Microbiology

Question 1

Name the two prokaryotic domains.

Answer 1

1. Archaea
2. Bacteria

Question 2

Describe the appearance of the following bacterial types:

• bacilli
• spirilla
• cocci

Answer 2

• Bacilli - long and rod-shaped.
• Cocci - spherical.
• Spirilla - helical or “spiral-shaped”.

Question 3

How would the shape of this bacterium be described?

Answer 3

This bacterium is a bacillus, meaning that it is rod-like in shape.

Question 4

What is a plasmid?

Answer 4

A small piece of circular DNA present in a prokaryotic cell.

Plasmids are extragenomic, meaning that they exist and replicate separately from the main circular chromosome.

Question 5

What polymer is the main component of bacterial cell walls?

Answer 5

Peptidoglycan

Note that fungal cell walls are composed of chitin, while plant cell walls are composed of cellulose.

Question 6

Name and describe the most common form of bacterial reproduction.

Answer 6

Bacteria generally produce offspring via binary fission, a form of asexual reproduction.

Question 7

Name three methods of bacterial gene transfer.

Answer 7

Bacteria can transfer genes via transformation, transduction, and conjugation.

For the AP Biology exam, remember that binary fission is a mechanism of bacterial reproduction, not gene transfer. It results in an exact copy of the parent organism.

Question 8

Describe how a bacterium might acquire new genetic material via transformation.

Answer 8

A bacterium picks up genetic material from its surroundings and integrates that material into its genome.

Transformation can also be induced for research purposes. Generally, this involves the insertion of a specifically engineered plasmid, called a vector, into the cell.

Question 9

Describe how a bacterium might acquire new genetic material via transduction.

Answer 9

A virus (or bacteriophage) transfers DNA between bacteria during the process of infection.

Usually, a viral capsid will contain only viral DNA or RNA, but it can occasionally can take up host material as well. When a new bacterium is infected, the DNA from the previous host can be integrated into the new host’s genome.

Question 10

Describe how a bacterium might acquire new genetic material via conjugation.

Answer 10

One bacterium transfers a piece of genetic material to another using a bridge, or sex pilus.

The most common example of conjugation involves a piece of material called the F plasmid. An F⁺ (plasmid-containing) cell will replicate its F plasmid, then extend a pilus to transfer it to an F^- cell.

Question 11

How do eukaryotic and prokaryotic cells differ in their processes of transcription and translation?

Answer 11

Eukaryotic transcription and translation occur separately, with the first process taking place in the nucleus and the second in the cytoplasm. In prokaryotes, both processes take place in the cytoplasm and can thus occur simultaneously.

Prokaryotic DNA does not contain introns, removing the need for the post-transcriptional splicing that occurs in eukaryotes.

Question 12

What is a transposon, and where in a bacterial cell could it be found?

Answer 12

It is a DNA sequence with the ability to excise itself from the genome and move to another location.

In both prokaryotes and eukaryotes, transposons are located in the chromosome(s).

Question 13

Define:

polycistronic

Answer 13

Refers to a molecule of prokaryotic mRNA that can be translated into multiple proteins.

In contrast, eukaryotic mRNA is monocistronic, meaning that each mRNA molecule codes for a single protein.

Question 14

What do streptomycin, tetracycline, and ampicillin have in common?

Answer 14

All three are examples of antibiotics.

Experiments described on the AP Biology exam may involve bacterial strains that are resistant to one or more of these chemicals.

Question 15

What term describes bacteria that can produce their own nutrients from carbon dioxide?

Answer 15

Autotrophs

This term is not limited to prokaryotes; for example, most plants and many species of algae are autotrophic.

Question 16

Name the two main types of autotrophs.

Answer 16

• Photoautotrophs
• Chemoautotrophs

Photoautotrophs use sunlight to gain energy via photosynthesis, while chemoautotrophs use the oxidation of inorganic compounds. Both groups build macromolecules from carbon dioxide.

Question 17

What term describes bacteria that need to obtain nutrients by consuming organic compounds?

Answer 17

Heterotrophs

Like autotrophs, heterotrophs include more than simply bacteria. Humans, for example, are heterotrophic.

Question 18

Name the two main types of heterotrophs.

Answer 18

• Photoheterotrophs
• Chemoheterotrophs

Photoheterotrophs use sunlight to gain energy via photosynthesis, while chemoheterotrophs use the oxidation of inorganic compounds. However, both groups need to consume organic macromolecules to obtain carbon.

Question 19

What group of bacteria cannot survive in the presence of oxygen?

Answer 19

Obligate anaerobes die in the presence of low to moderate oxygen concentrations. As such, they can only undergo anaerobic metabolism.

