MCAT Monthly Sample MCAT Passage April 21 (Biology)

[AptarePrep]

Here is a sample Biology passage for those of you studying for the MCAT. The solutions will be posted next week.

Auxotrophy is the inability of an organism to synthesize a particular organic molecule required for growth. An auxotroph is an organism that demonstrates auxotrophy (adj. auxotrophic). The opposite of auxotrophy is prototrophy. Prototrophy is characterized by the ability to synthesize all the compounds needed for growth. An auxotroph is not able to grow on minimal media. Instead, an auxotroph will only be able to group on media supplemented with the organic molecule they’re unable to synthesize.

Escherichia coli strain 679-680 will only grow on minimal media if it is supplemented with threonine and leucine. Escherichia coli strain Y44 will only grow on minimal media if it is supplemented with arginine and methionine.

In the first of three experiments, when the two strains are grown together in liquid culture supplemented with threonine, leucine, arginine, and methionine and then subsequently plated on unsupplemented minimal media, colonies of E. coli grow.

In a second experiment, strain 679-680 and Y44 were grown in a U-tube (strains are kept separate from each other by a fine-pored filter) containing liquid culture supplemented with threonine, leucine, arginine, and methionine. After incubation in the U-tube, when the strains are plated on unsupplemented minimal media colonies do not grow.

A third experiment involved an additional strain of E. coli, strain III. Strain III did not require supplementation to grow on minimal media. When this third strain was grown in a U-tube with Y44, after incubation, Y44 cells plated on minimal media with arginine would grow. When plated on minimal media supplemented with methionine, Y44 cells from the U-tube culture did not grow.

Questions

1) Which is the most likely explanation for the survival of bacteria grown on minimal media after 679-680 and Y44 strains were combined?

A. Cell fusion occurred.

B. Conjugation occurred

C. Transduction occurred

D. Transformation occurred.



2) Of the structures listed, which is most likely be present for 679-680 and Y44 strains to exchange DNA with each other?

A. Pili

B. Porin proteins

C. Cytoskeleton

D. Capsid proteins



3) Of the following, which is a likely explanation for the survival of strain Y44 grown on minimal media supplemented with arginine?

I. Y44 could synthesize its own methionine
II. Arginine was no longer toxic to Y44
III. Strain III secreted proteins that allowed Y44 to survive

A. I only

B. II only

C. III only

D. I, II, and III



4) In prokaryotes, genetic transfer is characterized by ___.
I. Being unidirectional
II. The inability to form true diploids



A. I only

B. II only.

C. I and II

D. Neither I nor II

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[AptarePrep]

Here are the solutions. Look for a new passage early next month.

Question 1) In the first of three experiments, physical contact between the two strains is not restricted and growth is observed when cells are subsequently plated on minimal media. However, when kept physically separate from each other, but media can be exchanged, they do not grow (second experiment). These two experiments together suggest that strain Y44 and strain 679-780 require physical contact to exchange genetic information, therefore conjugation is the most accurate answer.

For a bit of review:

Bacteria undergo binary fission to make exact copies of themselves. However, there are ways for bacteria to obtain additional DNA. Three ways that bacteria can exchange DNA include:

1. Conjugation: The process by which one bacterium (the donor) transfers genetic material to another (the recipient) through direct contact. The exchange of genetic information between two existing cells can be referred to as horizontal gene transfer. The donor bacterium carries a DNA sequence called the fertility factor, or F-factor. The F-factor allows the donor to produce a thin, tube-like structure called a pilus, which will enable the donor to directly contact the recipient. The pilus draws the two bacteria together and genetic information is transferred to the recipient (typically in the form of a plasmid). The genetic material transferred during conjugation often provides the recipient bacterium with some sort of genetic advantage. For instance, in many cases, conjugation serves to transfer plasmids that carry antibiotic resistance genes. In other cases, the recipient may receive a gene that now allows the organism to produce a toxin, allowing the cell to become virulent.
2. Transduction: the process by viruses (bacteriophages: viruses which infect bacteria) transfer genetic material from one bacterium to another. During the viral cycle, a virus infects a host bacterium and multiplies within the bacterium. Before release, the virus may remove a portion of the host cell's bacterial DNA and then later transfer it into the next host cell. Scientists have taken advantage of the transduction process to stably introduce genes of interest into various host cells using viruses.

