replication is highly intricate and involves a coordinated effort of numerous specialized proteins.
Comparative Gene Density- Bacteria vs. Humans:
About 4 million base pairs and roughly 3000 genes make up the genome of E. coli, which was sequenced in 1997. These figures are rather typical for bacteria, as the majority have genomes with a few thousand genes and a size of several million base pairs.
As a result, bacterial genomes have roughly 10% more genes than human genomes yet are just 0.1% larger. E coli has almost 1900 times of proteins and 2.4 million individual molecules. It is immediately apparent from comparing the two percentages that bacteria have a substantially higher “gene density” than humans, which is the number of genes per unit length of the genome.
In other words, bacterial DNA has between 500 and 1000 genes per million base pairs, while human DNA has an average of 10 genes per million base pairs.
Cellular Biomass Composition:
In both bacteria and eukaryotes, the percentage dry weight of RNA is higher than DNA though if the DNA is stretched in one cell all the way out, it would be about 2m long and all the DNA in all cells put together would be about twice the diameter of the Solar System. Living cell comprises different compounds and metabolites. Of these, water is the most abundant component accounting approximately 70% of cellular material. Rest of the cellular mass is dry weight comprising DNA, RNA, proteins, lipids, carbohydrates. Living cell mass of E coli is 9.5×10^-13 g and if 70% of cell is water, dry weight would be 2.8×10-13 g.
RNA Structure and Evolutionary Role:
RNA is similar to DNA consisting of long of chain of sugar linked by phosphate group. There is a cyclic base attached to each sugar and the base can pair with matching base to make double helix. This resembles DNA but helix is a bit contorted and often RNA is folded into complex structures stabilized by short helices interspersed with long single stranded loops. RNA is a highly capable genetic molecule that was once required to carry out hereditary processes independently. Before DNA appeared on the scene, RNA developed all the necessary mechanisms for storing and expressing genetic information, since it is today believed to be the first molecule of heredity. However, enzymes may readily break down single-stranded RNA due to its instability. In order to accurately transmit genetic information, DNA emerged as a far more stable form by effectively doubling the existing RNA molecule and substituting deoxyribose sugar for ribose.
Metabolic Flux Analysis and Biomass Composition
Given that cells are composed of many macromolecules and biopolymers, understanding their number and composition is crucial for calculating the metabolic fluxes of biosynthetic precursors and for any other metabolic or energetic analysis. To accurately account for the diversion of biosynthetic precursors toward biomass production, it is essential to measure both the cell’s external metabolic fluxes and the amino acid composition of the synthesized proteins when interpreting metabolic fluxes based on the measured ¹³C isotopomer distribution in proteinogenic amino acids.
For the spectrum of biological substances taken into consideration, determining the molecular composition of biomass typically involves applying various analytical procedures, each with its own sensitivity, interferences, and degree of confidence; also, some of these approaches provide redundant information. The quantitative identification of a constant biomass composition is necessary for the application of numerical approaches for metabolic flux analysis. To give the determined elemental composition, for instance, the percentages of macromolecular components that have been analyzed using various techniques should ideally sum up to 100%. Additionally, it is necessary to compute the confidence intervals for each element and component. If the biomass species being considered represent certain types of macromolecules like DNA or RNA, it is possible to calculate an average elemental composition for these species from the average content of individual building blocks. For e.g.: Average elemental composition for all proteins in an organism is calculated from amino acid content in all cellular proteins. This can be used for other microbial species since a moderate variation in amino acid content has only a small effect on overall elemental composition. Similarly average elemental composition of other macromolecules can be calculated from their average content in building blocks. Overall elemental composition of cell is used for whole biomass. Since there is significant variation in macromolecular composition with operating conditions, overall elemental composition of cell is the function of conditions at which they are grown. Experiment shows that during Nitrogen limited growth, the Nitrogen content may vary by factor of 2 depending on specific growth rate of biomass, whereas overall elemental composition remains approximately constant during Carbon limited growth although macromolecular composition of biomass may vary significantly