Nucleoid Nucleus Equivalent and Plasmids

The "cellular nucleus" in prokaryotes consists of a tangle of double-stranded DNA, not surrounded by a membrane and localized in the cytoplasm (Fig. 3.5). In E. coli (and probably in all bacteria), it takes the form of a single circular molecule of DNA. The genome of E. coli comprises 4.63 x 106 base pairs (bp) that code for 4288 different proteins. The genomic sequence of many bacteria is known.

The plasmids are nonessential genetic structures. These circular, twisted DNA molecules are 100-1000 x smaller than the nucleoid genome structure and reproduce autonomously (Fig. 3.6). The plasmids of human pathogen bacteria often bear important genes determining the phenotype of their cells (resistance genes, virulence genes).

Nucleoid Sem
Fig.3.2 Cocci are spherical bacteria. Those found in grapelike clusters as in this picture are staphylococci (Scanning electron microscopy (SEM)).
Plasmid Topology

Fig.3.4 Spirilla, in this case borrelia are spiral bacteria (light microscopy (LM), Giemsa stain).

Bacterial Nucleoid

Fig.3.4 Spirilla, in this case borrelia are spiral bacteria (light microscopy (LM), Giemsa stain).

— Bacteria During Cell Division

— Bacteria During Cell Division

Plasmid Topology

Fig. 3.5 The nucleoid (nucleus equivalent) of bacteria consists of a tangled circular DNA molecule without a nuclear membrane. Transmission electron microscopy (TEM) image of staphylococci.

DNA Topology in Bacterial Cells

The DNA double helix (one winding/10 base pairs) is also wound counterclockwise about its helical axis (one winding/15 helical windings). This so-called supercoiling is necessary to save space and energy. Only supercoiled DNA can be replicated and transcribed. Topoisomerases steer the supercoiling process. DNA gyrase and topo-isomerase IV are topoisomerases that occur only in bacteria. The 4-quinolones, an important group of anti-infection substances, inactivate these enzymes irreversibly.

Fig. 3.6 a Open circular form (OC). The result of a rupture in one of the two nucleic acid strands.

b Twisted (CCC = covalently closed circular), native form (TEM image).

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