Question

consider a diploid cell that contains three pairs of chromosomes designated \(\mathrm{AA}, \mathrm{BB}\), and \(\mathrm{CC}\). Each pair contains a maternal and a paternal member (e.g., \(A^{\mathrm{m}}\) and \(\mathrm{A}^{\mathrm{p}}\) ). Using these designations, demonstrate your understanding of mitosis and meiosis by drawing chromatid combinations as requested. Be sure to indicate when chromatids are paired as a result of replication and/or synapsis. You may wish to use a large piece of brown manila wrapping paper or a cut-up paper grocery bag for this project and to work in partnership with another student. We recommend cooperative learning as an efficacious way to develop the skills you will need for solving the problems presented throughout this text. Are there any possible combinations present during prophase of meiosis II other than those that you drew in Problem \(26 ?\) If so, draw them.

Short answer

Answer: Some possible chromatid combinations during prophase of meiosis II in a diploid cell containing three pairs of chromosomes include: - Cell 1: {A^m, B^m, C^m} - Cell 2: {A^p, B^m, C^m} - Cell 3: {A^m, B^p, C^m} - Cell 4: {A^p, B^p, C^m} - Cell 5: {A^m, B^m, C^p} - Cell 6: {A^p, B^m, C^p} - Cell 7: {A^m, B^p, C^p} - Cell 8: {A^p, B^p, C^p} Note that only four of these combinations will be present in the final haploid cells, as meiosis II results in four haploid cells. The combinations shown take into account all possible interactions between the chromatids of the three chromosome pairs.

Step-by-step solution

Understanding Meiosis#

Meiosis is a type of cell division that results in four daughter cells, each with half the number of chromosomes of the parent cell. It consists of two consecutive divisions: meiosis I and meiosis II. In meiosis I, homologous chromosomes pair up and undergo recombination, followed by separation into two daughter cells. In meiosis II, sister chromatids separate, resulting in four haploid cells.

Prophase of Meiosis II#

During prophase of meiosis II, the chromosomes condense, and the nuclear envelope breaks down. Centrosomes move to opposite poles of the cell, and spindle fibers begin to form. However, unlike during prophase of meiosis I, homologous chromosomes do not pair up through synapsis in prophase of meiosis II.

Chromatid Combination Diagram in Prophase of Meiosis II#

To draw the chromatid combinations during prophase of meiosis II, we first need to know the combinations from problem 26. Unfortunately, we do not have access to this information. Therefore, we will create a general solution based on the scenario presented in the exercise: 1. Label the maternal (A^m, B^m, C^m) and paternal (A^p, B^p, C^p) chromatids. 2. Consider all possible combinations of chromatids, keeping in mind that the maternal and paternal chromatids from the same chromosome pair (e.g., A^m and A^p) cannot be in the same haploid cell. Here is one example of possible chromatid combinations during prophase of meiosis II: - Cell 1: {A^m, B^m, C^m} - Cell 2: {A^p, B^m, C^m} - Cell 3: {A^m, B^p, C^m} - Cell 4: {A^p, B^p, C^m} - Cell 5: {A^m, B^m, C^p} - Cell 6: {A^p, B^m, C^p} - Cell 7: {A^m, B^p, C^p} - Cell 8: {A^p, B^p, C^p}

Determining Other Possible Combinations#

Based on the combinations generated in Step 3, we may identify other possible combinations if any exist. To determine this, we need to consider the possible chromatid arrangements and the process of meiosis: 1. All chromatids are still connected to their sister chromatids since they have not been separated yet, and homologous chromosomes do not undergo synapsis during prophase of meiosis II. 2. We generated eight combinations of chromatids, but it's essential to note that we can only obtain four haploid cells after meiosis II. Therefore, we would only see four possible chromatid combinations in real cells during prophase of meiosis II. 3. As prometaphase, metaphase, anaphase, and telophase of meiosis II would proceed, these combinations would evolve until we see final haploid cells, ensuring genetic diversity. By understanding the process of meiosis II and the possible chromatid arrangements during prophase, we do not have to draw any other combinations because our current combinations take into account all possible interactions between the chromatids of the three chromosome pairs.

Key concepts

Chromatid Combinations

In meiosis II, particularly during prophase, diverse chromatid combinations are made possible due to the random assortment of maternal and paternal chromatids. Unlike mitosis where sister chromatids are precisely divided to ensure each new cell is genetically identical to the parent cell, meiosis II contributes to genetic variation by shuffling the genetic deck. After the first meiotic division, each cell has a mix of maternal and paternal chromosomes. Then, during meiosis II, the chromatids – each a half of a duplicated chromosome – are sorted into new cells in a variety of combinations. This genetic shuffling results in cells that are distinct from the parent cell and from each other.

In our exercise, the chromatids labeled as A^m, B^m, C^m (maternal) and A^p, B^p, C^p (paternal) show the potential outcomes in prophase of meiosis II. To illustrate these combinations, it’s important to remember that each of the resulting cells will have a unique mix of these labeled chromatids. For example, while one cell may have A^m from the mother and B^p, C^p from the father, another might comprise entirely maternal or paternal chromatids, like A^p, B^p, C^p.

When drawing these combinations, it's critical to ensure there's an even split; each daughter cell should end up with a single copy of each chromosome – no duplicates – embodying a unique genetic code. This principle is at the heart of sexual reproduction and how it generates diversity within a species.

Cell Division

Cell division is a foundational concept in understanding not just meiosis II, but all of biological growth and reproduction. For organisms to grow, repair tissues, or reproduce, cells must divide. Meiosis II is a special type of cell division, vital for sexual reproduction because it ensures that each gamete (sperm or egg) contains exactly half the genetic material of the parent cell.

The process of meiosis II, which follows meiosis I, includes several stages: prophase II, metaphase II, anaphase II, and telophase II. Once the cell has undergone meiosis I, it’s primed for these subsequent phases. In prophase II, chromosomes condense and the nuclear envelope dissolves, while the spindle apparatus forms and prepares to pull sister chromatids apart. This step sets the stage for the precise separation of genetic material. Directly following prophase II, during metaphase II, sister chromatids line up at the cell's equator, ensuring that when they are pulled apart in anaphase II, each new daughter cell receives an identical set of genes. Finally, telophase II sees the actual formation of the new nuclei around each set of chromatids, and after cytokinesis, four unique haploid cells, each with their own distinct genetic makeup, are formed.

Genetic Diversity

Genetic diversity is the cornerstone of evolution and species survival, enabling populations to adapt to changing environments. Meiosis II greatly contributes to this genetic diversity in several ways. Firstly, during meiosis I, homologous chromosomes exchange genetic material in a process called crossing over, which shuffles the genetic information. Then, the random distribution of these recombined chromosomes into different cells during the first meiotic division increases variability.

The key event for increasing diversity during meiosis II is the random assortment of sister chromatids into gametes. Each chromatid carries different versions of genes due to the crossing over that occurred earlier. Because these chromatids sort independently, the combination of genes in the resulting gametes is unique each time meiosis occurs. This randomness of assortment means that even siblings from the same parents have significant genetic differences.

It's this genetic diversity that provides populations with a wide range of traits, some of which may be advantageous in specific environments or situations. This diversity is the raw material for natural selection, and ultimately, it's what drives the evolution of species. Thus, understanding the role meiosis II plays in genetic variation is not just about knowing the biology of reproduction but also appreciating the intricate mechanisms that underpin biodiversity itself.