AQA AS Level Biology Unit 2 - Adaptation and selection

topic of adaptation and selection in unit 2 aqa as level biology

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Genetic variation in bacteria

Through the proces of natural selection organisms adjust to suit the changing environment in which they live. This is known as asaptation. Adaptation increase the long-term reproductive success of a species by helping its members survive long enough to breed. Adaptation and selection are major factors in evolution. As such, they make a significant contribution tothe diversity of living organisms.

One of the most diverse and adaptable groups of organisms is bacteria. Their adaptation to a changing environment can be seen in their ability to develop resistance to antibiotics.

DNA is the means by which genetic information is passed from generation to generation. Diversity in organism arises from change to its DNA. The occur in 2 ways:

  •  changing the quantity or sturcture of the DNA of an organism. This is known as mutation
  • recombining the existing DNA of two individuals. This occurs during sexual reproduction

As in other organisms DNA is the genetic material in bacteria. Bacteria also increase diversity by changing their DNA in a simialr manner to other organisms, namely by mutations and conjugation. Although conjugation recombines the DNA of two individuals, it is not strictly sexual reproduction

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  • Mutations are changes in DNA that result in different characteristics.
  • Mutations arise in many ways
  • For example, one or more bases in a DNA sequence may be added, deleted or replace by other during replication.
  • A triplet of bases on a DNA molecules codes for a single amino acid. Any differences in the base sequence of a DNA molecules may result in a different protein, or no protein at all, being coded for.
  • The differnet amino acid sequence will lead to a different polypeptide, and hence a different protein, or no protein at all, being produced.
  • If this protein is an enzyme it is likely to disrupt the metabolic pathway leading to the production of other substances, including proteins.
  • As proteins are responsible for the characteristics of an organism, it follows that change to DNA are likely to alter an organism's characteristics
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Conjugation occurs when one bacterial cell transfers DNA to another bacterial cell. It takes place as follows.

  • One cell produces a thin projection that meets another cell and forms a thing conjugation tube between the two cells
  • The donor vell replicates one of its small circular pieces of DNA (plasmid)
  • The circular DNA is broken to make it linear before it passes along the tube into the recipient cell
  • Contact between the cells is bried leaving only time for a portion of the donor's DNA is be transferred
  • In this way the recipient acquires new characteristics from the donor cell

In conjugation, DNA in the form of genes can be passed from one species to another species. This is known as horizontal gene transmission. Where genes are passed down from one generation of a species to the next generation of the same species, the process known as vertical gene transmission.

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Diagram of conjugation


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Antibiotics are substances produced by living organisms. Although most are produced by bacteria, a few, such as penicillin are made by fungi. The term 'antibiotic' is often used more loosely to include synthetic and semi-synthetic drugs that destroy microorganisms

Alexander Flemming 1928 - penicillin

Antibiotics have proved highly effective in the treatment of bacterial diseases.

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How antibiotics work

One way in which antibiotics work is by preventing bacteria from making normall cell walls

In bacterial cells, as in plant cells, water constantly enters by osmosis. This entry of water would normall cause the cell to burst - a process called osmotic lysis. This it doesn't burst is due to the wall that surrounds the bacterial cells. This wall is made of tough material that is not easily stretched. A water enters the cell by osmosis, the contents expand and push against the cell wall. Being relatively inelastic, the cell walla resists expansion and so haalts further entry of water. It therby prevents osmotic lysis.

Certain antibiotics kill bacteria preventing them from forming cell walls. They inhibit the synthesis and assemble of important peptide cross-linkages in bacterial cell walls. This weakens the walls making them unable to withstand pressure. As a result they are unable to prevent water entering and so osmotic lysis occurs, killing the bacterium, As these antibiotics inhibit the proper formation of cell walls, ther are only effective when bacteria are growing. Penecillin is an antibiotic that works this way. Viruses have a different covering from bacteria and so are not killed by antibiotics

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Antibiotic resistance

Shortly after the dicovery of antibiotics it became apparent that the effectiveness of some antibiotics at killing bacteria was reduced, It was found that they populations of bacteria had developed resistance to antibiotics, such as, penecillin. The resistance was due not to a build-up of tolerance to the antibiotic but to a chance mutation within the bacteria. A mutation is a change withing the DNA that results in different characteristics, usually resulting from a change to some protein. In the case of resistance to penecillin, the mutation resulted in certain bacteria being able to make a new protein. This new protein was an enzyme which broke down the antibiotic penecillin before it was able to kill the bacteria. The enzyme was given the name penicillinase.

