MCAT Biology : Cellular Processes and Functions

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #6 : Mitosis And Meiosis

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.

 

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In cancer cells, karyotype analysis shows the specific arrangement of chromosomes. Often, as cancer cells divide uncontrollably, the number of chromosomes becomes deranged. In which of the following stages of mitosis is a non-disjunction most likely to occur?

Possible Answers:

Cytokinesis

Metaphase

Anaphase

Prophase

Telophase

Correct answer:

Anaphase

Explanation:

During anaphase two chromosomes over the metaphase plate are separated, and each daughter cell gets one copy. If this happens inappropriately and separation does not occur, a nondisjunction takes place.

Example Question #4 : Understanding Cell Division, Mitosis, And Meiosis

Combrestatin is a drug that prevents the polymerization of microtubules. Which of the following processes would be most affected by combrestatin?

Possible Answers:

Muscle contraction

Mitosis

Protein synthesis

Membrane depolarization

DNA synthesis

Correct answer:

Mitosis

Explanation:

Combrestatin interferes with the formation of microtubules, which make up the cytoskeletal architecture of a cell; therefore, the correct answer choice is involved with some microtubule-based process. DNA and protein synthesis do not involve microtubules, and would not be affected by the lack thereof. Muscle contraction depends on myosin, actin, troponin, etc., and not on microtubules. Membrane depolarization involves sodium/potassium channels, neurotransmitters, etc., and is not directly affected by microtubule inhibition.

The only answer that remains is mitosis, which involves microtubules in chromosomal segregation. The mitotic spindle in this separation is primarily composed of microtubules. The polymerization and depolymerization of microtubules is crucial for mitotic division. Combrestatin therefore prevents proper mitosis.

Example Question #7 : Mitosis And Meiosis

Which of the following lists the correct sequence of mitotic events?

Possible Answers:

Telophase, prophase, metaphase, anaphase

Prophase, telophase, metaphase, anaphase

Prophase, metaphase, anaphase, telophase

Metaphase, telophase, anaphase, prophase

Correct answer:

Prophase, metaphase, anaphase, telophase

Explanation:

The mitotic sequence occurs during the M phase of the cell cycle. The process is outlined below.

Prophase—chromosomes condense

Metaphase—chromosomes align in the center of the cell

Anaphase—sister chromatids begin to separate

Telophase—separated chromosomes decondense (relax)

Cytokinesis—the cytosol of the cell completely separates and the membranes fuse shut

Example Question #1351 : Biology

During which mitotic stage do the spindle fibers begin to form from microtubules?

Possible Answers:

Anaphase

Prophase

Telophase

Metaphase

Correct answer:

Prophase

Explanation:

The fibers of the spindle apparatus function by attaching to the centromeres of chromosomes and shortening, pulling the chromosomes to opposite ends of the cell. Since chromosomes line up on the apparatus during metaphase, we know that the spindle apparatus must have begun to form during prophase. The spindle attach during metaphase, and pull the chromatids apart during anaphase.

Example Question #9 : Mitosis And Meiosis

A mouse has a mutation in which its sister chromatids are unable to separate during cell division. What phase of mitosis should researchers target, in order to alleviate the condition using drug therapy?

Possible Answers:

Telophase

Anaphase

Prophase

Metaphase

Correct answer:

Anaphase

Explanation:

During anaphase, sister chromatids are separated and pulled to polar ends of the cell. Drug therapy would best be able to target the mutation during this phase of mitosis.

Chromosomes condense and exit the nucleus during prophase. Alignment along the equatorial plate occurs during metaphase. Separation occurs during anaphase, and the nuclei begin to re-form during telophase in preparation for cytokinesis.

Example Question #1 : Understanding Chromatin

Which answer choice incorrectly matches the type of chromatin with the phase of mitosis?

Possible Answers:

Euchromatin—telophase

Heterochromatin—prophase

Heterochromatin—metaphase

Heterochromatin—interphase

Euchromatin—interphase

Correct answer:

Euchromatin—telophase

Explanation:

DNA is stored in loosely wound euchromatin before mitosis. During mitosis, the DNA condenses into chromosomes, which are made of heterochromatin. It becomes more dense during prophase, and stays that way until the end of mitosis. Euchromatin is more lightly packed than heterochromatin.

