#nucleoid

The Scientific Research Notes of S. Sunkavally (years:2002-2011).

Prokaryotes are much simpler than Eukaryotes, but with one exception: the cell envelope is more Complex!

1- Nucleoid: Not a true nucleus. The package of the DNA. They have Histone-like proteins. Prokaryotic cells are haploid, even if they have more than one nucleoid.

2- Cytoplasmic Structures: No mitochondria! No chloroplasts! Storage in the cytoplasm occurs in form of insoluble granules called Inclusion bodies. One of the most common inclusion bodies consists of poly-β-hydroxybutyric acid (PHB), a lipid-like compound consisting of chains of β-hydroxybutyric acid units connected together. Another storage product formed by prokaryotes when carbon is in excess is glycogen. PHB and glycogen are used as carbon sources. There are also granules made up of polyphosphates. These granules are sometimes termed volutin granules or metachromatic granules because they stain red with a blue dye. They are characteristic features of the corynebacteria.

3- Cell Membrane: The major functions of the cytoplasmic membrane are:

Selective permeability and transport of solutes: By passive transort, active transport, and group translocations. 

Eelectron transport and oxidative phosphorylation, in aerobic species.

Excretion of hydrolytic enzymes.

Bearing the enzymes and carrier molecules that function in the biosynthesis of DNA, cell wall polymers, and membrane lipids.

Bearing the receptors and other proteins of the chemotactic.

 4- Cell Wall: The cell wall is rigid made up mainly of peptidoglycans. It protects the bacteria from bursting! Since the concentration of solutes in the cytoplasm is huge, water is supposed to drive into the cell and cause it to lyse. However, the cell wall does the job of protection. In addition, if a part of the cell wall was destroyed, the rest of it can regenerate it again. Add to that, the cell is wall is permeable to solutes but non-selectively.

5- Capsule & Glycocalyx: Capsule or Glycocalyx is defined as the polysaccharide-containing material lying outside the cell (with one exception of Bacillus anthracis where the capsule is polypeptide). The term "capsule" is more used when speaking of a condensed tight layer. The term "slime layer" is used when we speak of a loose one. They are antigenic (K antigens).

6- Flagella: Bacterial flagella are thread-like appendages composed entirely of protein which is made up of several thousand molecules of a protein subunit called flagellin. They are the organs of locomotion for the forms that possess them. Three types of arrangement: monotrichous (single polar flagellum), lophotrichous (multiple polar flagella), and peritrichous (flagella distributed over the entire cell). They are highly antigenic (H antigens), and some of the immune responses to infection are directed against these proteins.

7- Pili (Fimbriae): Many gram-negative bacteria possess rigid surface appendages called pili (L “hairs") or fimbriae (L “fringes"). They are shorter and finer than flagella; like flagella, they are composed of structural protein subunits termed pilins. Some pili contain a single type of pilin, others more than one. In some bacteria like Neisseria gonorrhoeae these pili show antigenic variation, which means they show diverse types of antigens! Which makes it hard on the immune system to respond. Minor proteins termed adhesins are located at the tips of pili and are responsible for the attachment properties “adhesion". Two classes can be distinguished: ordinary pili, which play a role in the adherence of symbiotic and pathogenic bacteria to host cells, and sex pili, which are responsible for the attachment of donor and recipient cells in bacterial conjugation.