Prokaryotes

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Structure of a Prokaryote

The term prokaryote means 'before the nucleus' and describes bacterial cells (of the kingdom Monera)

  • These cells do not possess a nucleus - instead the DNA is found free in the cytoplasm in a region called the nucleoid
  • These cells also lack all membrane-bound organelles - the only organelle they possess is ribosomes (not membrane-bound)
  • Bacterial cells are covered by a peptidoglycan cell wall and may be surrounded by an additional slime capsule


Structure of a Prokaryotic Cell

                                   2D Representation                                                                                                                   3D Representation


Prokaryotic Features

Cell Wall:  A rigid outer layer made of peptidoglycan that maintains shape and protects the cell from damage or bursting if internal pressure is high

Cell Membrane:  Semi-permeable barrier that controls the entry and exit of substances

Cytoplasm:  Fluid component which contains the enzymes needed for all metabolic reactions

Nucleoid:  Region of the cytoplasm which contains the genophore (the prokaryotic DNA)

Plasmid:  Additional DNA molecule that can exist and replicate independently of the genophore - it can be transmitted between bacterial species

Ribosome:  Complexes of RNA and protein that are responsible for polypeptide synthesis (prokaryotic ribosomes are smaller than eukaryotes - 70S)

Slime Capsule:  A thick polysaccharide layer used for protection against dessication (drying out) and phagocytosis

Flagella (singular flagellum):  Long, slender projection containing a motor protein which spins the flagella like a propellor, enabling movement

Pili (singular pilus):  Hair-like extensions found on bacteria which can serve one of two roles

  • Attachment pili:  Shorter in length, they allow bacteria to adhere to one another or to available surfaces
  • Sex pili:  Longer in length, they allow for the exchange of genetic material (plasmids) via a process called bacterial conjugation


Electron Micrograph of a Prokaryotic Cell (Escherichia coli)


Binary Fission

Binary fission is a form of asexual reproduction and cell division used by prokaryotic organisms

It is not the same as mitosis, there is no condensation of genetic material and no spindle formation

In the process of binary fission:

  • The circular DNA is copied in response to a replication signal
  • The two DNA loops attach to the membrane
  • The membrane elongates and pinches off (cytokinesis) forming two separate cells


The Process of Binary Fission