21st Century Guidebook to Fungi, SECOND EDITION, by David Moore, Geoffrey D. Robson and Anthony P. J. Trinci

Table of Contents

1. Chapter 4: Hyphal cell biology and growth on solid substrates
2. Chapter 5: Fungal cell biology
   1. Mechanisms of mycelial growth
   2. The fungus as a model eukaryote
   3. The essentials of cell structure
   4. Subcellular components of eukaryotic cells
   5. The nucleus
   6. The nucleolus and nuclear import and export
   7. Nuclear genetics
   8. Mitotic nuclear division
   9. Meiotic nuclear division
   10. mRNA translation and protein sorting
   11. The endomembrane systems
   12. Cytoskeletal systems
   13. Molecular motors
   14. Plasma membrane and signalling pathways
   15. Fungal cell wall
   16. Cell biology of the hyphal apex
   17. Hyphal fusions and mycelial interconnections
   18. Cytokinesis and septation
   19. Yeast-mycelial dimorphism
   20. Chapter 5 References and further reading about the cell cycle
   21. Buy a PDF of Chapter 5
3. Chapter 6: Structure and synthesis of fungal cell walls

5.3 The essentials of cell structure

Cells were discovered in 1665 by Robert Hooke who saw them in slices of cork using his seventeenth century optical microscope. Hooke coined the term ‘cell’ because he compared the compartments he saw in the cork to the small rooms in which monks lived. The word cell comes from the Latin cellula, a small room. It was another 170 years before the cell theory,  that all organisms are composed of one or more cells and all cells come from pre-existing cells, was first formulated (in 1839) by Matthias Jakob Schleiden and Theodor Schwann.

The essential life-sustaining activities of an organism occur within its cells, and almost all cells contain the genetic (hereditary) information necessary for performing and regulating those activities, and for transmitting the genetic information to the next generation of cells. Prokaryotes are cells, too, and both prokaryotic and eukaryotic cells have a membrane that envelops the cell and performs the crucial task of separating the cell from its environment. This cell membrane is also selectively permeable; that is, it regulates the chemicals that enter and leave the cell. By regulating the flow of ions, the membrane adjusts the electric potential and pH of the cell. By regulating the flow of water (directly or indirectly), the membrane adjusts the volume and osmotic potential of the cell. The material within (and enclosed by) the cell membrane is a complex mix of molecules and ions called cytoplasm. The cytoplasm also contains a variety of regions specialised to carry out one or more vital functions that are bounded by their own membranes in eukaryotes; these are generally called organelles.

There are two different kinds of genetic material: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is used for long-term information storage (although this function is served by RNA in some viruses). An organism’s genetic information is encoded in the sequence of its DNA or RNA. RNA is also characteristically used for information transport within the cell, e.g. as messenger and transfer RNAs (mRNA and tRNA), and ribosomal RNAs (rRNAs) serve essentially enzymatic functions during protein synthesis.

The genetic material in prokaryotes is organised as a simple circular DNA molecule which is packaged into the nucleoid region of the cytoplasm. This region is not separated by a membrane and this is a major distinguishing feature between prokaryotes and eukaryotes. In eukaryotes the  genetic material is distributed between different linear molecules called chromosomes which are housed in a nucleus surrounded by an organised nuclear membrane.

Prokaryotes do not have a nuclear membrane, and they also lack the membrane-bound organelles that characterise eukaryotic cells; however, ribosomes, and many of the other components of the protein synthesis mechanism, are present in both prokaryotic and eukaryotic cells, though there are some differences in detail (and some of those differences are important in providing drug targets for therapeutic antibiotics). In prokaryotes, the functions performed by specialised organelles in eukaryotes are performed by the plasma membrane (Table 1). Prokaryotic cells do have structurally distinct regions:

• cytoplasmic region contains the cell genome (DNA), ribosomes and various inclusions;
• appendages attached to the cell surface - pili (singular = pilus) are hair-like appendages responsible for attachment; motile bacteria have flagella but these are largely composed of the self-assembling protein flagellin and are not related to eukaryotic flagella (pilus and flagellum proteins of animal pathogens are major antigens, so this is a clinically-important distinction);
• cell envelope consisting of a capsule, a peptidoglycan cell wall (an important target for therapeutic antibiotics), and a plasma membrane.

Prokaryotes also frequently carry extrachromosomal (usually circular) DNA molecules called plasmids, which often carry genes conferring additional phenotypes, such as antibiotic resistance, pathogenicity, genome transfer and metabolism of exotic substrates. Plasmids are rare in most eukaryotes, but they do occur in fungi. Also, major eukaryotic organelles (e.g. mitochondria and chloroplasts) contain a genetic architecture separate from the nucleus comprising a (small) circular DNA molecule and an independent ribosome population and protein synthetic apparatus.

Table 1. Comparison of the main features of prokaryotic and eukaryotic cells
	Prokaryotes	Eukaryotes
Typical organisms	bacteria, archaea	protists, fungi, animals, plants
Typical size	about 1-5 µm	about 10-100 µm
Type of nucleus	nucleoid region; no true nucleus, no membrane enclosure (pro + karyotos = before having nucleus (literally, nuts))	true nucleus surrounded by double membrane (eu + karyotos = having true nucleus (literally, nuts))
DNA	circular DNA (usually)	linear DNA molecules (chromosomes) organised around histone proteins
RNA & protein synthesis	coupled in cytoplasm	RNA-synthesis inside the nucleus, protein synthesis in cytoplasm
Cytoplasmic ribosomes	Subunits = 50S & 30S	cytoplasmic  = subunits of 60S & 40S; organelle ribosomes more similar to prokaryotic ones
Cytoplasm structure	Cell is bounded by a membrane, very few structures internally	Cell is bounded by a membrane enclosing a cytoplasm containing many endomembranes, organelles and a cytoskeleton of microtubules and microfilaments
Cell movement	fflagella largely composed of flagellin protein	flagella and cilia contain microtubules largely made of tubulin protein; lamellipodia and filopodia containing actin
Mitochondria	none (redox electron transport takes place in the plasma membrane)	one to several dozen per cell (probably originating from endosymbiotic prokaryotes). Some eukaryotes lack mitochondria; in anaerobes mitochondria are replaced by hydrogenosomes.
Chloroplasts	none (photosynthetic electron transport takes place in folds of the plasma membrane stacked into thylakoids in cyanobacteria)	in algae and plants (probably originating from endosymbiotic cyanobacteria)
Organisation	usually single cells	single cells, colonies, higher multicellular organisms with specialised cells, tissues and organs
Cell division	binary fission (simple division)	nucleus/chromosomal division (karyokinesis) co-ordinated with cytoplasmic division (cytokinesis) in mitosis and meiosis
Table modified and adapted from the Wikipedia article entitled Cell (biology), [https://en.wikipedia.org/wiki/Cell_(biology)]

Updated July, 2019