Cell Cycle and Development Flashcards
Endomitosis is a phenomenon in which cells properly replicate their chromosomes but experience prematurely-terminated mitosis. As a result, the replicated chromosomes are retained within the original nucleus. In these cases, mitosis must arrest during:
A. metaphase.
B. anaphase.
C. prophase.
D. telophase.
C is correct. The question states that the replicated chromosomes are retained in the original nucleus. For this to be possible, either the nucleus was always intact, or the chromosomes have not migrated and it can easily re-form. The only mitotic phase that occurs before the chromosomes begin to migrate is prophase.
Edwards syndrome is a high-morbidity condition that affects about one in 6,000 births and results in internal organ disturbances. It is caused by a chromosomal abnormality in which cells contain an extra copy of chromosome 18. What is the term for this aneuploidy, and what may be a potential cause?
A. Monosomy, caused by a nondisjunction in one of the parental gametes
B. Monosomy, caused by a chromosomal deletion in one of the parental gametes
C. Trisomy, caused by a nondisjunction in one of the parental gametes
D. Trisomy, caused by a chromosomal deletion in one of the parental gametes
C is correct. The extra copy of chromosome 18 means that cells have three copies of that chromosome instead of the normal two. This phenomenon is known as trisomy, which can be caused by a nondisjunction event during meiosis. In the case of Edwards syndrome, chromosome 18 does not properly segregate, potentially causing one parental daughter cell to have two copies and the other to have none. When the “two-copied” gamete is joined with a wild-type gamete, the resulting zygote will have an extra chromosome.
A, B: Monosomy, as its name implies, is used to refer to cases in which cells have only one copy of a chromosome. Individuals with Edwards syndrome have three.
D: Deletion of an entire chromosome would result in monosomy, and even a partial deletion would not produce the genetic situation described.
Colchicine is a medication that is often used to treat gout. It is also used to make karyograms, for which cell division must be arrested at metaphase. What is a potential mechanism of this medication?
A. Colchicine inhibits DNA polymerase, blocking DNA replication.
B. Colchicine binds to tubulin and prevents the mitotic spindle from forming.
C. Colchicine disrupts histone-DNA interactions, which keeps DNA from condensing.
D. Colchicine prevents the nuclear envelope from re-forming.
B is correct. During metaphase, microtubules must locate and attach to kinetochores and pull chromosomes to the center of the cell. This requires proper functioning of tubulin-based microtubules, which corresponds to choice B.
A: DNA replication occurs during the S phase of the cell cycle. If this were colchicine’s mechanism of action, the cell would be arrested in interphase, not in metaphase.
C: DNA condensation occurs during prophase.
D: The nuclear envelope re-forms during telophase, not metaphase.
Meiosis I and II are two parts of a process that results in the creation of diverse cellular progeny. Which of the following are differences between meiosis I and meiosis II?
I. Meiosis I is a heterotypic division, while meiosis II is a homotypic division.
II. Crossing over occurs only during meiosis I.
III. DNA in meiosis I is highly conserved, but that in meiosis II often contains many mutations.
IV. A long S phase precedes meiosis I, but no S phase comes before meiosis II.
A. I only
B. I and II only
C. I, II, and III only
D. I, II, and IV only
D is correct. Meiosis I is a heterotypic, or reductional, division. In other words, parent cells begin as diploid, while daughter cells formed during meiosis I are haploid. Remember this for the MCAT – a very common misperception is that cells are still diploid at the beginning of meiosis II! Instead, meiosis II is a homotypic, or equational, division, making choice I correct. Statement II is also sensible, as genetic diversity is enhanced by crossing over between homologous chromosomes during meiosis I, but no such event occurs during meiosis II. Lastly, the cell replicates its DNA during the S phase prior to meiosis I, but no additional replication precedes meiosis II. This results in the overall transition from diploid to haploid.
III: DNA is always highly conserved. Although mutations are a source of diversity, they are usually dangerous for the organism. Consequently, meiosis II does not include intentional mutagenesis. Instead, genetic diversity arises from the random assortment of chromosomes and from crossing over.
