Microbiology

Question 1

How do eukaryotic and prokaryotic cells differ with respect to organelles?

Answer 1

Unlike eukaryotes, prokaryotes lack a nucleus, as well as all membrane-bound organelles.

Note that membrane-bound organelles include mitochondria, lysosomes, the ER, and the Golgi apparatus, but not ribosomes. Prokaryotes do contain ribosomes, a fact that appears often on the MCAT.

Question 2

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

Answer 2

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 3

How do eukaryotic and prokaryotic organisms differ in their cellular organization?

Answer 3

Prokaryotes are always unicellular, while eukaryotes can be either unicellular or multicellular.

On the MCAT, the most common example of a unicellular eukaryote is yeast, a fungus. Most other single-celled eukaryotes are classified as protists.

Question 4

How do eukaryotic and prokaryotic organisms differ in the composition of their ribosomes?

Answer 4

Eukaryotic ribosomes are slightly larger, with a 40S and a 60S subunit combining to yield 80S. Prokaryotic ribosomes have a 30S and a 50S subunit, which combine to form 70S.

The abbreviation “S” refers to the rate at which a molecule settles in a centrifuge.

Question 5

Determine if an organism with the following traits is a prokaryote or a eukaryote:

• composed of a single cell
• linear DNA
• has mitochondria

Answer 5

This organism is a eukaryote.

Only a eukaryote would possess mitochondria, since prokaryotes lack membrane-bound organelles. Eukaryotes also have linear, not circular, chromosomes. Note that both eukaryotes and prokaryotes can be unicellular.

Question 6

Name the two prokaryotic domains.

Answer 6

Archaea and Bacteria

Question 7

Decribe the appearance of the following bacterial types: bacilli, spirilla, and cocci.

Answer 7

Bacilli are long and rod-shaped, cocci are spherical, and spirilla are helical or “spiral-shaped.”

Question 8

How would the shape of this bacterium be described?

Answer 8

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

Question 9

What polymer is the main component of bacterial cell walls?

Answer 9

Bacterial cell walls are composed of peptidoglycan.

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

Question 10

Explain the structural differences between gram negative and gram positive bacteria.

Answer 10

Gram positive bacteria have a thick peptidoglycan wall, while gram negative species have a thinner cell wall surrounded by an outer membrane.

These designations come from a staining procedure using crystal violet, a purple dye. Due to their thick cell wall, gram positive cells retain the stain and appear purple. Gram negative cells allow the stain to be washed out and appear pink.

Question 11

S. pneumoniae is a gram positive bacterial species, while S. enterica is gram negative. Using this information alone, which species would likely be less responsive to treatment with antibiotics?

Answer 11

S. enterica would be more difficult to treat.

Gram negative bacteria contain a second membrane on the outside of their cell wall, rendering them resistant to many antibiotics, including penicillin.

Question 12

What term refers to the tubular structure used for locomotion in some bacteria?

Answer 12

The tubular structure that facilitates bacterial motion is the flagellum.

Question 13

How do eukaryotic and prokaryotic flagella differ in their protein composition?

Answer 13

Eukaryotic flagella are composed of tubulin while prokaryotic flagella are made up of flagellin.

Tubulin is the protein subunit of microtubules. Specifically, each eukaryotic flagellum consists of nine pairs of microtubules surrounding two individual ones.

Question 14

Name and describe the most common form of bacterial reproduction.

Answer 14

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

Question 15

A small bacterial population is introduced to a new environment with limited resources. For this population, list the following phases in order: stationary phase, exponential phase, death phase, lag phase.

Answer 15

1. lag phase
2. exponential phase
3. stationary phase
4. death phase

Question 16

Define:

polycistronic

Answer 16

The term “polycistronic” 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 17

Name three methods of bacterial gene transfer.

Answer 17

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

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

Question 18

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

Answer 18

In transformation, 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 19

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

Answer 19

In transduction, 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 20

What is a plasmid?

Answer 20

A plasmid is 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 21

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

Answer 21

In conjugation, 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 22

At a given time, 120 bacterial cells within a population are resistant to penicillin. After 20 minutes, 300 cells are penicillin-resistant, even though no binary fission took place. What process likely occurred?

Answer 22

Some of the cells likely underwent conjugation.

While the most common example of conjugation only involves transfer of the F plasmid, the process can also result in transfer of antibiotic resistance. If no cells divided and the number of resistant cells increased, conjugation probably occurred.

Question 23

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

Answer 23

All three are examples of antibiotics.

Experiment-based MCAT passages often include bacterial strains that are resistant to one or more of these chemicals.

Question 24

A mutant strain of E. coli has the genotype Lac^-. When transferred to a plate containing both glucose and lactose, what will occur?

Answer 24

The strain will survive by consuming glucose.

A Lac^- genotype denotes a mutation in the metabolism of lactose. Normally, lactose is digested into glucose and galactose. Since glucose is present, the strain can survive on that source; however, the strain would die if this glucose were removed.

Question 25

A mutant strain of *E. coli* has the genotype Phe⁺Trp^-. When transferred to a plate containing only glucose and phenylalanine, what will occur?

Answer 25

The strain will die. ## Footnote Wild-type *E. coli* can synthesize all essential amino acids. However, a Trp^- strain has a mutation in the synthesis of tryptophan. Since no tryptophan is present on the plate, this strain will be deficient and will not survive.

Question 26

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

Answer 26

A **transposon** is a DNA sequence with the ability to excise itself from the genome and move to another location. ## Footnote In both prokaryotes and eukaryotes, transposons are located in the chromosome(s).

