Flashcards in Adaptation and Selection (UNIT 2) Deck (31):
organisms adjust to suit the changing environment in which they live through the process of natural selection.
What does adaptation increase in a species and how?
The long term reproductive success of a species, by helping its members to survive long enough to breed.
What are the two ways in which changes in DNA occur?
-Changing in the quantity or structure of the DNA of an organism. MUTATION
-Recombining existing DNA of two individuals. Occurs during sexual reproduction.
Change in the quantity or structure of the DNA of an organism that results in different characteristics.
What are the two ways by which bacteria change their DNA to increase diversity?
What can a difference in the base sequence of a DNA molecule result in?
A different sequence of amino acids being coded for. Which will lead to a different polypeptide, and hence a different protein, or no protein at all, being produced.
If protein is an enzyme it is likely to disrupt the metabolic pathway leading to the production of other substances, incl. proteins.
As proteins are responsible for the characteristics of an organism, it follows that changes in DNA are likely to alter an organism characteristics.
When does conjugation occur?
When one bacterial cell transfers DNA to another bacterial cell.
Describe the process of conjugation.
-One cell produces a thin projection that meets another cell and forms a thin CONJUGATION TUBE between the two cells.
-The donor cell 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 brief, leaving only time for a portion of the donor's DNA to be transferred.
-In this way the recipient cell acquires new characteristics from the donor cell.
HORIZONTAL GENE TRANSMISSION
In conjugation, DNA in the form of genes can be passed from one species to another species.
VERTICAL GENE TRANSMISSION
When genes are passed down from one generation to the next generation of the same species.
Substance produced by living organisms that can destroy or inhibit the growth of micro-organisms.
Why do bacterial cells not usually burst due to entry of water by osmosis?
In bacterial cells, water constantly enters by osmosis. This entry of water would normally cause the cell to burst- OSMOTIC LYSIS.
They don't burst because of the wall that surrounds them.
This wall is made of a tough material that is not easily stretched.
As water enters the cell by osmosis, the contents expand and push against the cell wall. Being relatively inelastic , the cell wall resists expansion and so halts further entry of water.
PREVENTS OSMOTIC LYSIS
How do antibiotics prevent bacteria from making normal cell walls?
Certain antibiotic kill bacteria by preventing them from forming cell walls.
They inhibit the synthesis and assembly 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.
Why are antibiotics only effective when bacteria are growing?
They inhibit the proper formation of cell walls.
Why are viruses not killed by antibiotics?
They have a different covering from bacteria.
What is bacterial resistance caused by?
Chance mutations within the bacteria.
How did a mutation cause resistance to the antibiotic penicillin?
The mutation resulted in certain bacteria being able to make a new protein. This protein was an enzyme which broke down the antibiotic before it was able to kill the bacteria. (enzyme=penicillinase)
IT IS NOT THE PRESENCE OF ANTIBIOTICS THAT CAUSE BACTERIA TO MUTATE....
Mutations occur randomly and are very rare.
Why is the overall number of mutations large despite them being rare?
There are so many bacteria around.
Are mutations always advantageous?
NO, many mutations will be of no advantage to a bacterium. Most will be HARMFUL, and the bacterium will probably die.
Very occasionally it will be advantageous.
Why does the success of a mutation only matter in certain circumstances?
Because a mutation against a specific antibiotic is only an advantage in the presence of that antibiotic.
Explain how bacteria become resistant?
If mutation is advantageous and is in the presence of the correct antibiotic:
The antibiotic will kill all the normal bacteria without resistance, but not the mutant type.
MUTANT TYPE SURVIVE AND DIVIDE.
So bacteria produced from this survivor could be of mutant type and therefore have resistance.
The resistant gene is passed from one generation to the next i.e. by VERTICAL GENE TRANSMISSION.
Consequently the resistant form is selected for rather than non- resistant form when exposed to antibiotic.
Antibiotic resistant bacteria gradually predominate the population.
The frequency of the allele for penicillin resistance increase in the population.
How can antibiotic resistance be passed on to different species of bacteria?
The allele for antibiotic resistance is carried on the small circular loops of DNA called plasmids.
Plasmids can be transferred from cell to cell by CONJUGATION.
Resistance can be transferred between species by HORIZONTAL GENE TRANSMISSION.
What is the impact of horizontal gene transmission on certain bacteria?
Can lead to certain bacteria accumulating DNA that gives them resistance to a range of antibiotics. Called SUPER-BUGS.
What does increasing use of antibiotics lead to?
The greater the chance that a mutant bacterium will gain an advantage over the normal variety.
How are antibiotic resistant bacteria identified?
Species under examination is spread on to a clear nutrient jelly in a dish called a petri dish. At the same time, small discs, each possessing a different antibiotic, are placed on the jelly. As the bacteria grow they form a cloudy covering over the whole dish. If a ring, the bacteria are not resistant, if no ring, they are.
What is a problem with antibiotic treatments for TB?
The long period over which antibiotics must be taken, often 6-9 months.
When patients are ill, they readily take the antibiotics as they want to recover.
The antibiotics initially kill the least resistant strains of Mycobacterium (bacterium that causes TB).
After a number of months the patients feel better because vast majority of bacterium have been killed.
Temptation to stop taking antibiotics.
because bacteria remaining are most resistant to the antibiotic.
These resistant strains multiply and spread to others.
There is therefore selection pressure which leads to the development of bacteria that do not respond to the antibiotic.
how have multiple- resistant strains of TB developed?
Through horizontal gene transmission. Resistant strains interchange genes for resistance with other strains, by conjugation.
How have people overcome multiple resistant/ resistant strains of bacteria?
A cocktail of 3 to 4 antibiotics is used to ensure at least one will be effective. `
Why does the prevalence of MRSA in hospitals present a particular danger? (3)
-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 transmission of infections.
-Many antibiotics are used in hospitals and, as a result, any mutant antibiotic-resistant strain of the bacterium has an advantage over non resistant strains.
Can quickly develop into multiple resistant strains.