Adaptation and Selection

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Adaptation and Selection

Genetic variation in bacteria

  • Natural selection allows organisms to adjust to suit changing environments in which they live in by adaptation. 
  • Adaptation increases the long-term reproductive success of a species by helping it survive long enough to breed.
  • Adaptation and selection are major factors of evolution.
  • Bacteria are one of the most diverse and adaptable group of organisms, due to the ability to develop resistance to anitbiotics etc.
  • Changes to an organims DNA can occur in two ways:
  • By mutation, which is changing the quantity or structure of the DNA of an organism.
  • By sexual reproduction, by recombining the existing DNA of two individuals.
  • DNA is also the genetic material of a bacteria.
  • Bacteria also increase diversity by changing their DNA, namely by mutations and conjugation.
  • Conjugation is the recombination of two individuals DNA, but is not strictly sexual reporduction.

Mutations

  • Mutations are changes in DNA that result in different characteristics.
  • Mutations can arise by one or more bases in the DNA sequence either being added, deleted or replaced by others during replication.
  • This may then lead to a different amino acid being coded for and so a different sequence of amino acids in the DNA molecule.
  • This different amino acid sequence will lead to a different polypeotide, and hence a different protein, or no protein.
  • If this protein is an enzyme, it is likely to disrupt the metabolic pathway leading to the production of other substances.
  • Changes to DNA are likelt to alter an organism's characteristics.

Conjugation

  • This involves DNA from one bacterial cell being passed onto another.
  • The process is as follows:
  • One cell produces a thin projection that meets the other cell and forms a thin conjunction tube between them.
  • The donor cell replicated the plasmid.
  • The plasmid is broken to make it linear before it passes along the tube into the replicant cell.
  • Contact between cells is brief, so only a portion of the donor's DNA can be transferred.
  • This way the recipient cell aquires new characteristics from the donor cell.
  • Horizontal gene transmission = when DNA in the form of genes is passed on from one species to another species by conjugation.
  • Vertical gene transmission = when genes are passed down from one generation of a species to the next generation of the same species.

Antibiotics

  • Antibiotics are substances produced by living organisms that can destroy or inhibit the growth of microorganisms.
  • They are usually produced by bacteria and sometimes fungi, i.e penicillin.
  • Penicillin was the first antibiotic to be used to treat infections.

How antibiotics work

  • One way is by preventing bacteria from making normal cell walls.
  • In bacterial cells, water constantly enters by osmosis, which would normally cause cells to burst by the process of osmotic lysis; but this is prevented by the cell wall that surrounds the bacteria.
  • The cell wall is made out of tough material, which is not easily stretched; therefore as water enters by osmosis, the contents expand and push against the cell wall, which resists

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