MCAT Biology : Biology

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #1 : Other Microbiology Concepts

All of the following are examples of asexual reproduction, except __________.

Possible Answers:

parthenogenesis

fission

induction

budding

Correct answer:

induction

Explanation:

Organisms have the ability to reproduce in various ways outside of sexual reproduction. Mechanisms of asexual reproduction include binary fission, budding, regeneration, and parthenogenesis. Binary fission refers to mitotic division, leading to independent daughter offspring. Budding refers to the growth of a viable offspring from the main organism, before becoming mature and detaching. Regeneration refers to the ability to reproduce from a severed appendage. Parthenogenesis refers to development of an embryo without being fertilized.

Induction is involved in cell-cell communication during embryogenesis, causing one group of cells to change an adjacent group's behavior. It is not a method of asexual reproduction, and is therefore the correct answer. 

Example Question #83 : Microbiology

Which of the following can be considered extra-genomic DNA in eukaryotic cells?

Possible Answers:

Cytosolic chromosomes during mitosis

DNA in the nucleolus

DNA in the nucleus after envelope breakdown

Mitochondria DNA

Introns

Correct answer:

Mitochondria DNA

Explanation:

Extra-genomic DNA is DNA that does not originate from the nuclear genome. DNA from mitochondria is passed down from generation to generation from the mother to the offspring. It is considered extra-genomic because the genome for mitochondria comes from the actual cell constituent rather than the nucleus of the cell.

Other examples of extra-genomic DNA are plasmids and viral genomes that may become incorporated during the lysogenic viral cycle.

Any chromosomal DNA is considered genomic, and is contained in the organismal genome.

Example Question #84 : Microbiology

The cell is the most basic functional unit of life.  Everything that we consider to be living is made up of cells, and while there are different kinds of cells, they all have some essential features that link them all together under the category of "life."  One of the most important parts of a cell is the membrane that surrounds it, seperating it from the rest of the environment.

While organisms from the three main domains live in incredibly different environments, they all possess similar cell membranes.  This phospholipid bilayer protects the cell, giving it a way to allow certain things in while keeping other things out.  Though organisms from different domains have different kinds of fatty linkages in their membranes, they all serve this essential purpose.

Membranes contain all kinds of essential proteins and signal molecules that allow the inside of the cell to respond to the outside of the cell.  In a multicellular eukaryote, this ability can be used to allow cells to communicate.  In a bacterial colony, an extracellular signal could be used to signal other bacteria.  Signals cascade through a series of molecular pathways that go from the outside of the cell all the way to the nucleus and back out again, giving the cell control on a genetic level.  This allows cellular responses to be quick and effective, and it also allows the cell to control how long it stays in that state.

What characteristic of a cell could a scientist use to determine what domain to which an unknown cell belonged?

Possible Answers:

If the genetic material in the cell's nucleus was DNA or RNA

The presence of a cellular membrane

The presence of a cell wall

The kind of membrane fatty linkages

The environment of the cell

Correct answer:

The kind of membrane fatty linkages

Explanation:

The passage tells us that the type of fatty linkages in cell membranes can tell us what domain an organism belongs to.  For example, bacteria have ester linkages while archea have ether linkages.  The other answers are wrong for a few reasons.  All cells have DNA as their primary genetic material.  Viruses have RNA sometimes, but they aren't considered alive.  Both bacteria and plants have cell walls, so that won't tell us for sure what the domain is.  As stated in the passage, all cells have cellular membranes.  While the environment can tell us a bit about the cell, the passage doesn't say it is a sure way to know, so we can't make an assumption based on that.

Example Question #3 : Other Microbiology Concepts

What should a scientist look for if he is trying to determine whether a certain organism is a bacteria rather than a virus?

Possible Answers:

Whether the organism has a nucleus

Whether the organism causes disease

Whether the organism is smaller than a eukaryotic cell

Whether the organism contains genetic material

Whether the organism can be killed by antibiotics

Correct answer:

Whether the organism can be killed by antibiotics

Explanation:

Bacteria can be killed by antibiotics, whereas viruses cannot. 

Both bacteria and viruses have genetic material, can cause disease, and are smaller than eukaryotic cells. Neither of them has a nucleus.

Example Question #4 : Other Microbiology Concepts

Which of the following leukocytes is not a participant in the innate immune response?

Possible Answers:

Basophils

T-lymphocytes

Neutrophils

Macrophages

Correct answer:

T-lymphocytes

Explanation:

T-lymphocytes are members of the adaptive immune system, and are only activated after the innate immune system fails to eliminate the antigen. The adaptive immune system is a more specified response to the antigen, requiring previous exposure to the foreign pathogen and involving the secretion of antibodies. In contrast, the innate immune response does not require previous exposure or involve antibody release.

Example Question #5 : Other Microbiology Concepts

The following processes occur both in eukaryotes and prokaryotes except for which answer choice?

