World Builders™                                                                    Session Four  --  Microbiology             

                      
 Asexual Reproduction in  Prokaryotes

 Prokaryotes reproduce by division, and so all the descendents of a cell are identical.

More of the Same

      The earliest cells on earth reproduced by simply dividing into two cells. This is termed asexual reproduction because the cell just divides itself into two identical cells and does not need a partner to contribute genetic material.

    This is called binary fission.

    Some bacteria can reproduce very quickly, and may even divide every twenty minutes under ideal conditions.

     Prokaryotes are much smaller and more simple than the Eukaryotes that eventually evolved from them.  

     The genetic material (the DNA)  holds the directions about how to make the cell and what the cell should do.

    Remember, life processes are chemical processes.  Everything that the cell does uses energy.  The important molecule that fuels living organisms is called ATP.  Organisms produce ATP by using photosynthesis or taking in (eating) organic molecules.   Processing ATP also takes energy, but energy is left over for respiration and reproduction.

This is how Binary Fission happens.  Note that the cells use energy when they divide.

Interphase

(just living)

The cell has reached close to maximum size.
The cell has a good supply of ATP (fuel).

Temperature favors cell chemistry.
Nutrients are available in the environment

DNA Synthesis The cell makes a copy of its loop of DNA.
There are now two DNA loops in the cell.

This took energy and used up some of the ATP.
The cell rests for a little while.

Separation of genetic loops DNA strands separate and attach themselves to the cell wall.
Physical division of cell begins The cell starts to divide into two cells.

Each one will have its own loop of DNA.

Two identical cells have formed. One cell becomes two identical cells.
Each cell has a loop of DNA and some
     ribosomes.

Dividing the cell took energy.
The new cells rest and produce more ATP,


Conjugation

In addition to binary fission, bacteria sometimes share genetic material by a process called conjugation. There is a special set of genes that governs this

Plasmids: small circles of DNA floating in the cytoplasm

Bacteria swimming freely in water.

Note the loop of genetic material.

Note the blue ribosomes

Note the round orange plasmids.

Plasmids are free floating little loops of DNA.
They can reproduce themselves independently.

Gotcha!

One of the  bacteria sends out a "grappling hook" and catches the other one!

This little hook is called a pilus.

The pilus draws the bacteria close to each other.,
The pilus becomes a tube and some of the little plasmids (small independent loops of 'DNA) cross over from one cell to the other cell.
The tube closes and the bacteria go on their way with some genetic material that they did not have before.

These genes may find their way into the main DNA loop.

This really happens!  You have heard of infections that become resistant to penicillin and that then pass their resistance to other infections and diseases.  This is how they do it!

  • Advantages:

    • Widely dispersed populations can still reproduce.

    • Cells are identical to parents and should survive well if conditions don't change.

  • Disadvantages:

    • Cells are identical to parents and so are vulnerable to the same environmental stresses.

    • The characteristics of the cells change very slowly

    • there is very little innovation in survival strategies.

    • Unchanging cells may be slow to take advantage of new energy sources.


Microbiology  Information Menu

Design Microbiology Page

Top of Page 


CELL DIVISION: BINARY FISSION AND MITOSIS.  http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookmito.html
© 1996,1997, 1998, 1999, 2000, 2002, 2003.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at eviau@earthlink.net .