MCAT Biology : Mitosis and Meiosis

Study concepts, example questions & explanations for MCAT Biology

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Example Question #31 : Cellular Processes And Functions

Meiosis is a form of cell division that occurs in special types of cells called germ cells. It is different from mitosis because it takes a diploid cell and splits it into four, nonidentical haploid cells. In males, these haploid cells are called sperm and in females they are called eggs or ova. Meiosis has two steps: meiosis I and meiosis II. Both steps have their corresponding prophase, metaphase, anaphase, and telophase. Meiosis I phases are similar to mitotic phases, with a few key differences. Meiosis II phases are exactly identical to the mitotic phases. 

In humans, a cell in prophase has __________ chromosomes, in metaphase has __________ chromosomes, and in anaphase has __________ chromosomes.

Possible Answers:

 . . .  . . . 

 . . .  . . . 

 . . .  . . . 

 . . .  . . . 

Correct answer:

 . . .  . . . 

Explanation:

Humans are diploid organisms with a total of 46 chromosomes (2n = 46). They have 23 distinct chromosomes and each chromosome has a homologous chromosome, giving a total of 46 chromosomes. Upon completion of DNA replication in S phase, each chromosome gains an identical sister chromatid that is joined to the original chromosome at the centromere; however, this whole entity is still considered a single chromosome. When a cell enters prophase of mitosis, there are a total of 46 chromosomes, each with a sister chromatid. In metaphase, the cell still has 46 chromosomes and these chromosomes align along the midline of the cell. In anaphase, the sister chromatids get pulled apart to opposite ends; therefore, the sister chromatid separates from the chromosome and becomes its own chromosome. This means that in anaphase there are a total of 92 chromosomes in the cell.

These 92 chromosomes get pulled to opposite ends where 2 new daughter cells (with 46 chromosomes and no sister chromatids) are produced.

Example Question #32 : Cellular Processes And Functions

Meiosis is a form of cell division that occurs in special types of cells called germ cells. It is different from mitosis because it takes a diploid cell and splits it into four, nonidentical haploid cells. In males, these haploid cells are called sperm and in females they are called eggs or ova. Meiosis has two steps: meiosis I and meiosis II. Both steps have their corresponding prophase, metaphase, anaphase, and telophase. Meiosis I phases are similar to mitotic phases, with a few key differences. Meiosis II phases are exactly identical to the mitotic phases. 

Which of the following is true regarding meiosis and mitosis?

I. Prophase I of meiosis and prophase of mitosis both involve recombination

II. In humans, cells in metaphase of mitosis will have twice as many columns of chromosomes as cells in metaphase I

III. The daughter cells of mitosis have similar ploidy number as daughter cells of meiosis I

Possible Answers:

I and II

I only

I and III

II only

Correct answer:

II only

Explanation:

Mitosis and meiosis are both processes that involve cell division. In mitosis, a diploid parent cell divides and gives rise to two identical, diploid daughter cells. In meiosis, a diploid parent cell divides and gives rise to four identical, haploid daughter cells. Meiosis is divided into meiosis I and meiosis II. Meiosis I is unique because its prophase (prophase I) involves exchange of genetic material between chromosomes. This process is called recombination. Meiosis II is similar to mitosis. Note that both meiosis I and II have prophase, metaphase, anaphase, and telophase.

Metaphase in both mitosis and meiosis involves the alignment of the cell’s nuclear material along the midline. In mitosis, sister chromatids line along the midline. This means that in humans there will be a total of 46 columns of chromosomes along the midline (46 chromosomes with their respective sister chromatids). In meiosis I, however, it is slightly different. Recall that prophase I involves tetrad formation. This means that homologous chromosomes (with their respective sister chromatids) pair up along the midline, reducing the amount of columns of chromosomes in metaphase I to half of metaphase in mitosis. This means that in humans there will be only 23 columns of chromosomes in metaphase I.

Ploidy number refers to the amount of homologous chromosomes present. In humans, there are two sets of homologous chromosomes; therefore, humans are diploid. As mentioned mitosis produces two identical, diploid daughter cells (each cell has homologous chromosomes). In meiosis I two identical, haploid daughter cells are produced. These daughter cells have 23 chromosomes with sister chromatids; however, they lack their homologous pair and are, therefore, haploid. These two daughter cells undergo meiosis II and produce the final products of meiosis, which are four haploid daughter cells. Remember that the daughter cells of meiosis II will not have a sister chromatid because the sister chromatids are pulled to opposite poles during anaphase II.

Example Question #33 : Cellular Processes And Functions

Meiosis is a form of cell division that occurs in special types of cells called germ cells. It is different from mitosis because it takes a diploid cell and splits it into four, nonidentical haploid cells. In males, these haploid cells are called sperm and in females they are called eggs or ova. Meiosis has two steps: meiosis I and meiosis II. Both steps have their corresponding prophase, metaphase, anaphase, and telophase. Meiosis I phases are similar to mitotic phases, with a few key differences. Meiosis II phases are exactly identical to the mitotic phases. 

Which of the following eukaryotic process is similar to the bacterial asexual reproduction?

Possible Answers:

Transcription

DNA replication

Meiosis I

Mitosis

Correct answer:

Mitosis

Explanation:

Asexual reproduction is cell division that involves no sexual recombination. Recall that mitosis in eukaryotes produces two identical daughter cells with identical genetic make up. This is because there is no sexual recombination during mitosis.

Meiosis, on the other hand, undergoes sexual recombination during prophase I and is considered a form of sexual reproduction, which increases genetic variation. Meiosis occurs in the gonads, the site of sperm (in testes) and oocyte (in ovaries) production. DNA replication and transcription are not involved in cell division and reproduction.

Example Question #34 : Cellular Processes And Functions

Nuclear transport is a very important concept of study in modern cellular biology. Transport of proteins into the nucleus of an organism requires energy in the form of GTP, which is attached to a protein called Ras-related Nuclear protein (RAN).

RAN is a monomeric G protein found in both the cytosol as well as the nucleus and its phosphorylation state plays an important role in the movement of proteins into and out of the nucleus. Specifically, RAN-GTP and RAN-GDP binds to nuclear import and export receptors and carries them into or out of the nucleus. They also play a role in dropping off cargo that import and export receptors hold onto. RAN's functions are controlled by two other proteins: RAN guanine exchange factor (RAN-GEF) and RAN GTPase activating protein (GAP). RAN-GEF binds a GTP onto RAN, while RAN-GAP hydrolyzes GTP into GDP. As a result, there is a RAN-GTP and RAN-GDP concentration gradient that forms between the cytosol and nucleus.

During prophase, what most likely happens to the RAN-GTP and RAN-GDP concentration gradient? 

Possible Answers:

There is no change in the concentration gradient during prophase

The concentration gradient strengthens because the cell requires more proteins to diffuse into the nucleus during mitosis

The concentration gradient breaks down because the nuclear membrane breaks down

The concentration gradient breaks down because the cell no longer requires protein movement into or out of the nucleus

The concentration gradient strengthens because the nuclear envelope becomes even more impermeable

Correct answer:

The concentration gradient breaks down because the nuclear membrane breaks down

Explanation:

During mitosis, the nuclear envelope breaks down to allow the formation of chromosomes. Since all concentration gradients are dependent upon the impermeability of a membrane, when this envelope breaks down, the concentration gradient weakens and disappears. 

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