Question

A diploid cell contains three pairs of homologous chromosomes designated Cl and \(\mathrm{C} 2, \mathrm{M} 1\) and \(\mathrm{M} 2\), and \(\mathrm{S} 1\) and \(\mathrm{S} 2\). No crossing over occurs. What combinations of chromosomes are possible in (a) daughter cells following mitosis, (b) cells undergoing the first meiotic metaphase, (c) haploid cells following both divisions of meiosis?

Short answer

Answer: There are 8 possible combinations of chromosomes.

Step-by-step solution

Daughter cells following mitosis

During mitosis, the chromosomes are replicated, and each daughter cell receives an identical copy of the parent cell's chromosomes. Since the daughter cells are genetically identical to the parent cell, they will have the same chromosome combinations as the original diploid cell, which are: (Cl, M1, S1), (Cl, M1, S2), (Cl, M2, S1), (Cl, M2, S2), (C2, M1, S1), (C2, M1, S2), (C2, M2, S1), and (C2, M2, S2).

Cells undergoing the first meiotic metaphase

In the first meiotic division, homologous chromosomes are separated independently, meaning that different combinations of maternal and paternal chromosomes are possible in each gamete-producing cell. For the given chromosomes, there are 8 possible combinations that can be generated, which are: (Cl, M1, S1), (Cl, M1, S2), (Cl, M2, S1), (Cl, M2, S2), (C2, M1, S1), (C2, M1, S2), (C2, M2, S1), and (C2, M2, S2).

Haploid cells following both divisions of meiosis

After the second meiotic division, the sister chromatids are separated, and the resulting cells are haploid. Since there was no crossing over during the process, the possible combinations of chromosomes in the haploid cells will be the same as in the cells undergoing the first meiotic metaphase. Therefore, the possible haploid cells contain the following combinations of chromosomes: (Cl, M1, S1), (Cl, M1, S2), (Cl, M2, S1), (Cl, M2, S2), (C2, M1, S1), (C2, M1, S2), (C2, M2, S1), and (C2, M2, S2).

Key concepts

Mitosis

Mitosis is a crucial process where a single cell divides to produce two identical daughter cells, each containing a full set of chromosomes identical to the parent cell. This process is vital for growth, tissue repair, and asexual reproduction in organisms. Mitosis consists of several stages, with prophase, metaphase, anaphase, and telophase being the most notable. During these stages, the chromosomes replicate and align at the cell's center before being pulled apart by spindle fibers to opposite sides of the cell.

• As mitosis concludes, the cell splits into two, each receiving a complete set of chromosomes.
• There is no genetic variation introduced because the daughter cells are exact copies of the original diploid cell, maintaining its chromosomal combination.

Understanding mitosis is essential to comprehend cellular processes across various living organisms.

Meiosis

Meiosis is a specialized type of cell division that reduces the chromosome number by half, resulting in the creation of four haploid cells from an original diploid cell. This process is fundamental for sexual reproduction as it ensures genetic diversity through recombination and independent assortment. Meiosis is divided into two stages: meiosis I and meiosis II.

• Meiosis I involves the separation of homologous chromosomes, and meiosis II involves the separation of sister chromatids.
• Each stage contributes to generating unique chromosomal combinations in the resulting gametes, all of which are haploid.

The reductional division in meiosis is key to preventing the doubling of chromosomes in sexually reproducing organisms and results in each new generation having the correct number of chromosomes.

Chromosomal Segregation

Chromosomal segregation is a critical aspect of both mitosis and meiosis. It refers to the proper distribution of chromosomes into daughter cells, ensuring genetic stability and even distribution during cell division. In meiosis, chromosomal segregation introduces genetic variation due to the random assortment of chromosomes.

• During anaphase of mitosis, sister chromatids are pulled apart to opposite poles of the cell.
• In meiosis I, homologous chromosomes are segregated, and in meiosis II, the sister chromatids are separated, leading to diverse combinations in gametes.

Proper chromosomal segregation is necessary to maintain the correct number of chromosomes in daughter cells, and errors in this process can lead to genetic disorders or cell malfunction.

Diploid and Haploid Cells

Cells in organisms can exist as diploid or haploid, vital for understanding genetic variation and reproduction. Diploid cells (2n) possess two sets of chromosomes, one from each parent. They are predominant in somatic cells and contain the total number of chromosomes for a species, like 46 in humans.

• Diploid cells are essential for asexual reproduction and growth through mitosis.
• Haploid cells (n), on the other hand, have half the number of chromosomes, containing just one complete set.
• These are primarily the gametes, such as sperm and egg cells in animals, produced via meiosis.

The transition between diploid and haploid states is a cornerstone in sexual reproduction, ensuring genetic variation and evolutionary adaptability.