Problem 21 - Campbell Biology Concepts & Connections

Question

A mule is the offspring of a horse and a donkey. A donkey sperm contains 31 chromosomes and a horse egg cell contains 32 chromosomes, so the zygote contains a total of 63 chromosomes. The zygote develops normally. The combined set of chromosomes is not a problem in mitosis, and the mule combines some of the best characteristics of horses and donkeys. However, a mule is sterile; meiosis cannot occur normally in its testes (or ovaries). Explain why mitosis is normal in cells containing both horse and donkey chromosomes but the mixed set of chromosomes interferes with meiosis.

Step-by-Step Solution

Answer

Mitosis in mules is normal because chromosome duplication does not require pairing, while meiosis fails due to the odd number of chromosomes preventing proper pairing.

Step 1: Understand Mitosis

Mitosis is a process of cell division that results in two daughter cells, each having the same number and kind of chromosomes as the parent nucleus. This process ensures that each daughter cell produces exactly the same genetic material as the parent cell.

Step 2: Understand Meiosis

Meiosis is a process of cell division that reduces the chromosome number by half, resulting in four daughter cells, each genetically unique with half the number of chromosomes as the parent cell. This process is crucial for sexual reproduction.

Step 3: Chromosome Count in Mule

A mule has a combined chromosome set of 63 because it inherits 31 chromosomes from the donkey (via sperm) and 32 chromosomes from the horse (via egg).

Step 4: Mitosis in Mule Cells

During mitosis, the chromosomes are duplicated and evenly divided into two identical daughter cells. The mule's cells can undergo mitosis normally because the chromosome number (63) does not need to be matched with another cell, simply duplicated and divided.

Step 5: Problems in Meiosis

During meiosis, chromosomes need to pair up (synapse) accurately for successful division. The odd number of chromosomes (63) in mules results in unpaired chromosomes which cannot segregate evenly, causing problems in the formation of haploid gametes and leading to sterility.

Step 6: Conclusion

Thus, mules are sterile because their mixed set of chromosomes cannot pair up properly during meiosis, preventing the formation of viable gametes.

Key Concepts

Chromosome Number

Chromosomes are structures within cells that contain genetic material. In animals, each species has a characteristic number of chromosomes. A horse has 64 chromosomes (32 pairs), while a donkey has 62 chromosomes (31 pairs). In the case of mules, which are hybrids between horses and donkeys, they end up with 63 chromosomes. This uneven number of chromosomes can create certain complications, especially in processes like meiosis.

Despite the mixed set of chromosomes, a mule can grow and develop normally because mitosis, the process of general cell division, can handle the chromosome count without issues. However, this number becomes problematic in meiosis, which is responsible for producing reproductive cells.

Mitosis

Mitosis is a type of cell division that results in two daughter cells, each with the same genetic material as the original cell. This process is essential for growth, development, and tissue repair. Here’s how it works:

When a cell undergoes mitosis, it duplicates its chromosomes, so each daughter cell receives an exact copy of the genetic material. This ensures that all cells in an organism have the same number and type of chromosomes.

• Prophase: Chromosomes condense and become visible.
• Metaphase: Chromosomes align in the center of the cell.
• Anaphase: Chromosomes are pulled apart to opposite ends of the cell.
• Telophase: The cell divides into two, forming two identical daughter cells.

In mules, mitosis proceeds normally despite the mixed chromosome number. The 63 chromosomes are duplicated and distributed evenly, allowing for normal cell function and development.

Meiosis

Meiosis is a specialized type of cell division that reduces the chromosome number by half, producing four unique cells, each with half the genetic material of the parent cell. This process is crucial for sexual reproduction, ensuring genetic diversity.

Meiosis involves two rounds of division:

• Meiosis I: Homologous chromosomes pair up and separate, resulting in two cells with half the original number of chromosomes (though each chromosome consists of two sister chromatids).
• Meiosis II: The sister chromatids separate, resulting in four unique haploid cells.

However, in mules, meiosis encounters problems because of their uneven chromosome number (63). During meiosis I, chromosomes must pair up correctly for successful division. The odd number of chromosomes leads to unpaired chromosomes, causing issues in forming balanced gametes.

Sterility in Hybrids

Sterility is a common issue in hybrids, such as mules, due to difficulties in meiosis caused by uneven chromosome numbers. Mules inherit 31 chromosomes from a donkey and 32 from a horse, totaling 63. This odd number complicates the pairing process during meiosis.

For meiosis to proceed smoothly, chromosomes must pair up accurately. The unpaired chromosomes in mules fail to segregate properly, preventing the formation of viable gametes (sperm or eggs). As a result, mules cannot produce offspring, rendering them sterile.

This sterility highlights a significant biological barrier that prevents hybrid species like mules from reproducing. It ensures that genetic differences between parent species are maintained, preserving the unique characteristics of each species.