MCAT Biology : Mitosis and Meiosis

Study concepts, example questions & explanations for MCAT Biology

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Example Question #1 : Mitosis And Meiosis

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.

1

A karyotype of this individual for chromosomes 14 and 21 would thus appear as follows:

Pic2

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 important participants in both meiosis and mitosis.  Which of the following is false of meiosis?

Possible Answers:

Crossing over happens more frequently than in mitosis

Meiosis forms tetrads before its first division

It generates haploid cells, while mitosis generates diploid cells

It undergoes a reduction division during its second division

Meiosis is limited to germ cells

Correct answer:

It undergoes a reduction division during its second division

Explanation:

This is a tricky question because all of the choices are correct, except for one minor part. Meiosis sees cells undergo a reduction division in the first division. That means that after the first meiotic division, the diploid germ cell has become a haploid.

Example Question #2 : Mitosis And Meiosis

Cellular division is an essential part of the cell cycle. When a cell divides it passes genetic information to daughter cells. The amount of genetic information passed on to daughter cells depends on whether the cell undergoes mitosis or meiosis. Mitosis is the most common form of cell division. All somatic cells undergo mitosis, whereas only germ cells undergo meiosis. Meiosis is very important because it produces gametes (sperm and eggs) that are required for sexual reproduction. Human germ cells have 46 chromosomes (2n = 46) and undergo meiosis to produce four haploid daughter cells (gametes).

Which of the following would occur in humans if meiosis produced diploid gametes?

Possible Answers:

The number of chromosomes in subsequent generations would be 92

The number of chromosomes in subsequent generations would decrease exponentially

The number of chromosomes in subsequent generations would be 23

The number of chromosomes in subsequent generations would increase exponentially

Correct answer:

The number of chromosomes in subsequent generations would increase exponentially

Explanation:

Meiosis is the process by which haploid gametes are produced. The attachment of a haploid sperm to a haploid egg begins the process of development and fertilization. In humans a sperm containing 23 chromosomes joins with an egg containing 23 chromosomes to create an organism with 46 chromosomes.

If meiosis were to produce diploid gametes, then the diploid gametes would combine to form an organism that contains twice as many chromosomes as the parent. For example, a diploid sperm and egg (containing 46 chromosomes each) would create an organism with 92 chromosomes. This new organism would produce another organism with 184 chromosomes (double 92). The subsequent generations would have twice as many chromosomes as the previous generation, and the amount of chromosomes would increase exponentially.

Offspring 1: 92 chromosomes

Offspring 2: 184 chromosomes

Offspring 3: 368 chromosomes

The best answer is that the number of chromosomes in subsequent generations would increase exponentially.

Example Question #3 : Mitosis And Meiosis

During metaphase, the chromosomes of the cell are __________.

Possible Answers:

aligned at the midplane of the cell

separated at each end of the poles

duplicated

beginning to shorten and thicken

difficult to see

Correct answer:

aligned at the midplane of the cell

Explanation:

During metaphase, chromosomes align along the metaphase plate, in preparation for anaphase. This phase of the cell cycle is highly visible as a distinct band of chromosomes lined up in the center of the cell.

Example Question #4 : Mitosis And Meiosis

Mit

Which stage of mitosis is shown in the illustration?

Possible Answers:

Prophase

Telophase

Cytokinesis

Anaphase

Metaphase

Correct answer:

Metaphase

Explanation:

This cell is in metaphase. This can be determined because the chromosomes are lined up on in the middle. This is known as the metaphase plate. After alignment the microtubules attach, and the chromosomes are ready to be seperated, which is the next phase (anaphase).

Example Question #5 : Mitosis And Meiosis

The __________ is the site of attachment of spindle fibers to sister chromatids during mitosis.

Possible Answers:

centrosome

centromere

kinetochore

centriole

Correct answer:

kinetochore

Explanation:

Centrosomes are portions of the cell that help nucleate microtubules and form the mitotic spindle. Centrioles are composed of tubulin and are portions of the centrosome. The centromere is the portion of the chromosome where the two sister chromatids are linked. The kinetochore is a protein structure that helps associate the mitotic spindle to the sister chromatids. The outer portion of the kinetochore interacts with the microtubules, while the inner portion associates with the centromeric DNA. 

Example Question #2 : 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.

 

Untitled

 

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:

Metaphase

Anaphase

Prophase

Cytokinesis

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 #7 : 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:

Mitosis

Muscle contraction

DNA synthesis

Membrane depolarization

Protein 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 #8 : Mitosis And Meiosis

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

Possible Answers:

Prophase, telophase, metaphase, anaphase

Telophase, prophase, metaphase, anaphase

Metaphase, telophase, anaphase, prophase

Prophase, metaphase, anaphase, telophase

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 #2 : Mitosis And Meiosis

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

Possible Answers:

Anaphase

Metaphase

Telophase

Prophase

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 #10 : 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:

Anaphase

Metaphase

Prophase

Telophase

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.

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