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During the cell cycle, there are several checkpoints to ensure proper cell growth and division. Which checkpoint ensures that each duplicated chromatid is appropriately attached to cellular machinery in order to segregate properly?
During mitosis, the metaphase checkpoint ensures that each duplicated chromatid is attached to the spindle apparatus. If the metaphase checkpoint is not satisfied, the cell will not enter anaphase. This checkpoint is essential for preventing aneuploidy, a condition in which there are an abnormal number of chromosomes in a cell.
The G1 checkpoint determines if the cell will enter the S phase to replicate DNA and prepare for cell division. Quiescent cells, cells that do not frequently divide, are often stopped at the G1 checkpoint for long periods. The G2 checkpoint is used to prevent the cell from entering mitosis if there were errors in the replication of DNA during the S phase. Failure to pass the G2 checkpoint can result in apoptosis. There is no G0 checkpoint.
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Most cells spend the vast majority of their time in __________.
Most cells spend about 90% of their time in interphase. Note that mitosis and meiosis comprise only about 10% of the cell cycle.
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Out of all phases in the cell cycle, most cells spend the majority of their life in __________.
Cell division, or mitosis, is a rather small portion of a cell's life and includes prophase, metaphase, anaphase and telophase. The majority of a cell's life is spent in interphase.
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Which of the following best describes the two primary purposes of Active S cyclin-CDK phosphorylation in the cell cycle?
Active S cyclin-CDK phosphorylation in the cell cycle is primarily intended to ensure that each portion of the cell's genome is copied once and only once. Daughter cells that do not copy a complete genome will likely die; however, carrying extra copies of certain genes will also negatively affect daughter cells, and phosphorylation of proteins that make up pre-replication complexes safeguards against this.
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During the G1 phase of the cell cycle, in which form will Eukaryotic DNA be found?
Chromosomes and Chromatid are both incorrect because eukaryotic DNA is condensed into these tightly packed chromosomal structures during M phase of mitosis. Plasmids are not found in eukaryotes and an unfolded continuous strand of DNA would be too long to fit within a nucleus. Thus, Chromatin is the correct answer choice; chromatin is a protein-DNA complex in a loosely packed form which allows for gene transcription which is necessary during the G1 phase of the cell cycle.
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What is the initial checkpoint in a mammalian cell cycle whose major function is to allow or disallow a cell to mitotically divide?
The G1 Checkpoint is the correct answer, because if a cell gets a signal at this checkpoint then the cell goes on to complete the S, G2, and M phases and will end up dividing. If this signal is not received at the G1 checkpoint then the cell enters the non-dividing G0 phase.
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There are several checkpoints during the cell cycle to ensure proper growth and division. Which checkpoint of the cell cycle is considered the "point of no return?"
Once the cell passes the G1 checkpoint, the cell becomes committed to the cell cycle and enters the S phase where DNA is replicated. The checkpoint is to ensure the cell has grown enough and has enough resources to begin DNA replication. The next checkpoint is the G2 checkpoint, where the cell checks and makes sure the DNA replicated correctly before beginning mitosis. If the cell does not pass this checkpoint, it commences apoptosis and dies.
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Crossing over occurs during which stage of meiosis?
During prophase I homologous chromosomes will line up with one another, forming tetrads. During this lining up, DNA sequences can be exchanged between the homologous chromosomes. This type of genetic recombination is called crossing over, and allows the daughter cells of meiosis to be genetically unique from one another.
Crossing over can only occur between homologous chromosomes. Cells become haploid after meiosis I, and can no longer perform crossing over.
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During which step of cell division does crossing over occur?
When chromatids "cross over," homologous chromosomes trade pieces of genetic material, resulting in novel combinations of alleles, though the same genes are still present. Crossing over occurs during prophase I of meiosis before tetrads are aligned along the equator in metaphase I.
By meiosis II, only sister chromatids remain and homologous chromosomes have been moved to separate cells. Recall that the point of crossing over is to increase genetic diversity. If crossing over did not occur until sometime during meiosis II, sister chromatids, which are identical, would be exchanging alleles. Since these chromatids are identical, this swap of material would not actually change the alleles of the chromatids.
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What is the evolutionary purpose of cells that undergo crossing over?
