3.2.10 Adaptation and selection are major components of evolution

AntibioticsAntibiotics

Antibiotics are chemicals that inhibit the growth of bacteria and fungi. Some common antibiotics you may have heard of are penicillin, amoxycillin and ampicillin. There are many ways that antibiotics work but you need to know that they inhibit the formation of the bacterial cell wall.

Without the ability to form a cell wall bacteria are susceptible to osmotic lysis. This is where the contents of the bacterial cell have a lower water potential than the surroundings so water enters the cell. The cell wall would normally keep the cell from bursting without it water enters the cell unchecked causing it to burst.

Genetic variation in bacteria

MRSA stands for methicillin resistant staphylococcus aureus. This means that the bacterium (staphylococcus aureus) is resistant to standard antibiotics and is thus difficult to break down. MRSA posseses plasmids (small circular DNA elements) that have genes for antibiotic resistance contained on them. This enables them to be able to synthesise the cell wall in the presence of antibiotics.conjugation

Plasmids are aquired by a process called conjugation. Conjugation is an important mechanism for allowing bacteria to adapt to changes in their environment. You can see in the picture to the right that the bacteria have formed a bridge between each other through which they can exchange genetic information. In this manner advantageous genes are able to become established and spread rapidly through a population of bacteria.

This is an example of horizontal gene transmission.

In larger organisms genes are passed from parent to offspring – vertical gene transmission. Bacteria accomplish this when they divide by binary fission to reproduce themselves.

How does antibiotic resistance arise? Mutations to DNA of bacteria are responsible for the formation of new genes that have not existed before. Overuse of antibiotics in the environment causes the antibiotic resistance gene to confer an advantage to any bacterium that posseses the gene. Any cells that don’t posses the gene are likely to be killed by the antibiotics leaving the resistant individuals untouched. Subsequent generations will be made up of resistant cells only.

This is what the exam board say you need to be able to do:

• apply the concepts of adaptation and selection to other examples

• evaluate methodology, evidence and data relating to antibiotic resistance

• discuss ethical issues associated with the use of antibiotics

• discuss the ways in which society uses scientific knowledge relating to antibiotic resistance to inform decision-making.

Here is the powerpoint we used in the lesson: Genetic_variation_in_bacteria

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