Nucleoid

In the image you see the scheme of a procariot cell. In the center, the nucleoid (DNA) marked with the number 7.

Nucleoid, nuclear region or nuclear body is the region that contains DNA in prokaryotes. This region is irregular in shape.

In prokaryotic cells, DNA is a single molecule, generally circular and multiform (closed) and double-stranded, which is located in a sector of the cell known as the nucleoid (which means &# 34;similar to nucleus"), which does not imply the presence of a nuclear membrane. This system serves to store genetic information, in contrast to the existing system in eukaryotic cells, where DNA is stored within a double-membrane organelle called the nucleus.

Structure

The formation of a nucleoid must address two essential problems, which are: the condensation of a large DNA in the small cell space of a bacterium and the organization of the DNA in a three-dimensional shape that suits its function
Chromosome DNA is not only condensed but also functionally organized in a manner compatible with all DNA processes: replication, recombination, segregation, and transcription.
The final form of the nucleoid has been shown to arise from a hierarchical organization of DNA. Long-range and short-range DNA-DNA connections formed within macrodomains and between macrodomains contribute to condensation and functional organization.

Nucleoid-Associated Proteins

Proteins associated with nucleoid (NAP) of the bacteria (left) in different colors. In gray bacterial DNA.

Bacteria possess a group of DNA-binding proteins known as nucleoid-associated proteins (NAPs) that are analogous in function to eukaryotic histones. These proteins allow for greater packing, without the need to increase the degree of supercoiling of the DNA.

The NAPs are characterized by: their low molecular weight and being rich in charged amino acids. Among the best known are H-NS (Histone-like Nucleoid-structuring), FIS (Factor inversion stimulation), HU (Heat Unstable protein), IHF (Integration host factor), Lrp (Leucine-responsive regulatory protein).

NAPs have the ability to bind to DNA in a specific, but also non-sequence-specific manner. As a result, NAPs are dual-function proteins of both DNA sequence and DNA structure.
The NAPs participate in the compaction of the bacterial chromosome through the following mechanisms: inducing and stabilizing the bends in the DNA; condensing DNA through a bridge that could occur between nearby or distant DNA segments; or by restraining supercoils in the DNA. The nucleoid after supercoiling is a helical ellipsoid with regions of highly condensed DNA along its long axis.

Each species is characterized by a specific set of NAPs, and only HU-like proteins are found in all bacteria.

Chromosome

In many bacteria, the chromosome is a closed double-stranded (circular) DNA molecule, which encodes genetic information in haploid form. DNA ranges in size from 500,000 to several million base pairs (bp) encoding 500 to several thousand genes.

Display

Although the appearance may change, it is clearly visible against the cytosol. Sometimes even strands that could be DNA are visible. Using the Feulgen stain, which specifically stains DNA, the nucleoid can be observed under a light microscope.
The nucleoid can be clearly visualized using high magnification electron microscopy techniques.

Composition

The nucleoid is formed by condensation and functional arrangement of a single chromosomal DNA, with the help of: chromosomal architectural proteins and RNA molecules, as well as the nucleoid itself. DNA supercoiling.
Experimental evidence shows that the isolated nucleoid is composed primarily of 80% DNA, with 10% RNA and 10% protein. These last two components act as messenger RNA and as regulatory proteins of the genome.