MCAT Biology Review
Chapter 2: Reproduction
1. Which of the following is the correct sequence of the development of a mature sperm cell?
1. Spermatid → spermatocyte → spermatogonium → 2° spermatocyte → spermatozoan
2. Spermatogonium → 1° spermatocyte → 2° spermatocyte → spermatid → spermatozoan
3. Spermatozoan → 1° spermatocyte → 2° spermatocyte → spermatogonium → spermatid
4. Spermatogonium → 1° spermatocyte → 2° spermatocyte → spermatozoan → spermatid
2. Which of the following correctly pairs the stage of development of an egg cell with the relevant point in a woman’s life cycle?
1. From birth to menarche—prophase II
2. At ovulation—metaphase I
3. At ovulation—metaphase II
4. At fertilization—prophase II
3. Some studies suggest that in patients with Alzheimer’s disease, there is a defect in the way the spindle apparatus attaches to the kinetochore fibers. At which stage of mitotic division would one first expect to be able to visualize this problem?
4. A researcher wishes to incorporate a radiolabeled deoxyadenine into the genome of one of the two daughter cells that would arise as a result of mitosis. What is the latest stage of cellular development during which the radiolabeled deoxyadenine could be added to achieve this result?
5. Certain ovarian tumors called granulosa cell tumors are known to produce excessive levels of estrogen. A physician who diagnoses a granulosa cell tumor should look for a secondary cancer in which of the following parts of the reproductive tract?
1. Fallopian tube
6. Upon ovulation, the oocyte is released into the:
1. fallopian tube.
3. abdominal cavity.
7. Cancer cells are cells in which mitosis occurs continuously, without regard to quality or quantity of the cells produced. For this reason, most chemotherapies attack rapidly dividing cells. At which point(s) in the cell cycle could chemotherapy effectively prevent cancer cell division?
1. S stage
1. I only
2. I and II only
3. II and III only
4. I, II, and III
8. Which of the following INCORRECTLY pairs a structure of the male reproductive system with a feature of the structure?
1. Seminal vesicles—produce alkaline fructose-containing secretions
2. Epididymis—surrounded by muscle to raise and lower the testes
3. Vas deferens—tube connecting the epididymis to the ejaculatory duct
4. Cowper’s glands—produce a fluid to clear traces of urine in the urethra
9. What is the last point in the meiotic cycle in which the cell has a diploid number of chromosomes?
1. During interphase
2. During telophase I
3. During interkinesis
4. During telophase II
10.Which of the following does NOT likely contribute to genetic variability?
1. Random fertilization of an egg by a sperm
2. Random segregation of homologous chromosomes
3. Crossing over between homologous chromosomes during meiosis
4. Replication of the DNA during S stage
11.Which of the following statements correctly identifies a key difference between mitosis and meiosis?
1. In metaphase of mitosis, replicated chromosomes line up in single file; in metaphase II of meiosis, replicated chromosomes line up on opposite sides of the metaphase plate.
2. During anaphase of mitosis, homologous chromosomes separate; during anaphase of meiosis I, sister chromatids separate.
3. At the end of telophase of mitosis, the daughter cells are identical to each other; at the end of meiosis I, the daughter cells are identical to the parent cell.
4. During metaphase of mitosis, centromeres are present directly on the metaphase plate; during metaphase of meiosis I, there are no centromeres on the metaphase plate.
12.Which of the following is true regarding prophase?
1. The chromosomes separate and move to opposite poles of the cell.
2. The spindle apparatus disappears.
3. The chromosomes uncoil.
4. The nucleoli disappear.
13.An individual who is phenotypically female is found to have only one copy of a disease-carrying recessive allele on the X chromosome, yet she demonstrates all of the classic symptoms of the disease. Geneticists determine she has a genotype that likely arose from nondisjunction in one of her parents. What is the likely genotype of this individual?
1. 46,XX (46 chromosomes, with XX for sex chromosomes)
14.During which phase of the menstrual cycle does progesterone concentration peak?
1. Follicular phase
3. Luteal phase
15.Which of the following would NOT be seen during pregnancy?
1. High levels of hCG in the first trimester
2. High levels of progesterone throughout the pregnancy
3. Low levels of FSH in the first trimester
4. High levels of GnRH throughout the pregnancy
Answers and Explanations
Diploid cells called spermatogonia differentiate into primary spermatocytes, which undergo the first meiotic division to yield two haploid secondary spermatocytes. These undergo a second meiotic division to become immature spermatids. The spermatids then undergo a series of changes leading to the production of mature sperm, or spermatozoa.
From the time of birth until shortly before ovulation, all egg cells are arrested at the prophase stage of meiosis I. These cells are referred to as primary oocytes. At ovulation, the egg cell has completed meiosis I and is now arrested in metaphase II as a haploid cell called a secondary oocyte. When a sperm penetrates the outer layers of the secondary oocyte, it completes meiosis II to become a mature ovum.
The spindle apparatus first interacts with the kinetochore fibers near the end of prophase. While the spindle apparatus aligns the chromosomes at the equatorial plate during metaphase, choice (B), the initial connection of the microtubule to the kinetochore occurs in prophase.