There are two types of transduction: generalized and specialized. In generalized transduction, the bacteriophages can pick up any portion of the host's genome. In contrast, with specialized transduction, the bacteriophages pick up only specific portions of the host's DNA.

1. Transformation: Transformation is a process by which foreign genetic material is taken up by a cell. The process results in a stable genetic change within the transformed cell.

Question 2) The correct answer is pili.

Porin proteins: These proteins are contained in the outer membrane of Gram-negative bacteria. They mediate the diffusion of small hydrophilic molecules through the outer membrane.
Cytoskeleton: The prokaryotic cytoskeleton is the collective name for all structural filaments in prokaryotes. Advances in visualization technology and structure determination led to the discovery of prokaryotic filaments in the early 1990s. Analogues for all major cytoskeletal proteins in eukaryotes been found in prokaryotes, and cytoskeletal proteins with no known eukaryotic homologues have also been discovered. Cytoskeletal elements play essential roles in cell division, protection, shape determination, and polarity determination in various prokaryotes.
Capsid proteins: Capsid proteins create a shell around the genetic material of a virus.



Question 3) I Only.

The most likely explanation for the survival of strain Y44 grown on minimal media supplemented with arginine is that Y44 could synthesize its own methionine after growing in a U-tube with 679-680.

Arginine was no longer toxic to Y44 is incorrect. Arginine was never toxic to Y44. Rather, Y44 could not synthesize arginine and therefore needed a supplement in the media.

Strain III secreted proteins that allowed Y44 to survive is incorrect. In order for Y44 to be able to grow on medial subsequent to U-tube growth, the strain would need to have acquired material that it could pass on as it divided.

For a little more background:
The U-tube experiment is used to demonstrate whether physical contact between bacterial cells is necessary for genetic transfer. A U-shaped tube is used. In one arm of the U, a suspension of a bacterial strain unable to synthesize particular nutrients (e.g., arginine in this example). A strain genetically unable to synthesize different required metabolites (e.g., methionine in this example) is placed in the other arm. Liquid may be transferred between the arms by the application of pressure or suction, but bacterial cells cannot pass through the center filter. After several hours of incubation, the cells are plated on minimal media to see if colonies will grow. If colonies grow, that implies that genetic information was transferred from one organism to the other which rescues its inability to synthesize a certain metabolite. In this passage, strain 679-680 could synthesize arginine and methionine, but not threonine and leucine. Strain Y44 could synthesize threonine and leucine, but not arginine and methionine. The observation that Y44 can grow in the absence of methionine after incubation in the U-tube with 679-680 suggests that genetic information was passed from strain 679-680 to Y44. This genetic transfer would be conjugation independent as the two strains would not be able to come in direct contact with each other. The filter between the two arms allows solution to pass through, but not the bacterial cells. Viral transduction could explain how strain Y44 received the ability to synthesize methionine. A lysogenic bacteriophage could pass through the filter and introduce the necessary genetic information. To learn more about U-tube experiments, read about the work of 1) Davis and U-Tube experiments, 2) Lederberg and Tatum, and 3) Zinder and Lederberg.





Question 4) The correct answer is I and II.

Genetic transfer in prokaryotes is unidirectional. There is a donor cell (often referred to as F++; the organism that can synthesize a pilus for conjugation) and a recipient (F−−).

During genetic transfer between prokaryotes, fragments of a chromosome or a complete plasmid may be transferred to a recipient cell. However, to be a true diploid, a cell must have two complete sets of chromosomes. This does not occur.

 

[jhmmd]

^^This looks strangely familiar lol thank you for the review