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Subject: biology
A cell is observed to be actively synthesizing proteins and duplicating its organelles, but its DNA content remains unchanged. In which phase of the cell cycle is this cell most likely operating?
Answer: G1 phase
During the G1 phase of interphase, the cell grows, makes proteins, and duplicates organelles, while DNA replication (which occurs in the S phase) has not yet begun.
Which of the following best explains why cytokinesis in plant cells involves the formation of a cell plate, unlike the cleavage furrow seen in animal cells?
Answer: Plant cells have a rigid cell wall that prevents the cell membrane from pinching inward.
Plant cells possess a rigid cell wall that prevents the formation of a cleavage furrow, which is a pinching mechanism. Instead, they build a cell plate that develops into a new cell wall to divide the cell.
What is the primary reason meiosis is essential for maintaining a constant chromosome number in sexually reproducing organisms across generations?
Answer: It halves the chromosome number in gametes, so fertilization restores the diploid number.
Meiosis reduces the chromosome number from diploid (2n) to haploid (n) in gametes. This reduction is critical because when two haploid gametes fuse during fertilization, the diploid chromosome number characteristic of the species is restored, preventing a doubling of chromosomes in each generation.
Which statement accurately describes the key difference in chromosome movement during anaphase of mitosis compared to anaphase I of meiosis?
Answer: In mitosis anaphase, sister chromatids separate; in meiosis I anaphase, homologous chromosomes separate.
In anaphase of mitosis, the centromeres split, and sister chromatids are pulled apart. In contrast, during anaphase I of meiosis, whole homologous chromosomes separate and move to opposite poles, while the sister chromatids of each chromosome remain attached.
How do crossing over and independent assortment collectively contribute to genetic variation in sexually reproducing organisms?
Answer: Crossing over shuffles alleles between homologous chromosomes, and independent assortment randomly combines parental chromosomes in gametes.
Crossing over, occurring in prophase I, involves the exchange of DNA segments between non-sister chromatids, creating new allele combinations on individual chromosomes. Independent assortment, in metaphase I, refers to the random orientation of homologous pairs, leading to a vast number of different chromosome combinations in each gamete. Together, these mechanisms ensure genetic diversity.
What is the primary consequence if a cell loses its ability to regulate cell cycle checkpoints?
Answer: The cell will undergo uncontrolled division, potentially leading to tumor formation.
Cell cycle checkpoints are crucial for stopping the cycle if DNA is damaged or replication is incomplete. The loss of this control allows cells to divide uncontrollably, which is the fundamental mechanism behind tumor formation and cancer.
Which of the following best describes the primary purposes of mitosis in a multicellular organism?
Answer: To facilitate growth, repair, and replacement of damaged or old cells.
Mitosis produces two genetically identical diploid daughter cells. Its primary roles in multicellular organisms are for growth (increasing organism size), repair (healing wounds), and replacement (shedding old skin cells). It can also be used for asexual reproduction in some organisms.
A human cell is found to contain 92 chromatids. According to the definition of a chromosome, how many chromosomes does this cell possess at this moment?
Answer: 46 chromosomes, because each replicated chromosome still consists of two sister chromatids.
Even after DNA replication in the S phase, where each chromosome consists of two sister chromatids, it is still counted as one chromosome as long as the sister chromatids remain attached at the centromere. A human diploid cell normally has 46 chromosomes, so after replication, it has 46 chromosomes, each with two chromatids, totaling 92 chromatids.
Nondisjunction during meiosis can lead to genetic disorders such as Down syndrome. What is the fundamental cellular event that causes nondisjunction?
Answer: The failure of homologous chromosomes or sister chromatids to separate properly during anaphase.
Nondisjunction is defined as the failure of homologous chromosomes to separate during anaphase I or the failure of sister chromatids to separate during anaphase II. This results in gametes having an abnormal number of chromosomes, which can lead to conditions like trisomy 21 (Down syndrome) upon fertilization.
A {0} is a single long DNA molecule wound around proteins called histones.
Answer: chromosome
A chromosome is the fundamental unit of genetic material, consisting of DNA wrapped around proteins.
Before a cell divides, each chromosome is copied into two identical copies called sister {0}.
Answer: chromatids
Sister chromatids are the two identical copies of a replicated chromosome that remain attached to each other.
Sister chromatids remain attached to each other at a region called the {0}.
Answer: centromere
The centromere is the constricted region of a chromosome that holds sister chromatids together and serves as an attachment point for spindle fibers.
Human body cells are {0}: they contain 46 chromosomes arranged as 23 homologous pairs.
Answer: diploid
Diploid cells (2n) contain two complete sets of chromosomes, one from each parent.
Gametes are {0}: they contain 23 chromosomes, one from each pair.
Answer: haploid
Haploid cells (n) contain a single set of chromosomes, such as gametes.
{0} are haploid sex cells that contain 23 chromosomes, one from each pair.
Answer: Gametes
Gametes are the reproductive cells (sperm and egg) that carry half the genetic information.
Fertilization fuses two haploid gametes into a diploid {0}, restoring the full number of 46 chromosomes.
Answer: zygote
A zygote is the diploid cell formed by the fusion of two haploid gametes during fertilization.
Most of the cell cycle is {0}, when the cell is growing and preparing for division.
Answer: interphase
Interphase is the longest phase of the cell cycle, during which the cell grows, duplicates organelles, and replicates its DNA.
{0} is nuclear division that produces two genetically identical diploid daughter cells.
Answer: Mitosis
Mitosis is a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus.
{0} is the division that produces gametes, yielding four haploid daughter cells.
Answer: Meiosis
Meiosis is a specialized type of cell division that reduces the chromosome number by half, creating four haploid cells, each genetically distinct from the parent cell.
{0} carry the same genes at the same positions but may carry different alleles of those genes.
Answer: Homologous chromosomes
Homologous chromosomes are a pair of chromosomes (one from each parent) that are similar in length, gene position, and centromere location.
In prophase I, homologous chromosomes pair up along their length in a process called {0}.
Answer: synapsis
Synapsis is the pairing of homologous chromosomes during prophase I of meiosis, forming a bivalent.
While paired in prophase I, non-sister chromatids exchange segments of DNA at crossover points called chiasmata, a process known as {0}.
Answer: crossing over
Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes during meiosis, leading to genetic recombination.
The random orientation of homologous pairs at the cell equator in metaphase I is called {0}.
Answer: independent assortment
Independent assortment is the random distribution of homologous chromosomes into daughter cells during meiosis, contributing to genetic variation.
If a homologous pair fails to separate in anaphase I, or sister chromatids fail to separate in anaphase II, the failure is called {0}.
Answer: nondisjunction
Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division, leading to an abnormal number of chromosomes in daughter cells.
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