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Example Questions
Example Question #1351 : Mcat Biological Sciences
There are two models for the operation of the Golgi apparatus in eukaryotic cells. As it is difficult to visualize the operation of cells at the molecular level in real time, scientists typically rely on static electron micrographs to see the morphology of organelles. As a result, the dynamic operation of these organelles can sometimes be unclear.
Cisternal Maturation Hypothesis
In the cisternal maturation hypothesis, the cisternae of the Golgi apparatus evolve. Proteins leave the endoplasmic reticulum, and enter the cis-Golgi. The cisterna of the cis-Golgi then matures, with its enzymatic contents and internal environment changing as it becomes the medial-Golgi, and, eventually, the trans-Golgi.
In this model, the proteins never physically leave their membrane-bound cisternae during their transit across the Golgi. Instead, the entire unit of contents remains within the evolving cisternae.
Vesicular Transport Hypothesis
In contrast to the cisternal maturation hypothesis, the vesicular transport hypothesis posits that the cis-, medial-, and trans-Golgi cisternae are more static structures. Instead of evolving around their contents, the contents are physically shuttled via vesicular intermediates from each cisterna to the next.
In the case of vesicular transport, vesicles are shuttled along microtubules. Motor proteins facilitate this movement, with unique proteins being used for each direction of movement along a microtubule.
In the Golgi apparatus, mannose-6-phosphate is often added to proteins to mark them for delivery to lysosomes. Which membrane-bound structure fuses with lysosomes following uptake of large and small extra-cellular particles, respectively?
Pinosome and vacuole
Endosome and vacuole
Vacuole and endosome
Endosome and pinosome
Pinosome and endosome
Endosome and pinosome
An endosome is a specific type of intracellular vesicle formed from the uptake of large particles from the extracellular environment. Similarly, a pinosome is an intracellular vesicle formed from the uptake of small particles or fluids. Endosomes result from endocytosis; pinosomes result from pinocytosis.
These vesicles are transported to the lysosomes in the cell, where they fuse and deposit their contents into the lysosome structure for degradation by hydrolytic enzymes.
Example Question #1 : Cell Cycle
During which of the following portions of the cell cycle is DNA polymerase most active?
G1
G0
M
S
S
By referring to DNA polymerase activity, this question is asking about the portion of the cell cycle that involves DNA replication. S phase is the portion of Interphase during which the cell replicates its DNA.
Example Question #2 : Cell Cycle
During which of the following portions of the cell cycle are mRNA and proteins mainly produced?
S
G1
M
G2
G1
By referring to mRNA and proteins, this question is asking about the portion of the cell cycle that involves gene transcription and protein translation. This occurs mostly during growth and organelle replication in the G1 portion of Interphase.
Example Question #1 : Cell Cycle
Human chromosomes are divided into two arms, a long q arm and a short p arm. A karyotype is the organization of a human cell’s total genetic complement. A typical karyotype is generated by ordering chromosome 1 to chromosome 23 in order of decreasing size.
When viewing a karyotype, it can often become apparent that changes in chromosome number, arrangement, or structure are present. Among the most common genetic changes are Robertsonian translocations, involving transposition of chromosomal material between long arms of certain chromosomes to form one derivative chromosome. Chromosomes 14 and 21, for example, often undergo a Robertsonian translocation, as below.
A karyotype of this individual for chromosomes 14 and 21 would thus appear as follows:
Though an individual with aberrations such as a Robertsonian translocation may be phenotypically normal, they can generate gametes through meiosis that have atypical organizations of chromosomes, resulting in recurrent fetal abnormalities or miscarriages.
Chromosomes are often made visible using Giemsa staining. This stain shows specific banding patterns for chromosomes, and helps scientists organize them under a microscope. Considering that chromosomes are the standard unit of organization for a cell’s DNA, during which phase of the cell cycle would chromosomes most likely be visible?
G2 phase
G1 phase
S phase
M phase
Chromosomes are visible during all portions of the cell cycle
M phase
Chromosomes are most likely to be visible when a cell is organizing its genetic material, as it would just before it undergoes cytokinesis and becomes two daughter cells. S phase is a tempting answer, as this is when DNA is duplicated, but S phase only encompasses the enzymatic replication of DNA, and not its organization which is characteristic of the much shorter M phase.
Example Question #1 : Cell Cycle
Scientists use a process called Flourescent In-Situ Hybridization, or FISH, to study genetic disorders in humans. FISH is a technique that uses spectrographic analysis to determine the presence or absence, as well as the relative abundance, of genetic material in human cells.
To use FISH, scientists apply fluorescently-labeled bits of DNA of a known color, called probes, to samples of test DNA. These probes anneal to the sample DNA, and scientists can read the colors that result using laboratory equipment. One common use of FISH is to determine the presence of extra DNA in conditions of aneuploidy, a state in which a human cell has an abnormal number of chromosomes. Chromosomes are collections of DNA, the totality of which makes up a cell’s genome. Another typical use is in the study of cancer cells, where scientists use FISH labels to ascertain if genes have moved inappropriately in a cell’s genome.
Using red fluorescent tags, scientists label probe DNA for a gene known to be expressed more heavily in cancer cells than normal cells. They then label a probe for an immediately adjacent DNA sequence with a green fluorescent tag. Both probes are then added to three dishes, shown below. In dish 1 human bladder cells are incubated with the probes, in dish 2 human epithelial cells are incubated, and in dish 3 known non-cancerous cells are used. The relative luminescence observed in regions of interest in all dishes is shown below.