Cancers are caused by cells that divide uncontrollably. In the quest to find better cancer medications, many researchers are investigating mitotic mechanisms as potential targets. All of the following are potential mitotic chemotherapeutic drug targets EXCEPT:
A. DNA polymerase.
B. microtubules.
C. kinetochores.
D. centrioles.
A is correct. Although DNA polymerases might be acceptable targets due to their role in DNA replication, this occurs during the S phase of interphase, not during mitosis.
B, C, D: Microtubules, kinetochores, and centrioles are all critical structures involved in chromosome migration during mitosis.
Interphase, which comprises the bulk of the cell cycle, is itself split into multiple phases. Which segment of interphase generally spans the longest period of time?
A. G1
B. G2
C. G0
D. S
A is correct. During the G1 phase, the cell conducts protein and organelle synthesis at a high rate while the cell grows in size. The transition from the G1 to the S phase is termed the “restriction point” and constitutes the rate-limiting step in the cell cycle.
The G2-M checkpoint:
A. ensures that the cell is of sufficient size to undergo mitosis.
B. ensures that the DNA has been replicated accurately.
C. functions only in gametes.
D. sends cells directly to the G0 phase if it is not passed.
B is correct. This position, often known specifically as the G2-M DNA damage checkpoint, occurs soon after the S phase of the cell cycle. It is crucial to ensuring that the recently-replicated DNA was copied accurately; if not, mitosis will not occur.
A: This is the function of the G1-S checkpoint. Here, the cell assesses whether it has enough organelles and is large enough to proceed to DNA replication.
D: While this is a possibility following the G1 phase, it will not occur immediately after G2.
Cyclin and cyclin-dependent kinases (Cdks) play an essential role in the regulation of the cell cycle. If a cell from a mutated line displays permanently reduced levels of both cyclin and Cdks, which consequence is most likely?
A. The cell cycle would be prevented from occurring.
B. The cell would rely on other growth factors to stimulate the cycle.
C. The cell would lack the DNA proofreading mechanism that typically follows the S phase.
D. The cell would complete the cycle more rapidly than usual.
A is correct. Cyclin and Cdks are crucial cell cycle regulators. Depending on the phase of the cycle, certain Cdks will be expressed at higher concentrations than others; this signals the cell to prepare for the next transition. In other words, the levels of these proteins are dynamic throughout the cell cycle. If this cell has perpetually low concentrations of cyclin and Cdks, it lacks the signaling molecules required to initiate and prep the cell for upcoming phases.
B: As these proteins are described as “essential,” they likely cannot be replaced by alternative molecules.
C: DNA proofreading does not directly relate to cyclin.
D: This might occur if levels of cyclin and specific Cdks were high, not low.
If a cell has not yet reached an adequate size to progress further, in which phase will it remain until the issue is rectified?
A. G1
B. S
C. G2
D. M
A is correct. The G1 phase is responsible for protein and organelle production in addition to cell growth. Two major checkpoints mark the cell cycle: one during G1 and the other in G2. In order to pass the G1 checkpoint, the cell must have reached a sufficient size and synthesized enough organelles to move into the S phase.
C: The checkpoint between G2 and mitosis ensures that DNA replication has proceeded accurately.
If trauma causes a cell to dramatically shorten the length of its S phase, what compensatory mechanism might occur?
A. The cell would not produce viable daughter cells.
B. The cell would enter the G0 phase.
C. The cell would spent a longer period of time in the G2 phase.
D. None; the cell would be unable to correct any errors that occurred.
C is correct. The S phase is responsible for DNA replication. A drastically impaired S phase, then, is likely to yield a cell with an incomplete or inaccurately replicated genome. As a result, this cell would be prevented from passing the G2-M checkpoint, which ensures correctness in DNA replication. Instead of quickly entering mitosis, it will spend additional time in the G2 phase to correct the errors generated by the traumatic event.
A: While this is possible, we cannot be certain that the damage is irredeemable. In contrast, choice C is virtually guaranteed to be true.
B: The G0 phase is a period of “rest” that is favored by specific cell types, especially neurons and certain kinds of muscle. It would not be entered after impaired DNA replication.
