Bacterial chromosomes consists of large DNA molecules, RNA transcripts attached to DNA by RNA polymerase, and assorted proteins the duplicate DNA, repair DNA damage, and control patterns of gene expression.

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From: Encyclopedia that Microbiology (Third Edition), 2009

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K. Drlica, A.J. Bendich, in Encyclopedia of Microbiology (Third Edition), 2009

Historical Introduction

Bacterial chromosomes were discovered much later than their eukaryotic bio counterparts, largely due to their little size. Moreover, bacter chromosomes carry out not undergo the striking metaphase condensation that provides eukaryotic chromosomes so straightforward to see. Indeed, it was not until the early on 1940s that bacteria were plainly shown to undergo spontaneous mutation and also to have actually mutable genes. At around that time, Avery and also associates discovered the chemical nature of hereditary material: extracted DNA brought a character for polysaccharide synthetic from one strain of Pneumococcus to another. At first, the result was no universally welcomed as evidence for hereditary exchange, partly since the so-called ‘transforming principle’ exerted its result after one unknown variety of steps and also partly due to the fact that Avery lacked a molecular frame for explaining how DNA could function as genetic material. In 1952, Hershey and Chase announced the phage DNA, no protein, is injected right into bacterial cells throughout infection, and a year later on Watson and also Crick listed the structural frame for DNA. In ~ that point DNA became widely accepted as the carrier of hereditary information, and a find for bacter chromosomes began.

By 1956, nucleoids, as bacterial chromosomes space called, can be seen in life cells together discrete, compact structures (for recent instance see Figure 1). Gentle exploit methods ultimately yielded large, undamaged DNA molecules; by the early on 1970s the became possible to isolate a compact type of the chromosome because that biochemical study. DNA supercoiling had actually been found in the mid-1960s, and also within a te enzymes referred to as DNA topoisomerases that introduce and also remove supercoils were found. The existence of DNA topoisomerases gave credence come the idea that chromosomal DNA is under torsional tension inside cells. Throughout the 1980s the dynamic, regulated nature the supercoiling emerged as a significant structural feature that necessary to be thought about whenever the activities of the chromosome were discussed. The advance of rapid approaches for determining nucleotide sequences led to finish sequences for many bacterial genomes in the 1990s. DNA sequence analyses brought about the conclusion the all living organisms re-superstructure a usual ancestor, and inferences might be drawn about the nucleotide sequence background of chromosomes.


Figure 1. Bacterial nucleoids. Nucleoids the Escherichia coli K-12 were visualized in a confocal scanning laser microscopic lense as developed by GJ Brankenhoff (Nature 317: 748–749, 1985). Elongated cells were obtained by expansion in broth. Climate the nucleoids were stained v the DNA-specific fluorochrome DAPI (0.1 μg ml−1) added to the development medium. Under these conditions the stain had no result on growth. The cells were observed either lively (a) or after fixation v 0.1% osmium tetroxide (b). Because the cell border is not easily visualized, it has actually been sketched in for reference (c). Multiple nucleoids to be present since these fast-growing cell contain DNA in a state of multifork replication. In live cell the nucleoid has actually a cloud-like appearance and also a smooth boundary through the cytoplasm (protuberances, if present, would certainly be smaller than 200 nm). Magnification because that panels (a) and (b) is 9000×. Picture courtesy the Dr. Conrad Woldringh, department of Molecular cell Biology, university of Amsterdam, The Netherlands.

An emerging theme is the dynamic nature of bacterial chromosomes. In terms of nucleotide sequence, enormous gene shuffling has occurred over the course of evolution. Through respect to three-dimensional structure, parts of the chromosome move to certain regions that the cell at specific times throughout the cabinet cycle, if the mass of the DNA threads with replication forks. At the level that DNA conformation, transforms can take place within minutes after alterations in cellular setting occur. These transforms are influenced, and in some situations directed, by protein contents of chromosomes.

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In the adhering to sections we sketch major concepts concerning chromosome structure. Us emphasize that a bacterial chromosome is not indistinguishable to a bacterial genome: a chromosome is a dynamic protein–RNA–DNA structure that can vary in conformation, size, DNA content, and form with expansion conditions, vice versa, a genome is the genetic information contents of the organism, its DNA sequence; a genome go not readjust with development conditions.