Adaptation and Selection (UNIT 2)

Question 1

ADAPTATION

Answer 1

organisms adjust to suit the changing environment in which they live through the process of natural selection.

Question 2

What does adaptation increase in a species and how?

Answer 2

The long term reproductive success of a species, by helping its members to survive long enough to breed.

Question 3

What are the two ways in which changes in DNA occur?

Answer 3

• Changing in the quantity or structure of the DNA of an organism. MUTATION
• Recombining existing DNA of two individuals. Occurs during sexual reproduction.

Question 4

MUTATION

Answer 4

Change in the quantity or structure of the DNA of an organism that results in different characteristics.

Question 5

What are the two ways by which bacteria change their DNA to increase diversity?

Answer 5

Mutations

Conjugation

Question 6

What can a difference in the base sequence of a DNA molecule result in?

Answer 6

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.

Question 7

When does conjugation occur?

Answer 7

When one bacterial cell transfers DNA to another bacterial cell.

Question 8

Describe the process of conjugation.

Answer 8

• 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.

Question 9

HORIZONTAL GENE TRANSMISSION

Answer 9

In conjugation, DNA in the form of genes can be passed from one species to another species.

Question 10

VERTICAL GENE TRANSMISSION

Answer 10

When genes are passed down from one generation to the next generation of the same species.

Question 11

ANTIBIOTICS

Answer 11

Substance produced by living organisms that can destroy or inhibit the growth of micro-organisms.

Question 12

Why do bacterial cells not usually burst due to entry of water by osmosis?

Answer 12

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

Question 13

How do antibiotics prevent bacteria from making normal cell walls?

Answer 13

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.

Question 14

Why are antibiotics only effective when bacteria are growing?

Answer 14

They inhibit the proper formation of cell walls.

Question 15

Why are viruses not killed by antibiotics?

Answer 15

They have a different covering from bacteria.

Question 16

What is bacterial resistance caused by?

Answer 16

Chance mutations within the bacteria.

Question 17

How did a mutation cause resistance to the antibiotic penicillin?

Answer 17

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)

Question 18

IT IS NOT THE PRESENCE OF ANTIBIOTICS THAT CAUSE BACTERIA TO MUTATE….

Answer 18

Mutations occur randomly and are very rare.

Question 19

Why is the overall number of mutations large despite them being rare?

Answer 19

There are so many bacteria around.

Question 20

Are mutations always advantageous?

Answer 20

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.

Question 21

Why does the success of a mutation only matter in certain circumstances?

Answer 21

Because a mutation against a specific antibiotic is only an advantage in the presence of that antibiotic.

Question 22

Explain how bacteria become resistant?

Answer 22

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.

Question 23

How can antibiotic resistance be passed on to different species of bacteria?

Answer 23

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.

Question 24

What is the impact of horizontal gene transmission on certain bacteria?

Answer 24

Can lead to certain bacteria accumulating DNA that gives them resistance to a range of antibiotics. Called SUPER-BUGS.

Question 25

What does increasing use of antibiotics lead to?

Answer 25

The greater the chance that a mutant bacterium will gain an advantage over the normal variety.

Question 26

How are antibiotic resistant bacteria identified?

Answer 26

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.

Question 27

What is a problem with antibiotic treatments for TB?

Answer 27

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.
BAD
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.

Question 28

how have multiple- resistant strains of TB developed?

Answer 28

Through horizontal gene transmission. Resistant strains interchange genes for resistance with other strains, by conjugation.

Question 29

How have people overcome multiple resistant/ resistant strains of bacteria?

Answer 29

A cocktail of 3 to 4 antibiotics is used to ensure at least one will be effective. `

Question 30

Why does the prevalence of MRSA in hospitals present a particular danger? (3)

Answer 30

• 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.

Question 31

Why is antibiotic resistance on the increase? (8)

Answer 31

• Used to treat minor ailments with short lived and trivial symptoms.
• Sometimes used to treat viral diseases, because although they are ineffective against the virus, they may help prevent the development of secondary bacterial infections.
• Incomplete course
• Patients stockpile unused antibiotics from previous prescriptions and then take them later in smaller doses than they should.
• Doctors accept patients demands for antibiotic treatments, even when not absolutely necessary.
• Used to treat minor ailments in domesticated animals.
• Used to prevent disease among intensively reared animals, such as chickens.
• Antibiotics are used by farmers and companies to reduce disease and hence increase productivity of animals.