World Builders™                                                                    Session Four  --  Microbiology             

       Getting a Few More Genes     
         
                 DNA Exchange in Prokaryotes

 Prokaryotes sometimes share genetic material informally but they generally just divide.

Conjugation

As we saw in the page on Binary Fission, prokaryotes have a simple and accurate way to reproduce themselves.  However, although faithful replication is essential, change is sometimes needed when conditions change.

     Prokaryotes have ways to get additional genetic material from other prokaryotes.  This method is called conjugation. 

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

The cells may have plasmids, small circles of DNA, floating in their cytoplasm.  Sometimes they share their plasmids with other cells.

This is how conjugation works.

Bacteria are 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!

     There are two other ways that cells can pick up extra and  DNA.  One is transformation  In transformation the cell absorbs DNA that is floating around in its environment.  

     In transduction a cell is invaded by a virus.  A virus is even smaller than a prokaryote, but it also has DNA.  Some of the viral DNA may become part of the genome of the prokaryote or the archaean organism.  Perhaps the new DNA will help the organism to survive, perhaps it will have no effect, perhaps it will put the cell at a disadvantage   Only time will tell. 


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