It must be stressed that it is not the presence of antibiotics that causes the bacteria to mutate. Mutations occur randomly and are very rare. However as there are so many bacteria around the total number of mutations is large. Many of these mutations will be of no advantage to a bacterium. Indeed most will be harmful in which case the bacterium will probably die. Very occasionally a mutation will be advantageous. Even then it dependds upon the circumstances.

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For example

A mutation that leads to the production of penicillinase is only an advantage when the bacterium is in the presence of of penecillin. If this is the case then the penicillin will kill all the normal bacteria without penicillinase but not the mutant type with penicillinase. Only the mutant individual will survive and divide. This means that the bacteria produced from this survivor could be of the mutant type and will therefore be resistant to penicillin. The gene for penicillinase and hence antibiotic resistance is passesd on from one generation to the next. i.e by vertical gene transmission. Consequently the resistant form is selected for rather than the no resistant form when exposed to penicillin. These penicillin-resistant bacteria therefore gradually predominate in the population. The frequency of the allele for penecillin resistance increase in the population.

More significanly the allele for antibiotic resistance is carried on the small circular loops of DNA called plasmids. Resistance can therefore find its way into other bacterial species by horizontal gene transmission. Horizontal gene transmission can lead to certain bacteria accumulating DNA that gives them resistance to a range of antibiotics. These are so called 'superbugs'

New mutation that give bacteria resistance to antibiotics arise randomly all the time however,the more we use antibiotics the greater the change that the mutant bacterium will gain an advantage ovr the normal variety. In time, and with continued use of antibiotic, the greater is the chance that the mutant will out-compete and replace the normal variety

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Antibiotic resistance and TB

One problem for TB is the long period for which the antibiotics must be taken - often 6-9 months. When individual are ill they willingly take the antibiotics as they are keen to recover. The antibiotics initially destroy the least resistant strains of mycobacterium. After a number of months the patients feel better because the vast majority of mycobacterium have been killed. The temptation then is for patients to consider themselves cured and to stop taking the antibiotics. This is possibly the worst course of action because the few bacteria that remain are those that are most resistant to the antibiotics. The resistant strains of mycobacterium survive, multiply and spread to others. There is therefore a selection pressure that leads to  the development of strains of mycobacterium that do not respond to the antibiotic. These strains can then interchange genes for resistance with other strains by conjugation. It is in this way that multiple-antibiotic-resistant strains of TB have developed. To over come the problem a cocktail of three of four antibiotics is used to ensure that at least one will be effective. Health workers often directly observe patients taking these antibiotics in order to ensure that they complete the course of treatment.

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Antibiotic resistance and MRSA

Many people carry a bacterium belonging to the genus staphylococcus in their throats. Sraphylococcus aureus is a species that cause only minor health symptoms in healthy individuals. If its becomes a major health risk, it can be cleared up with a treatment of antibiotics. MRSA (methincillin-resistant staphylococcus aureus) is the name given to any strain of their bacterium that is resistant to one or more antibiotics, MRSA is especially prevalent in hospitals and can present a particular danger because:

  • people in hospital tend to be older, sicker and weaker than the general population making them more vulnerable to infection
  • many people live close together and are examined by doctors and nurses who have just touched other patients. This is perfect for the transmissions of infections
  • many antibiotics are used in hospitals and asa aresult any mutant antibiotic resistant strain of bacterium has an advantage over non-mutant strains. With many different antibiotics being used strains can more easily develop multiple resistance to antibiotics.

MRSA is therfore very difficult to treat not least because some strains have developed resistance to almost every known antibiotic.

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Implications of antibiotic use

The more widely antibiotics are used, the greater the risk hat resistance will develop. Antibiotic resistance is on the increase for a number of reasons:

  • Antibiotic are used to treat minor ailments whose symptoms are trivial and/or short lived
  • Antibiotics are sometimes used to treat viral diseases because although they are ineffective against viruses, the may help prevent the development of secondary bacterial infections to which patients may be vulnerable
  • Patients do not always complete the course of antibiotic as prescribed
  • Patients stockpile unused antibiotics from previous prescriptions and then take them later in smaller doses than they should
  • Doctors accept patient's demands for antibiotic treatments even wehn they aren't necessary
  • antibiotics are used to treat minor ailments in domesticated animal
  • antibiotics are used to prevent disease among intensively reared animal e.g chickens
  • farmers use them to reduce disease in animals
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