Possible Answers:

Oxidative phosphorylation

mRNA processing

Regulation of intracellular osmoles

Regulation of transcription

Correct answer:

mRNA processing

Explanation:

Both bacteria and eukaryotic cells regulate their cytosolic osmoles in order to both maintain the integrity of their cell membranes and to maintain proper functioning of cytosolic proteins. Although bacteria have the added protection of a cell wall, they still rely on ion gradients to regulate the uptake and release of nutrients. Both regulate transcription using transcription factors and undergo aerobic respiration. Bacteria do NOT process their mRNA. Rather, it can be transcribed and translated simultaneously.

Example Question #1 : Natural Selection

Which of the following is an example of natural selection?

I.  Horses are bred for strength and endurance, and over time, the population of horses is more robust.

II.  A late spring storm kills all the young plants in a region, but they are spared outside the storm zone.

III.  Ancient ancestors of giraffes instinctively wanted to have longer necks to reach food higher in the trees, leading to the present appearance of giraffes.

IV.  A flower that happens to be more attractive to pollinators is more likely to have reproductive success. 

V.  A mutation of a bacterium caused by exposure to ultraviolet light causes the originally red colonies to be yellow instead.

Possible Answers:

I

III

II

IV

V

Correct answer:

IV

Explanation:

It is always difficult to rephrase "survival of the fittest" in some new, clever way. The flowers which BY CHANCE have developed a different color, pattern, or odor that better attracts pollinators are indeed more likely to experience reproductive success and pass on these genes to their offspring. Competing plants might do well for a while, but they are already disfavored, and further environmental changes may put them even more at risk (or have no effect, or again favor them over the presently more attractive plants).

The horse choice is an example of intentional breeding—artificial selection.

The storm option does not imply any condition in any of the plants which conferred an advantage against freezing to death, or even any difference between species of plants; it is more akin to a question about mass extinction than to one about evolution.

The giraffe choice relates to the Lamarckian fallacy of being able to pass on acquired characteristics; species that are more successful just plain "luck out" relative to environmental stresses.  

The bacterial response discusses a mutation without likely survival implications for the bacterium.

Example Question #2 : Natural Selection

Vertebrates are evolutionarily adapted to terrestial life. Which one of the following adaptations is LEAST likely to contribute to this land-based predominance?

Possible Answers:

Internal lungs

Short loops of Henle

Impermeable outer skin

Internal fertilization

Development of legs

Correct answer:

Short loops of Henle

Explanation:

Vertebrates have adapted to terrestial living due to their ability to maintain water inside their bodies, despite no longer being immersed in water. The loop of Henle in the nephrons is designed to concentrate urine, reabsorbing water without unnecessarily excreting it. The longer the loops descend into the medulla, the more concentrated the urine becomes. Shorter loops would not concentrate urine as much, and thus would not contribute to a vertebrate's adaptation to land-based life.

Internal lungs, impermeable skin, and internal fertilization would all protect vital processes from interference by the external environment.

Example Question #1 : Evolution

Members of a species of red fox have teeth of varying sharpness. Foxes with very sharp teeth are able to kill large prey for food, while foxes with very dull, strong teeth are able to crush eggs and small animals. Foxes with teeth of medium sharpness, however, cannot get food and many die before they are able to reproduce. Over time, the fox population shows a greater proportion of individuals with either very sharp or very dull teeth. Which type of natural selection best describes this situation?

Possible Answers:

Disruptive selection

Directional selection

Stabilizing selection

Artificial selection

Vestigial selection

Correct answer:

Disruptive selection

Explanation:

In this scenario, the two extreme phenotypes are selected for, while intermediate phenotypes are selected against. This is disruptive natural selection. Over time, disruptive selection results in a decreased frequency of "middle" phenotypes and an increased frequency of the two groups at the extreme ends. This is a process that can eventually lead to speciation.

The opposite is process stabilizing selection, in which the extreme variations are selected against in favor of more "average" phenotypes. Directional selection occurs when only one end of the spectrum is favored, such as sharp teeth but not dull teeth. Artificial selection involves human intervention in selecting desirable traits. Vestigial selection is not a type of natural selection.

Example Question #4 : Natural Selection

In a certain species of feline, all males are much larger than females. Members of either sex that are of intermediate size struggle to find mates. What principle best describes this phenomenon?

Possible Answers:

Bottleneck effect

Directional selection

Genetic drift

Stabilizing selection

Disruptive selection

Correct answer:

Disruptive selection

Explanation:

Large size is favored in males and small size is favored in females, but intermediate size is always disfavored. The result is an increase in the two extreme phenotypes (either large or small) and a decrease in the average phenotype. This type of trend is known as disruptive selection.

Stabilizing selection occurs when the extreme phenotypes are disfavored, and the average or intermediate phenotype is preferable. Directional selection occurs when only one extreme phenotype is advantageous, for example if only large felines were able to find mates. Genetic drift is the phenomenon by which the allele frequencies of a population change by chance, due to independent assortment or other random events. The bottleneck effect occurs when an outside event, such as disease or natural disaster, diminishes the original population such that the allele frequencies of the new population differ from those of the original.

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