Question 27

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

Answer 27

Bacteria that can synthesize their own nutrients are called **autotrophs**. ## Footnote This term is not limited to prokaryotes; for example, most plants and many species of algae are autotrophic.

Question 28

Name the two main types of **autotrophs**.

Answer 28

Autotrophs can be divided into **photoautotrophs** and **chemoautotrophs**. ## Footnote Photoautotrophs use sunlight to gain energy via photosynthesis, while chemoautotrophs use the oxidation of inorganic compounds. Both groups build macromolecules from carbon dioxide.

Question 29

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

Answer 29

Bacteria that must consume external organic compounds are called **heterotrophs**. ## Footnote Like autotrophs, heterotrophs include more than simply bacteria. Humans, for example, are heterotrophic.

Question 30

Name the two main types of **heterotrophs**.

Answer 30

Heterotrophs can be divided into **photoheterotrophs** and **chemoheterotrophs**. ## Footnote 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 31

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

Answer 31

**Obligate anaerobes** die in the presence of low to moderate oxygen concentrations. As such, they can only undergo anaerobic metabolism. ## Footnote The term "obligate" means that a strain has to undergo a specific process. Obligate aerobes, then, can only use aerobic respiration.

Question 32

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

Answer 32

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

Question 33

*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 33

*E. coli* is **facultatively anaerobic**. ## Footnote 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 34

# Define: **symbiosis**

Answer 34

Broadly, **symbiosis** refers to any close relationship between two different species, including mutualism, commensalism, and parasitism. ## Footnote Bacteria in the human intestine are mutualistic, meaning that both humans and bacteria benefit from the relationship.

Question 35

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 35

The relationship between *Legionella* and humans is an example of **parasitism**. ## Footnote Unlike mutualism or commensalism, parasitism involves the harm of one species involved. Here, humans suffer while *Legionella* benefits.

Question 36

# Define: **chemotaxis**

Answer 36

**Chemotaxis** is the movement of a cell toward a beneficial stimulus or away from a toxic one. ## Footnote Both prokaryotes and eukaryotes exhibit chemotaxis. For example, human immune cells can move toward the site of an infection.

Question 37

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

Answer 37

Question 38

What is the defining characteristic of a **virus**?

Answer 38

A virus can only replicate when inside a host cell. Viral replication, transcription, and translation all require use of the host cell's machinery. ## Footnote Since they are not composed of one or more cells, viruses are not classified as living organisms.

Question 39

What macromolecule comprises the viral **capsid**?

Answer 39

The viral capsid is made from **protein**. ## Footnote In some viruses, the capsid is encased within a lipid envelope.

Question 40

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

Answer 40

The virus is a **bacteriophage**. ## Footnote A bacteriophage is a virus that infects bacterial cells, such as *E. coli*. Experiment-based passages will sometimes shorten this to "phage."

Question 41

# Define: **prophage**

Answer 41

A **prophage** is a virus that exists within the bacterial genome in a temporarily harmless state. ## Footnote Prophages are found during the lysogenic cycle, in which viral genetic material integrates into the bacterial chromosome without immediately lysing the host cell.

Question 42

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 42

The student must be wrong about **ribosomes** and the **nuclear membrane**. Viral particles do not contain a nucleus or any other organelles. ## Footnote Viruses can contain either single- or double-stranded DNA or RNA, as well as enzymes like reverse transcriptase.

Question 43

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

Answer 43

Viruses are significantly smaller than prokaryotes, which in turn are smaller than eukaryotic cells. ## Footnote Though viruses vary in size, the largest are around 250 nanometers in diameter. In comparison, bacterial cells can be several micrometers in length.

Question 44

How does a **bacteriophage** infect a host cell?

Answer 44

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. ## Footnote 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 45

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

Answer 45

Viruses that infect eukaryotes enter the cell either by **fusing with the cell membrane** or via **endocytosis**. ## Footnote Before infection can occur, the virus must identify the host cell using receptors on the cell surface.

Question 46

Explain the difference between the **lysogenic** and **lytic** cycles.

Answer 46

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

Question 47

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

Answer 47

Damaging factors such as UV light and free radicals trigger progression into the lytic phase. ## Footnote In other words, viruses often leave the lysogenic phase when outside events signify a stressful or unsafe environment.

Question 48

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

Answer 48

The virus must be in the **lytic** phase. ## Footnote The lytic phase is the state in which a virus lyses, or kills, the host cell.

Question 49

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 49

This virus is a **retrovirus**. ## Footnote 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 50

Name one common example of a retrovirus.

Answer 50

The most commonly mentioned retrovirus is the **human immunodeficiency virus**, or **HIV**. ## Footnote HIV infects helper T cells, resulting in a damaged immune system.

Question 51

Which category contains disease-causing structures that are smaller than viruses?

Answer 51

Small, basic disease-causing molecules are classified as **subviral particles**. Like viruses, they are not technically living. ## Footnote Subviral particles that could be mentioned on the MCAT include prions and viriods.

Question 52

What is a **prion**?

Answer 52

A **prion** is a disease-causing protein. As subviral particles, prions are not living, but can cause nearby proteins to fold into incorrect conformations. ## Footnote The most well-known disease caused by prions is likely mad cow disease (bovine spongiform encephalopathy).

Question 53

What is a **viroid**?

Answer 53

A viroid is a disease-causing molecule of single-stranded RNA. The smallest of the subviral particles, viroids generally infect plants and alter transcription of their genes.