Crossing over is a process that happens between homologous chromosomes in order to increase genetic diversity. During crossing over, part of one chromosome is exchanged with another. The result is a hybrid chromosome with a unique pattern of genetic material. Gametes gain the ability to be genetically different from their neighboring gametes after crossing over occurs. This allows for genetic diversity, which will help cells participate in survival of the fittest and evolution.
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What structures exchange genetic material during crossing over?
During crossing over, homologous chromosomes come together in order to form a tetrad. This close contact allows the nonsister chromatids from homolgous chromosomes to attach to one another and exchange nucleotide sequences. The word "nonsister" implies that the chromatids have the same genes, but are not exact copies of one another, as they come from separate chromosomes.
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Crossover of homologous chromosomes in meiosis occurs during which phase?
The crossing over of homologous chromosomes occurs in prophase I of meiosis. Prophase I of meiosis is characterized by the lining up of homologous chromosomes close together to form a structure known as a tetrad. A tetrad is composed of four chromatids.
Anaphase I is marked by the separation of homologous chromosomes, whereas in anaphase II there is the separation of sister chromatids. In anaphase I sister chromatids are still intact and connected at the centromere. Prophase II is similar to prophase in mitosis in that there is the break down of the nuclear membrane and the formation of spindle fibers in preparation for the separation of sister chromatids.
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During crossing over, two homologous chromosomes pair to form which of the following choices?
The tetrad, which divides into non-sister chromatids, exchanges genetic information in order to make the genetic pool more variant, and result in combinations of phenotypic traits that can occur outside of linked genotypic coding.
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Chromosomal crossover occurs in which phase of meiosis?
During prophase I, homologous chromosomes pair with each other and exchange genetic material in a process called chromosomal crossover. The exchange occurs in segments over a small region of homology (similarity in sequence, ie., the same alleles). The new combinations of DNA created during crossover provide a significant source of genetic variation.
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Crossing over is a phenomenon that happens during Meiosis I in the attempt to create genetic diversity. Crossing over typically occurs between which of the following structures?
Crossing over occurs when chromosomal homologs exchange information during metaphase of Meiosis I. During this stage, homologous chromosomes line up on the metaphase plate and exchange genetic information.
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When in meiosis does crossing over occur?
Crossing over occurs during prophase I when parts of the homologous chromosomes overlap and switch their genes.
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How is oogenesis different from spermatogenesis?
When a primary oogonium undergoes meiosis, it will only result in one viable germ cell, or egg. The other smaller cells are called polar bodies and typically disappear following division. Spermatogenesis will result in four separate sperm cells, each capable of producing offspring.
Both eggs and sperm are haploid, and both processes can involve crossing over during meiosis.
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When would an extra copy of chromosome 21 be introduced into the genetic pool?
Down's syndrome is caused by an extra copy of chromosome 21. This trisomy is a result of nondisjunction, which can occur during either meiosis I or meiosis II. Nondisjunction most often occurs during anaphase I of meiosis. Note that most other trisomies and monosomies are lethal.
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Which of the following choices correctly lists the order of development of the listed cell types of oogenesis in humans?
Primary oocyte
Secondary oocyte
Oogonium
Ovum
Oogonia are the primordial oocytes formed inside females either during or shortly after birth. At this time, the formation of primary oocytes occurs during dictyate, which lasts until the surge of luteinizing hormone (LH) just before the onset of puberty. After menarche, a few of these cells will further develop each period into secondary oocytes, which are halted in metaphase II until fertilization. At the end of meiosis II, both polar bodies created to discard extra haploid sets of chromosomes disintegrate, leaving behind the oocyte which can then mature into an ovum. Thus, the correct order is the following order: oogonium, primary oocyte, secondary oocyte, and ovum.
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Many secondary processes enable the primary processes of human gametogenesis to occur smoothly. One important secondary process during human oogenesis is ovarian follicle atresia.
Which of the following is closest to the definition of "ovarian follicle atresia"?
Follicular atresia is a hormone-controlled, apoptotic (cell-suicide) process by which immature follicles degenerate and are resorbed into the main body of the ovary, leaving one out of typically 20 primary follicles standing as a secondary follicle. This process, moderated by follicle stimulating hormone and tumor necrosis factor alpha (TNF), assists the body in forming the corpus luteum out of the remaining follicle following ovulation, as the body would otherwise not be able to generate enough progesterone to continue the process.
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