Mitosis follows the following sequence: prophase, metaphase, anaphase, telophase, cytokinesis. Interphase refers to the time period between mitotic divisions. During interphase, most DNA is euchromatin, but some regions remain as heterochromatin to prevent unwanted transcription; thus DNA exists as both types of chromatin during interphase, but only as heterochromatin during mitosis. Matching euchromatin to telophase is the answer, as this is a false statement.

Example Question #1351 : Biology

Something goes wrong during meiosis in a male and the four daughter sperm cells that are produced all have the wrong amount of chromosomes. Two of the sperm have one extra chromosome and the other two have one missing chromosome. At what step did the problem most likely occur, and what was it?

Possible Answers:

Metaphase I—nondisjunction

Telophase II—translocation

Telophase I—deletion

Anaphase I—inversion

Prophase I—trisomy

Correct answer:

Metaphase I—nondisjunction

Explanation:

If nondisjunction occurs in Metaphase I, then one extra chromosome composed of two tetrads would go into one of the cells starting metaphase II while the other would have one less. This extra chromosome would then undergo the rest of meiosis normally, leaving an extra chromosome, composed of one tetrad in two of the daughter sperm. These are the two that are missing from the other daughter sperm. 

Example Question #11 : Mitosis And Meiosis

When does genetic crossover occur during meiosis? 

Possible Answers:

Prophase I

Telophase II

Anaphase I

Metaphase I

Correct answer:

Prophase I

Explanation:

This is a simple memorization problem. Crossover occurs when the nucleus decondenses. The chromosomes are able to crossover during prophase I when chromosome pairs are aligned next to one another. Crossover cannot occur later in meiosis, as the chromosomes have already been separated.

Example Question #14 : Mitosis And Meiosis

The concept of genomic imprinting is important in human genetics. In genomic imprinting, a certain region of DNA is only expressed by one of the two chromosomes that make up a typical homologous pair. In healthy individuals, genomic imprinting results in the silencing of genes in a certain section of the maternal chromosome 15. The DNA in this part of the chromosome is "turned off" by the addition of methyl groups to the DNA molecule. Healthy people will thus only have expression of this section of chromosome 15 from paternally-derived DNA.

The two classic human diseases that illustrate defects in genomic imprinting are Prader-Willi and Angelman Syndromes. In Prader-Willi Syndrome, the section of paternal chromosome 15 that is usually expressed is disrupted, such as by a chromosomal deletion. In Angelman Syndrome, maternal genes in this section are deleted, while paternal genes are silenced. Prader-Willi Syndrome is thus closely linked to paternal inheritance, while Angelman Syndrome is linked to maternal inheritance.

Figure 1 shows the chromosome 15 homologous pair for a child with Prader-Willi Syndrome. The parental chromosomes are also shown. The genes on the mother’s chromosomes are silenced normally, as represented by the black boxes. At once, there is also a chromosomal deletion on one of the paternal chromosomes. The result is that the child does not have any genes expressed that are normally found on that region of this chromosome.

 

 

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A scientist is investigating a cell undergoing division, and notes that chromosome 15 is aligned with the other chromosomes in the center of the cell, as if along a line. Each of the 23 pairs of chromosomes is arranged as a tetrad along this plate. Which phase of cell division is this cell most likely undergoing?

Possible Answers:

Anaphase II

Anaphase of mitosis

Metaphase I

Metaphase II

Metaphase of mitosis

Correct answer:

Metaphase I

Explanation:

Metaphase I is the best answer. Because the chromosomes are arranged as tetrads, we know that it is in meiosis I. Only in meiosis I do we separate homologous chromosome pairs, rather than sister chromatids.

Additionally, because the chromosomes are aligned in the center of the cell, metaphase is the most likely phase of meiosis. Anaphase involves the separation of chromosomes, and telophase is the terminal phase after full separation is accomplished.

Example Question #21 : Cellular Processes And Functions

Which of the following processes occur in meiosis but do not occur in mitosis?

I. Spindle formation

II. Separation of homologous chromosomes

III. Separation of sister chromatids

IV. Recombination

Possible Answers:

I, II, and III

II and IV

I, II, III, and IV

II only

Correct answer:

II and IV

Explanation:

Mitosis and meiosis are similar processes that yield very different results. One of the major differences is that meiosis separates homologous chromosomes prior to separating sister chromatids. This is what leads to the reduction of ploidy. Both processes involve spindle formation (the microtubule apparatus that pulls the chromosomes/chromatids apart). Recombination is a phenomenon unique to meiosis that results in increasing genetic diversity.

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