Adaption and Selection.

Biology AQA new AS level unit 2, adaption and selection.

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Genetic Variation in Bacteria.

  • Adaption - organisms adjust to suit changing environment they live in through natural selection. Increases long-term reproductive success by helping members live long enough to breed.
  • Bacteria are diverse & adaptable organisms that develop resistance.
  • Diversity arises from changes in DNA which occur in 2 ways: 1. changing quantity/structure of DNA (mutation), recombining existing DNA of 2 individuals (sexual reproduction).
  • Bacteria also increase diversity by conjugation.
  • Mutations - changes in DNA lead to different characteristics. 1 or more bases may be added, deleted or replaced leading to different sequence, polypeptide and hence protein. If it's an enzyme it will disrupt production of other subs.
  • Conjugation - Occurs when 1 bacterial cell transfers DNA to another:
  • 1. 1 cell produces thin production that meets another cell & forms conjugation tube between 2 cells.
  • 2. Donor cell replicates 1 plasmid which is broken into linear shape then passes along tube. As the contact is brief only 1 portion of DNA is transferred.
  • 3. Recipient cell acquires new characteristics.
  • In conjugation genes passed from 1 species to another = horizontal gene transmission; genes passed down generations = vertical gene transmission.
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These are substances that can destroy/inhibit growth of microorganisms, mostly produced by bacteria.

They work by stopping bacteria making cell walls - they inhibit synthesis and assembly of peptide cross-linkages so walls are weakened and cannot withstand pressure that occurs when water tries to move in by osmosis. The walls are inelastic so resist expansion. Without this they are unable to stop water moving in so osmotic lysis occurs and kills bacterium. It's only effective if bacteria are growing. Resistance:

  • Due to chance mutation in bacteria.
  • Occurs with penicillin - mutation resulted in certain bacteria being able to make new protein, an enzyme that broke down AB before it could work (penicillinase).
  • Presence of AB does not lead to mutation.
  • Genefor penicillinase then passed down by vertical gene transmission. Frequency of alleles increases so resistant form becomes dominant.
  • Alleles carried on plasmids which transfer by conjugation which leads to some bacteria accumulating DNA with resistance.
  • To identify resistant form place in petri dish with different antibiotics, as bacteria grow they form a cloudy covering.
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Antibiotic Resistance.

Resistance and TB:

  • Need to be taken for long period and initially destroy least resistant strains.
  • As vast majority killed patient feels better, if they stop taking them then only the most resistant bacteria remain which multiply and spread.
  • Strains can interchange genes for resistance by conjugation.
  • Cocktail of drugs used so at least 1 will be effective.

Resistance and MRSA:

  • Staphylococcusis a bacterium in many people's throats, staph. aureus is a strain that gives only minor symptoms in healthy people. Treated with antibiotics.
  • MRSA (methicillin-resistant staphylococcus) - any strain of bacterium resistant to at least 1 of the antibiotics.
  • Prevalent in hospitals as people are older, sicker and weaker so more vulnerable to infection; many people close together and staff touch patients so transmit it; many antibiotics used in hospitals so any mutant resistant strain has advantage over non-mutant strain, strains can more easily develop multiple resistance.
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Implications of Antibiotic Use.

The more widely they're used, the greater the risk that resistance will develop. Resistance is increasing due to:

  • Antibiotics used for minor ailments.
  • Used to treat viral diseases as prevent possibility of secondary bacterial infections.
  • Don't always complete course.
  • Patients stockpile unused ones from previous prescriptions then take them later in smaller doses.
  • Patients demand them.
  • Used for domesticated animals.
  • Used to prevent disease in intensively reared animals.
  • Farmers use them to reduce disease and increase productivity.
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