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Adaptation and Selection
Genetic variation in bacteria
Throught he process of natural selection, organisms adapt, increasing long-term reproductive
success of the species by helping its members survive long enough to breed.
One of the most diverse and adaptable group of organisms is bacteria. Their adaptation to a
changing environment can be seen in their ability to develop resistance to antibiotics.
Diversity in organisms arises from changes to its DNA which occur in two ways:
Changing the quantity or structure of the DNA of an organism, known as mutation
Recombining the existing DNA of two individuals which occurs during sexual reproduction
These are changes in DNA that result in different characteristics that occur by chance and at
random. They arise in many ways, for example one or more bases in a DNA sequence may be
added, deleted or replaced by others during replication, resulting in a different sequence of amino
acids and a different polypeptide.
This occurs when one bacterial cell transfers DNA to another bacterial cell. It takes place as
One cell produces a thin projection that meets another cell, forming a thin conjugation
tube between the two cells
The donor cell replicates one of its small, circular pieces of DNA (plasmid)
The plasmid is broken to make it linear before it passes along the tube into the recipient
Contact between the two cell is brief, leaving only time for a portion of the donor's DNA to
In this way the recipient cell acquires new characteristics from the donor cell
One way in which antibiotics work is by preventing bacteria from making normal cell walls so
osmotic lysis occurs.
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Shortly after the discovery of antibiotics it was discovered that some populations of bacteria had
developed resistance to antibiotics such as penicillin. The resistance was not due to a build-up of
tolerance to the antibiotic, but a chance mutation within the bacteria which resulted in certain
bacteria being able to make a new protein. The new protein was an enzyme which broke down the
antibiotic penicillin before it was able to kill the bacteria.…read more