The term “obligate” means that a strain has to undergo a specific process. Obligate aerobes, then, can only use aerobic respiration.

Question 20

What group of bacteria can survive in either oxygen-rich or oxygen-poor environments?

Answer 20

Facultative anaerobes can use either aerobic or anaerobic respiration, depending on the presence of oxygen.

Question 21

E. coli sometimes uses the process of fermentation, and is able to survive outside the body for a significant period of time. What term best describes the metabolism of E. coli?

Answer 21

Facultatively anaerobic

Fermentation, whether lactic acid or ethanol, is associated with anaerobic metabolism. However, if the strain can survive outside the body, it must not be killed by the presence of oxygen, as an obligate anaerobe would be.

Question 22

What is the defining characteristic of a virus?

Answer 22

A virus can only replicate when inside a host cell. Viral replication, transcription, and translation all require use of the host cell’s machinery.

Since they are not composed of one or more cells, viruses are not classified as living organisms.

Question 23

With regard to size, how does a viral particle compare to a bacterial or eukaryotic cell?

Answer 23

Viruses are significantly smaller than prokaryotes, which in turn are smaller than eukaryotic cells.

Though viruses vary in size, the largest are around 250 nanometers in diameter. In comparison, bacterial cells can be several micrometers in length.

Question 24

Label the following parts of a virus on the diagram below: capsid, nucleic acid, tail sheath, tail fibers.

Answer 24

Question 25

What macromolecule comprises the viral capsid?

Answer 25

Protein

In some viruses, the capsid is encased within a lipid envelope.

Question 26

A student concludes that a certain viral particle contains single-stranded DNA, ribosomes, a nuclear membrane, and enzymes. Assuming that the particle actually is a virus, about which of these structures must he be wrong?

Answer 26

The student must be wrong about ribosomes and the nuclear membrane. Viral particles do not contain a nucleus or any other organelles.

Viruses can contain either single- or double-stranded DNA or RNA, as well as enzymes like reverse transcriptase.

Question 27

A certain virus contains a single-stranded RNA genome. Upon infection, it transcribes this material back into DNA. What name is given to this type of virus?

Answer 27

Retrovirus

Retroviruses undergo the process of reverse transcriptase, in which they transform RNA into DNA to be integrated into a host genome. For this purpose, they contain the enzyme reverse transcriptase.

Question 28

Name one common example of a retrovirus.

Answer 28

Human immunodeficiency virus

HIV

HIV infects helper T cells, resulting in a damaged immune system.

Question 29

A certain virus infects E. coli. How would this virus be classified?

Answer 29

Bacteriophage

A bacteriophage is a virus that infects bacterial cells, such as E. coli. Experiment-based passages will sometimes shorten this to “phage.”

Question 30

How does a bacteriophage infect a host cell?

Answer 30

Using its tail fibers, the phage attaches to the outside of the bacterial cell. Genetic material is then injected through the sheath and into the bacterium.

Note that the majority of the viral particle, including the capsid, never enters the host cell! New capsid proteins are assembled in the infected host cell.

Question 31

How does an animal virus (such as those that infect humans) infect a host cell?

Answer 31

Viruses that infect eukaryotes enter the cell either by fusing with the cell membrane or via endocytosis.

Before infection can occur, the virus must identify the host cell using receptors on the cell surface.

Question 32

Explain the difference between the lysogenic and lytic cycles.

Answer 32

• In the lysogenic cycle, viral DNA is integrated into the genome of the host, remaining in a fairly harmless state.
• In the lytic cycle, the virus replicates and lyses, or kills, the host cell, releasing new viral particles.

Viruses in the lysogenic cycle, known as prophages, generally progress into the lytic cycle after certain events trigger this change.

Question 33

Define:

prophage

Answer 33

A virus that exists within the bacterial genome in a temporarily harmless state.

Prophages are found during the lysogenic cycle, in which viral genetic material integrates into the bacterial chromosome without immediately lysing the host cell.

Question 34

What factors might cause a virus to move from the lysogenic to the lytic phase?

Answer 34

Damaging factors such as UV light and free radicals trigger progression into the lytic phase.

In other words, viruses often leave the lysogenic phase when outside events signify a stressful or unsafe environment.

Question 35

If virally infected cells are dying, the virus must be in which phase?

Answer 35

Lytic phase.

The lytic phase is the state in which a virus lyses, or kills, the host cell.

Question 36

Some species of the bacterial genus Legionella live in human cells, often causing a severe lung disease. What symbiotic relationship do these organisms display?

Answer 36

The relationship between Legionella and humans is an example of parasitism.

Unlike mutualism or commensalism, parasitism involves the harm of one species involved. Here, humans suffer while Legionella benefits.