Test: Embryology - 1 - MCAT MCQ

During oogenesis, the process of creating female gametes, the primary oocytes in the ovaries undergo DNA replication during fetal development but are arrested at prophase I until puberty. At the time of arrest, the primary oocytes have 46 chromosomes, which have replicated to form 92 chromatids (two chromatids per chromosome). Therefore, during childhood, the primary oocytes in the ovaries of a female would have 46 chromosomes and 92 chromatids. After puberty, only one of these primary oocytes will undergo meiosis each month, resulting in the release of an egg with 23 chromosomes and 46 chromatids (one chromatid per chromosome) during ovulation.

Oogenesis and spermatogenesis are the processes of meiosis that occur in females and males, respectively, leading to the formation of gametes (eggs and sperm). There are several differences between these two processes.

First, oogenesis forms two polar bodies, while spermatogenesis only forms one. During oogenesis, the primary oocyte undergoes two rounds of meiosis, resulting in the formation of one mature egg and two polar bodies. The polar bodies do not have the potential to be fertilized and serve mainly to discard excess genetic material. In spermatogenesis, each primary spermatocyte undergoes meiosis and forms four functional sperm cells.

Second, oogenesis produces a larger gamete than spermatogenesis. The female gamete, or egg, is much larger and contains abundant cytoplasm to support the developing embryo. In contrast, the male gamete, or sperm, is relatively small and streamlined to facilitate motility and reach the egg for fertilization.

Third, both processes do not produce the same number of viable gametes. Oogenesis results in the formation of one viable egg cell per primary oocyte, whereas spermatogenesis produces four viable sperm cells per primary spermatocyte.

Lastly, both oogenesis and spermatogenesis are arrested at an intermediate step until puberty. In females, oogenesis is arrested at prophase I until puberty, while in males, spermatogenesis is arrested at the primary spermatocyte stage. The onset of puberty triggers the continuation of these processes.

Contact inhibition is a normal cellular mechanism that regulates cell growth and prevents cells from growing uncontrollably. When cells come into contact with each other, they stop dividing and form a monolayer. This mechanism helps maintain the proper organization and structure of tissues. In cancer, however, cells lose contact inhibition and continue to divide and grow even when they come into contact with neighboring cells.

During cell division, the nuclear envelope, which surrounds the nucleus, needs to break down to allow for proper segregation of chromosomes. This breakdown occurs during prometaphase, which is a stage of mitosis or meiosis. Prometaphase follows prophase and precedes metaphase.

In prometaphase, the following events take place:

• The nuclear envelope starts to disintegrate, and the membrane breaks down into smaller vesicles.
• The microtubules of the mitotic spindle attach to the kinetochores of the chromosomes.
• The chromosomes become condensed and are prepared for proper alignment during metaphase.
• The disappearance of the nuclear envelope during prometaphase allows the microtubules to interact with the chromosomes directly, facilitating their movement and alignment. This breakdown is crucial for the subsequent stages of cell division.

Anaphase (option B) is the stage during which sister chromatids separate and move towards opposite poles of the cell. The nuclear envelope is already absent during this phase.

S phase (option C) is the synthesis phase of the cell cycle, where DNA replication occurs. The nuclear envelope is still intact during this phase.

Cytokinesis (option D) is the final stage of cell division, where the cytoplasm divides, and two daughter cells are formed. The nuclear envelope reforms around the separated chromosomes in each daughter cell during cytokinesis.

Centrioles are small, cylindrical structures found in eukaryotic cells. They are typically found in pairs and are located within a specialized region of the cell called the centrosome. Centrioles play a crucial role in cell division, particularly in the formation of the mitotic spindle during mitosis and the organization of microtubules.

Prokaryotic cells, on the other hand, do not possess centrioles. Prokaryotes, such as bacteria, lack membrane-bound organelles typically found in eukaryotic cells. They undergo cell division through a process called binary fission, which does not involve the formation of a mitotic spindle or the presence of centrioles.

During mitosis, the movement of chromosomes is facilitated by the interaction between microtubules and kinetochores. Centrioles, located in the centrosomes, organize and send out microtubules that form the mitotic spindle. The microtubules attach to kinetochores, which are protein structures located at the centromeres of sister chromatids.

As the mitotic spindle forms, the microtubules extend and connect to the kinetochores of each sister chromatid. The microtubules exert forces on the kinetochores, pulling the sister chromatids apart towards opposite poles of the cell. This separation ensures that each daughter cell receives an equal and complete set of chromosomes.

Option B is incorrect because homologous chromosome separation occurs during meiosis, not mitosis. Option C is incorrect as centrioles on chromatids do not exist. Option D is also incorrect because each centrosome does not bind an individual chromatid, but rather the microtubules from both centrosomes collectively interact with all the chromatids.

In option D, there is no RB mutation, meaning that the RB protein is intact and able to properly inhibit the cell cycle progression. Additionally, the CDK in this scenario cannot bind to cyclin. Without the formation of the cyclin/CDK complex, CDK activity is inhibited, preventing uncontrolled cell cycle progression. This scenario, where RB is functional and CDK cannot bind to cyclin, is less likely to lead to the formation of a neoplasm compared to the other options.

Karyotyping involves the analysis of chromosomes to detect any abnormalities or mutations in their structure or number. This technique is typically performed during metaphase of the cell cycle.

During metaphase, chromosomes are fully condensed and aligned along the equatorial plane of the cell. At this stage, the chromosomes are most visible and can be easily observed and analyzed under a microscope. Cells undergoing mitosis or meiosis can be arrested at metaphase by various methods, allowing for the collection and examination of chromosomes for abnormalities.

Colchicine is a drug that affects the function of tubulin, a protein involved in the formation of microtubules. Microtubules are important components of the cytoskeleton and play a crucial role in various cellular processes, including cell division.

During cell division, microtubules form the mitotic spindle, which helps separate the duplicated chromosomes during anaphase. By binding to tubulin, colchicine disrupts the assembly and stability of microtubules, leading to the arrest of cells in anaphase.

Loss of function mutations in apoptosis-controlling proteins can contribute to the formation of a cancerous mass. Apoptosis is a process of programmed cell death that helps eliminate damaged or abnormal cells. Mutations that disrupt the normal function of apoptosis-controlling proteins can prevent cells from undergoing apoptosis when necessary, allowing them to survive and accumulate, which can contribute to the formation of a tumor or cancerous mass.