Prokaryotes

Course- B.Sc. (Agri.)

Subject- Principles of plant pathology

Unit-3

Prokaryote

A prokaryote is a single-celledorganism that lacks a membrane-boundnucleus (karyon), mitochondria, or any othermembrane-bound organelle. All theintracellular water-soluble components(proteins, DNA and metabolites) are locatedtogether in the same volume enclosed by thecell membrane, rather than in separate cellularcompartments. Prokaryotes, however, dopossess protein-based microcompartments,which are thought to act as primitiveorganelles (protein-bounded and lipid-bounded organelles). Some prokaryotes alsohave multicellular stages in their life cycles,such as myxobacteria, or create large colonies,like cyanobacteria

Fig. 1

Fig.- 2

Capsule - Found in some bacterial cells, this additional outer

covering protects the cell when it is engulfed by other

organisms, assists in retaining moisture, and helps the cell

adhere to surfaces and nutrients.

Cell Wall - Outer covering of most cells that protects the

bacterial cell and gives it shape.

Cytoplasm - A gel-like substance composed mainly of water that

also contains enzymes, salts, cell components, and various

organic molecules.

Cell Membrane or Plasma Membrane - Surrounds the cell's

cytoplasm and regulates the flow of substances in and out of

the cell.

• Pili - Hair-like structures on the surface of the cell

that attach to other bacterial cells. Shorter pili called

fimbriae help bacteria attach to surfaces.

Flagella - Long, whip-like protrusion that aids in

cellular locomotion.

Ribosomes - Cell structures responsible for protein

production.

Plasmids - Gene carrying, circular DNA structures

that are not involved in reproduction.

Nucleiod Region - Area of the cytoplasm that

contains the single bacterial DNA molecule.

Most prokaryotes reproduce asexually through a

process called binary fission. During binary fission,

the single DNA molecule replicates and the original

cell is divided into two identical cells.

Binary fission begins with the single DNA molecule

replicating and both copies attaching to the cell

membrane.

Next, the cell membrane begins to grow between the

two DNA molecules. Once the bacterium just about

doubles its original size, the cell membrane begins

to pinch inward.

A cell wall then forms between the two DNA

molecules dividing the original cell into two

identical daughter cells.

Reproduction in bacteria

Fig. 3

Reproduction

• When conditions are favourable bacteria can reproduce every 20 minutes

• In 48 hours a single bacterial cell could become culture 4000 times the mass of the earth

• Thankfully the food supply is limited and this does not happen

Binary Fission

• When a cell has almost doubled in size it will replicate its DNA and divide in half

• This produces 2 identical daughter cells

• Binary fission is a form of asexual reproduction

• Asexual reproduction does not invovle the exchange of genetic material

Conjugation

• Is a form of sexual reproduciton

• Genetic Information is exchanged between individuals

• A long bridge of proteins forms between two bacterial cells

• DNA is transferred from one cell (donor) to another (recipient)

1- Donor cell produces

pilus. 2- Pilus attaches to

recipient cell and brings

the two cells together.

3- The mobile plasmid is

nicked and a single strand

of DNA is then

transferred to the recipient

cell.

4- Both cells synthesize a

complementary strand to

produce a double stranded

circular plasmid and also

reproduce pili; both cells

are now viable donors.

Fig. 4

Spore Formation

• Bacteria form spores during unfavourableconditions

• One type is called an endospore

– Cell forms a thick internal wall the encloses its DNA

• Spores can remain dormant for months until conditions improve

Fig. 5

Fig. 6

Morphology of bacterial cellCoccus (plural cocci or coccuses) can

be used to describe any bacterium that

has a spherical shape. It is one of the

three distinct types of bacteria shapes,

the other two being bacillus (rod-

shaped) and spirillum (spiral-shaped)

cells

Spirochaetes (also spelled spirochetes)belong to a phylum of distinctivediderm (double-membrane) bacteria,most of which have long, helicallycoiled (corkscrew-shaped) cells.Spirochaetes are chemoheterotrophicin nature, with lengths between 5 and250 µm and diameters around 0.1–0.6 µm

Fig. 7

Fig. 8

Vibrio is a genus of Gram-negativebacteria possessing a curved-rod shape(comma shape), several species ofwhich can cause foodborne infection,usually associated with eatingundercooked seafood. Typically foundin saltwater, Vibrio species arefacultative anaerobes that test positivefor oxidase and do not form spores. Allmembers of the genus are motile andhave polar flagella with sheaths.

Fig. 9

Reference

Books

Plant Pathology by G N Agrios

Web Sources

http://agridr.in/tnauEAgri/eagri50/PATH171/index.html

http:// agriinfo.com

http:// wekipedia.org

Image References

Fig.1 http://en.wikipedia.org/wiki/Bacterial_cell_structure

Fig-2 http://micro.magnet.fsu.edu/cells/bacteriacell.html

Fig-3 http://media-1.web.britannica.com/eb-media//93/130893-050-5B4B3040.jpg

Fig-4 http://upload.wikimedia.org/wikipedia/commons/thumb/3/3e/Conjugation.svg/2000px-Conjugation.svg.png

Fig-5 http://images.tutorvista.com/content/cell-structure/prokaryote-and-eukaryote-cell.jpeg

Fig-6 http://3.bp.blogspot.com/-LYYiLl2sLQU/Ux34riE3DvI/AAAAAAAABy4/7LoImQQpflI/s1600/gram-bacteria1-copy.png

Fig-7http://en.wikipedia.org/wiki/Bacterial_cellular_morphologies#mediaviewer/File:Bacterial_morphology_diagram.svg

Fig-8http://upload.wikimedia.org/wikipedia/commons/thumb/6/69/Bacterial_morphology_diagram.svg/2000px-Bacterial_morphology_diagram.svg.png

Fig-9

http://upload.wikimedia.org/wikipedia/commons/thumb/d/db/Vibrio_cholerae_01.jpg/240px-Vibrio_cholerae_01.jpg