To ensure that the labeled deoxyadenine will be incorporated into the DNA of one of the daughter cells, we have to insert the nucleotide before DNA replication has been completed. Because replication occurs during S stage, we could introduce the deoxyadenine during G1 or S stage. Because G1 precedes S, the latest point the deoxyadenine could be added is the S stage.
Estrogen is known to cause growth of the endometrial lining during the follicular phase of the menstrual cycle, and its levels stay high during the luteal phase to promote vascularization and glandularization of this tissue. Excessive levels of estrogen may provide a strong enough signal for cell growth to promote tumor formation, or even cancer. The other tissues listed in this question require estrogen for development, but are not strongly associated with rapid tissue growth due to estrogen.
This subtle point about ovulation eludes most students and remains hard to believe until the organs are examined in anatomy class in medical school. The ruptured ovarian follicle releases an oocyte into the abdominal cavity, close to the entrance of the fallopian tube. With the aid of beating cilia, the oocyte is drawn into the fallopian tube, through which it travels until it reaches the uterus. If it is fertilized in the fallopian tube, it will implant in the uterine wall. If fertilization does not occur, it will be expelled along with the uterine lining during menstruation.
The question is asking us to determine at which points in the cell cycle we can prevent or at least lower the number of cells undergoing mitosis. One idea would be to prevent DNA synthesis during the S stage of the cell cycle. Without the DNA being replicated, two viable daughter cells could not be formed. Another idea would be preventing the mitotic cycle from occurring altogether in prophase by preventing spindle apparatus formation, preventing the nuclear membrane from dissolving, or other processes during this phase. Similarly, a treatment that would act on cells in the metaphase stage of the cell cycle would also interfere with the mitotic cycle. Therefore, any of the three solutions presented would be a viable option.
The epididymis is the site of sperm maturation at the posterior side of the testis. In the epididymis, sperm gain mobility and are stored until ejaculation. It is the vas deferens (ductus deferens) that is surrounded by muscle that raises and lowers the testis to maintain a constant temperature suitable for sperm production, not the epididymis.
The first meiotic division (reductional division) separates homologous chromosomes to opposite poles of the cell during anaphase I. Near the end of telophase I, cytokinesis occurs, resulting in two haploid (n) daughter cells. Thus, during interkinesis and anaphase II, the daughter cells are already haploid, eliminating choices (C) and (D). The cell is diploid during interphase, choice (A), but remains diploid up until the end of telophase I.
The safest way to answer this question correctly is to go through each answer choice and eliminate the ones that contribute to genetic variability. The random fertilization of a sperm and an egg, the random segregation of homologous chromosomes during anaphase I, and crossing over between homologous chromosomes during prophase I all contribute to genetic variability during sexual reproduction because they result in novel combinations of genetic material, eliminating choices (A), (B), and (C). S stage, choice (D), should specifically not cause increased genetic variability; the DNA should be copied precisely, without error, meaning that both strands of DNA should be identical.
The key differences between mitosis and meiosis primarily appear during meiosis I. Of note, synapsis and crossing over occurs during prophase I, and homologous chromosomes are separated during meiosis I (rather than sister chromatids, as in mitosis). While the location of the centromeres relative to the equatorial plate may seem a trivial point, it is representative of the fact that homologous chromosomes line up on opposite sides of the equatorial plate in meiosis, in contrast to the alignment of each chromosome directly upon the metaphase plate in mitosis.
In prophase, the chromatin condenses into chromosomes, the spindle apparatus forms, and the nucleoli and nuclear membrane disappear. Choice (A) describes anaphase, whereas choices (B) and (C) describe telophase.
Nondisjunction refers to the incorrect segregation of homologous chromosomes during anaphase I, or of sister chromatids during anaphase II. In either case, one daughter cell ends up with two copies of related genetic material, while the other receives zero. Immediately, this should eliminate choices (A) and (B), which show a normal complement of chromosomes (46). An individual who has only one recessive disease-carrying allele, and yet still expresses the disease, likely does not have a dominant allele for the given trait. This is seen in males, who are hemizygous for many X-linked genes, and can also be seen in women with Turner syndrome (45,X), who have only one X chromosome. Thus, choice (C) is the answer.
Progesterone peaks during the luteal phase, as it supports the endometrium for potential implantation of a blastula. Progesterone levels are relatively low during follicular phase and ovulation, eliminating choices (A) and (B). Withdrawal of progesterone actually causes menses, eliminating choice (D).
During the first trimester of pregnancy, the corpus luteum is preserved by human chorionic gonadotropin (hCG); hence, progesterone secretion by the corpus luteum is maintained during the first trimester. This eliminates choice (A). During the second trimester, hCG levels decline, but progesterone levels rise because the hormone is now secreted by the placenta itself, eliminating choice (B). High levels of progesterone and estrogen inhibit GnRH secretion, thus preventing FSH and LH secretion and the onset of a new menstrual cycle. This eliminates choice (C) and validates choice (D).