Question

A student is looking at cells under the microscope. The cells are from an organism that has a diploid chromosome number of 14. In one particular case, the cell has seven replicated chromosomes (sister chromatids) aligned at the metaphase plate of the cell. Which of the following statements accurately describes this particular cell? a. The cell is in metaphase of mitosis. b. The cell is in metaphase of meiosis I. c. The cell is in metaphase of meiosis II. d. All of the above are correct. e. Both b and c are correct.

Short answer

The correct answer is e. Both b (The cell is in metaphase of meiosis I.) and c (The cell is in metaphase of meiosis II.) are correct.

Step-by-step solution

Understand the Phases of Cell Division

Mitosis and meiosis are two types of cell division processes. They are distinguished by the way chromosomes move within the cell. Metaphase is a stage in both mitosis and meiosis where chromosomes align in the middle of the cell before being separated into each of the two newly forming cells. In metaphase of mitosis, each chromosome aligns at the cell equator separately and there is no interaction between the sister chromatids of different chromosomes. However, in metaphase of meiosis I, homologous chromosomes interact and align together at the cell equator. On the contrary, in metaphase of meiosis II, similar to metaphase of mitosis, chromosomes align separately at the cell equator.

Analyze the Chromosome Count

The cell described in the problem has seven replicated chromosomes or sister chromatids aligned at the metaphase plate. Each sister chromatid is a duplicate of a single chromosome. Therefore, there are seven pairs of these, totaling to fourteen. The diploid number of chromosomes in this organism is also noted as 14.

Draw Conclusions Based on Information

Going by the facts, and considering the cell is in the metaphase stage with seven chromosomes aligned at the metaphase plate, one could conclude that the cell is either in the metaphase of meiosis II or metaphase of mitosis because in both these stages, sister chromatids align separately at the cell equator. However, the cell cannot be in the metaphase of meiosis I because in this stage, homologous chromosomes align together, which is not the case here.

Key concepts

Mitosis and Meiosis

When exploring the marvels of cellular replication, students encounter the fundamental processes of mitosis and meiosis. Both mechanisms ensure that cells divide successfully, but their roles and procedures are distinct. Mitosis is the process through which a single cell divides to produce two identical daughter cells, each carrying a duplicate set of chromosomes identical to the parent cell. This process is essential for growth, repair, and asexual reproduction. During mitosis, the diploid chromosome number is maintained, meaning that if a cell starts with 14 chromosomes, each daughter cell will also have 14 chromosomes.

Conversely, meiosis is a special type of cell division necessary for sexual reproduction. It consists of two sequential phases, meiosis I and II, ultimately leading to the formation of four non-identical sex cells (gametes), each with half the number of chromosomes of the parent cell. This reduction in chromosome number ensures genetic diversity in offspring and is the reason why these cells are haploid.

Understanding these two types of cell division is essential when approaching the study of cellular behaviors and genetic inheritance, as it lays the groundwork for grasping more complex biological concepts.

Sister Chromatids Alignment

One of the most striking moments of cell division is the purposeful alignment of sister chromatids. These are essentially identical copies of a single chromosome, created during the S phase of interphase, and held together at a region called the centromere. When a student observes cells under a microscope during the metaphase of mitosis or meiosis II, they will notice these pairs of sister chromatids aligned neatly along a central plane in the cell—this plane is known as the metaphase plate.

In metaphase of mitosis, each chromosome's sister chromatids align with chromosomes pulled by spindle fibers attached to opposite poles of the cell. This precise arrangement is pivotal as it ensures that when the sister chromatids separate during the following anaphase stage, each new daughter cell receives a complete set of chromosomes. Similarly, in metaphase of meiosis II, which follows the separation of homologous chromosomes in meiosis I, sister chromatids line up along the metaphase plate before being pulled apart into different cells. These stages are essential in maintaining genetic stability and diversity.

Diploid Chromosome Number

The diploid chromosome number is a foundational concept within genetics, referring to the total number of chromosomes within a cell, including two sets of each chromosome, one from each parent. In sexually reproducing organisms, like humans, this characteristic diploid state preserves the species-specific number of chromosomes from generation to generation.

For instance, if an organism has a diploid chromosome number of 14, each of its somatic cells (non-sex cells) contains 14 chromosomes, arranged in 7 pairs. These pairs are called homologous because one chromosome of each pair is inherited from the mother and the other from the father. During fertilization, haploid gametes merge to form a diploid zygote, ensuring the offspring has the correct number of chromosomes. Consequently, studying the diploid number offers insights into the intricate dance of chromosomes during cell division and the transfer of genetic material across generations.