D: Like choice A, this is too extreme to be the best answer. Additionally, the cell does not lack compensatory mechanisms entirely; this type of situation is what the checkpoints are for.
In what way is apoptosis distinct from necrosis?
A. Apoptosis is a naturally-triggered event, while necrosis involves uncontrolled cell death.
B. Apoptosis produces inflammation, while necrosis is not associated with any noticeable symptoms.
C. Apoptosis is typically caused by bacterial or fungal infections, while necrosis is a process stimulated by signals within the cell.
D. Apoptosis usually requires medical treatment, while necrosis typically resolves itself without external attention.
A is correct. Apoptosis, sometimes known as “programmed cell death,” represents a naturally-occurring physiological process. In contrast, necrosis is generally triggered by an environmental stressor, such as extreme cold, injury, or lack of blood flow to an area. This promotes the death of otherwise healthy cells.
B: Apoptosis is typically thought to be non-inflammatory.
C: Again, apoptotic pathways are directly triggered by internal factors, not infection. In fact, these descriptions are reversed.
D: This choice also switches the characteristics of apoptosis and necrosis.
What tissues will arise from the cells marked by the dark arrow? (the arrow is pointing at ectodermal cells)
A. Epidermis, lining of internal organs, blood, spleen
B. Lining of internal organs (stomach, lungs, intestines, etc.)
C. Muscle, cardiac and skeletal systems, blood, heart, spleen
D. Epidermis, nervous system, lens of the eye, hair
D is correct. The arrow points to ectodermal cells. This germ layer differentiates into “external” features including the epidermis, lens of the eye, fingernails, and hair. It also gives rise to the nervous system via neurulation.
A: The tissue types listed here come from all three germ layers.
B: This choice relates much more to the endoderm than the ectoderm.
C: Muscle, the cardiovascular system, and the skeletal system are classic mesodermal tissues. The arrow above indicates the ectoderm.
Which cellular features increase in the direction of the arrow labeled B? (from multipotent to totipotent)
A. DNA methylation and the expression of pluripotent genes
B. “Open” chromatin and DNA methylation
C. “Open” chromatin and lineage potential
D. DNA methylation and the expression of lineage-specific genes
C is correct. As cells progress from totipotency to pluripotency to multipotency, their lineage potential decreases, meaning that they lose the ability to give rise to a broad range of tissue types. Additionally, their DNA becomes increasingly methylated. Remember, DNA methylation inhibits transcription, thus deceasing the prevalence of euchromatin or “open chromatin.” Moving in the opposite direction, then, would increase both lineage potential and the overall tendency of chromatin to fall into a loose arrangement.
A clinical biologist is viewing a tissue sample from a patient with an osteosarcoma. The sample displays multinucleate cells and a “striped” appearance and appears to be innervated with somatic nerves. These cells most probably derived from which germ layer?
A. The ectoderm
B. The mesoderm
C. The endoderm
D. This answer cannot be known without more information.
B is correct. The description in the question stem most closely correlates to skeletal muscle. This tissue type, along with the skeletal system, the cardiovascular system, and the kidneys, derives from the mesodermal layer.
A: The ectoderm develops into external features (hair, nails, part of the eye, etc.) along with the nervous system.
C: The endoderm gives rise to the linings of several internal organs.
A scientist studying cell-to-cell communication discovers that a particularly biologically relevant signal acts in a paracrine manner. What must be true of the cell or cells involved?
A. A single target cell is responding to a signal generated by itself.
B. The cellular response involves a hormone that travels long distances through the bloodstream.
C. The signal travels a very small distance through the bloodstream from its origin to the target cell.
D. The signal travels a very small distance from its origin to the target cell, but this response does not involve hormone movement through the bloodstream.
D is correct. Paracrine activity involves the secretion of local signaling molecules known as paracrine factors. These factors diffuse to nearby target cells and exert a variety of effects.
A: This choice describes an autocrine signal.
B: The description here corresponds to endocrine signaling. Paracrine molecules are not technically hormones.
C: Again, paracrine signaling does not involve hormones. Instead of traveling in the blood, paracrine factors are released into the extracellular space and diffuse to local targets.
