MCAT Biology : Biology

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #3 : Mitosis And Meiosis

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

Possible Answers:

Telophase, prophase, metaphase, anaphase

Metaphase, telophase, anaphase, prophase

Prophase, telophase, metaphase, anaphase

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 #1 : Mitosis And Meiosis

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

Possible Answers:

Prophase

Telophase

Metaphase

Anaphase

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 #7 : 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:

Metaphase

Prophase

Telophase

Anaphase

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 : Chromatin And Chromosomes

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

Possible Answers:

Heterochromatin—prophase

Heterochromatin—metaphase

Euchromatin—interphase

Euchromatin—telophase

Heterochromatin—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:

Prophase I—trisomy

Telophase II—translocation

Telophase I—deletion

Anaphase I—inversion

Metaphase I—nondisjunction

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 #1352 : Biology

When does genetic crossover occur during meiosis? 

Possible Answers:

Anaphase I

Metaphase I

Prophase I

Telophase II

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 #1353 : Biology

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.

 

 

Untitled

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:

Metaphase II

Anaphase of mitosis

Anaphase II

Metaphase of mitosis

Metaphase I

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 #1354 : Biology

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, III, and IV

II and IV

II only

I, II, and III

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.

Example Question #1355 : Biology

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).

Meiosis is a form of __________ reproduction and mitosis is a form of __________ reproduction.

Possible Answers:

asexual . . . asexual

sexual . . . sexual

asexual . . . sexual

sexual . . . asexual

Correct answer:

sexual . . . asexual

Explanation:

The key difference between sexual and asexual reproduction is recombination. If a process involves shuffling of genetic material between chromosomes (recombination), then it is sexual reproduction. Recall that crossing over, a type of genetic recombination, occurs during prophase I of meiosis. This leads to the production of daughter cells that are distinct from the parents; therefore, meiosis is a form of sexual reproduction.

In mitosis there is no genetic recombination and the daughter cells are identical to the parent cells; therefore, mitosis is a form of asexual reproduction.

Example Question #1356 : Biology

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 is not a characteristic of meiosis?

Possible Answers:

It functions to grow and repair the human body

It is preceded by the G2 phase

It produces egg and sperm cells with no homologous chromosomes

It produces four haploid daughter cells

Correct answer:

It functions to grow and repair the human body

Explanation:

Meiosis is the process by which a diploid cell divides into four haploid daughter cells. The daughter cells produced are called gametes (sperm in males and egg in females). By definition, a haploid cell contains only one set of chromosomes; therefore, the egg and sperm cells will not contain any homologous chromosomes.

Recall that mitosis is part of the cell cycle and that it is preceded by three phases: G1 phase, S phase, and G2 phase. Meiosis is immediately followed by the G2 phase. The main function of meiosis is to produce haploid gametes that can be used for sexual reproduction. Growth and repair of tissues in the human body is accomplished by mitosis, not meiosis.

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