If the bladder cells are experiencing uncontrolled division in dish 1, they are likely spending abnormally long periods of time in which phase of the cell cycle?
S phase
M phase
G1 phase
G2 phase
G0 phase
M phase
Cell division occurs in M phase. Thus, if the cells are experiencing uncontrolled growth, they are probably remaining in M phase abnormally long.
Example Question #1351 : Mcat Biological Sciences
There are several methods for analyzing the number of cells undergoing proliferation. Cells or whole organisms can be treated with BrdU, a uracil analog, which is incorporated during DNA synthesis. Cells or tissues can be fixed and BrdU can be detected using BrdU-specific antibodies. Similarly, cells can be fixed without any pretreatment, and proliferation can be detected by antibodies specific for MKI67 or pH3 (phosphorylated histone 3). MKI67 can be detected at all active phases of the cell cycle (not interphase) while pH3 can be detected during mitosis only.
Which detection method should be used to detect the greatest number of healthy cells?
MKI67
Each method will detect the same number of proliferating cells
pH3
BrdU
BrdU
Because cells can be treated with BrdU in advance of analysis, a large window of time can be analyzed. MKI67 and pH3 only measure proliferation at a "snapshot" of time.
BrdU analysis will essentially provide a total number of cells that are capable of proliferating since the initial treatment with BrdU. Any cells that have undergone DNA replication since this addition will be detected, while quiescent or dead cells will not be detected. In contrast, MK167 and pH3 will show only cells that are in the act of proliferation when the sample is taken. Normally replicating cells that are simply in interphase will not be detected, even though they are healthy.
Example Question #1 : Cell Cycle
In 2013, scientists linked a cellular response called the unfolded protein response (UPR) to a series of neurodegenerative diseases, including such major health issues as Parkinson’s and Alzheimer’s Disease. According to their work, the unfolded protein response is a reduction in translation as a result of a series of enzymes that modify a translation initiation factor, eIF2, as below:
In the above sequence, the unfolded protein sensor binds to unfolded protein, such as the pathogenic amyloid-beta found in the brains of Alzheimer’s Disease patients. This sensor then phosphorylates PERK, or protein kinase RNA-like endoplasmic reticulum kinase. This leads to downstream effects on eIF2, inhibition of which represses translation. It is thought that symptoms of neurodegenerative disease may be a result of this reduced translation.
Which of the following times in the cell cycle is the activity of eIF2 likely to be highest?
G0
G2
M
G1
S
G1
We know that inhibition of eIF2 represses translation. The question asks for the time when eIF2 is most active, thus the time when translation is likely at its highest.
In M phase, the cell is actively dividing. That may make this a tempting answer, but G1 is actually when all of the non-genetic material is doubled. S phase sees the doubling of the genetic complement, and G2 is a major checkpoint stage to ensure everything is ready to go for mitosis.
G1 is the best answer, as this is the period when organelles and cellular proteins are being synthesized.
Example Question #1352 : Mcat Biological Sciences
During the eukaryotic cell cycle, what is the purpose of the G2/M checkpoint?
To allow for formation of the spindle fibers
To ensure that the cell is ready to divide
To ensure that all chromosomes are aligned properly for cell division
To ensure that the cell is large enough for cell division
To ensure that DNA hasn't been damaged during S phase
To ensure that DNA hasn't been damaged during S phase
The G2 phase occurs after S phase, but before M phase (mitosis). The purpose of the G2/M check point is to ensure there is no DNA damage that occurred during S phase (DNA synthesis). If damage is found, the cell will try to repair and DNA breaks. If the DNA cannot be repaired, the cell will undergo apoptosis. Many cancer suppressor genes, such as p53, are involved in this process of checking the DNA quality.
Example Question #1 : Cell Cycle
Which of the following choices will be affected by a cell containing a nonfunctional copy of the protein p53?
I. Apoptotic pathways
II. DNA repair pathways
III. Ability to arrest the cell cycle
I and III
I and II
II only
I, II, and III
I, II, and III
p53 is sometimes referred to as "the guardian of the genome" due to its huge role in suppressing the propagation of cells containing permanent DNA damage. If DNA damage is detected, p53 becomes activated and acts to promote cell cycle arrest. This gives the cell a chance to repair its genome by activating the appropriate DNA repair pathways (for which p53 is also responsible).
p53 also plays a role in promoting apoptosis. If the DNA damage is irreparable, the cell will go through apoptosis to prevent propagation of this damage.
Damage to the p53 gene can lead to unmitigated cell division and tumor formation, marking p53 as a proto-oncogene.
Example Question #4 : Cell Cycle
Which of the following factors might cause cell cycle growth arrest?
Failure of the chromosomes to line up on the equatorial plate during mitosis
Lack of appropriate growth factors
All of these answers
DNA damage
All of these answers
All of the choices are potential reasons for cell cycle arrest.
DNA damage activates pathways (commonly through the protein p53) in an attempt to repair the damage or activate apoptotic pathways if the DNA damage cannot be fixed. This causes arrest of the cell cycle at the G2 checkpoint.
Lack of appropriate growth factors will keep the cell from progressing through the cell cycle. Prolonged lack of growth is sometimes referred to as G0 of the cell cycle, and occurs when the cell cannot pass the G1 checkpoint.
Failure to properly align the chromosomes along the equatorial plate during mitosis will prevent the cell from activating pathways to degrade the cohesin that holds the sister chromatids together. This is a method to ensure proper segregation of chromosomes, and is known as the metaphase checkpoint.
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