Question

The nuclear DNA content of a single sperm cell in Drosophila melanogaster is approximately 0.18 picogram. What would be the expected nuclear DNA content of a primary spermatocyte in Drosophila? What would be the expected nuclear DNA content of a somatic cell (non-sex cell) in the G1 phase? What would be the expected nuclear DNA content of a somatic cell at metaphase?

Short answer

Answer: The expected nuclear DNA contents are as follows: - Primary spermatocyte: 0.72 picograms - Somatic cell in G1 phase: 0.36 picograms - Somatic cell at metaphase: 0.72 picograms

Step-by-step solution

Determine the DNA content in a primary spermatocyte

In Drosophila, the DNA content of a single sperm cell is 0.18 picogram. During spermatogenesis, primary spermatocytes undergo meiosis to produce four sperm cells. Therefore, the DNA content of a primary spermatocyte must be four times that of a single sperm cell. We can calculate this as follows: Primary spermatocyte DNA content = 4 * DNA content of a sperm cell Primary spermatocyte DNA content = 4 * 0.18 picograms

Calculate the DNA content of a primary spermatocyte

Now, let's calculate the DNA content of a primary spermatocyte: Primary spermatocyte DNA content = 4 * 0.18 picograms Primary spermatocyte DNA content = 0.72 picograms

Determine the DNA content of a somatic cell in the G1 phase

During the G1 phase of the cell cycle, the DNA content of a somatic cell has not been duplicated yet. Since a somatic cell is diploid, it contains twice the DNA content of a haploid sperm cell (which has half the DNA content). Therefore, the DNA content of a somatic cell in the G1 phase can be calculated as: G1 phase somatic cell DNA content = 2 * DNA content of a sperm cell G1 phase somatic cell DNA content = 2 * 0.18 picograms

Calculate the somatic cell DNA content in the G1 phase

Now, let's calculate the DNA content of a somatic cell in the G1 phase: G1 phase somatic cell DNA content = 2 * 0.18 picograms G1 phase somatic cell DNA content = 0.36 picograms

Determine the DNA content of a somatic cell at metaphase

At metaphase, the DNA content of a somatic cell has been duplicated during the S phase of the cell cycle. Therefore, the DNA content of a somatic cell at metaphase is twice that of a somatic cell in the G1 phase. We can calculate this as follows: Metaphase somatic cell DNA content = 2 * G1 phase somatic cell DNA content Metaphase somatic cell DNA content = 2 * 0.36 picograms

Calculate the somatic cell DNA content at metaphase

Now, let's calculate the DNA content of a somatic cell at metaphase: Metaphase somatic cell DNA content = 2 * 0.36 picograms Metaphase somatic cell DNA content = 0.72 picograms In summary: - The expected nuclear DNA content of a primary spermatocyte in Drosophila is 0.72 picograms. - The expected nuclear DNA content of a somatic cell in the G1 phase is 0.36 picograms. - The expected nuclear DNA content of a somatic cell at metaphase is 0.72 picograms.

Key concepts

Spermatogenesis in Drosophila

Spermatogenesis is the process by which sperm cells are produced from male primordial germ cells by way of mitosis and meiosis. In Drosophila melanogaster, a model organism commonly used in genetic studies, this process is particularly interesting due to its unique features.

Each primary spermatocyte that forms after the initial rounds of mitotic divisions undergoes meiosis to create four haploid sperm cells. Unlike in humans, Drosophila spermatogenesis includes a phase of chromosomal condensation leading to the sausage-shaped DNA in sperm heads, which is why the DNA content is a defining factor for studying this process.

Considering a single sperm cell has 0.18 picograms of DNA, one can infer that a primary spermatocyte will have four times this amount prior to meiosis - a crucial detail in understanding the reproductive biology of Drosophila and the nuances of meiosis.

Cell Cycle Phases

To fully understand the changes in nuclear DNA content, it is essential to be familiar with the cell cycle phases. The cell cycle is a series of events that leads to cell division and replication. There are four main phases: G1, S, G2, and M.

During the G1 phase, the cell grows and carries out normal functions. In the context of DNA content, a somatic cell in the G1 phase has not yet initiated DNA synthesis and thus contains a single copy of the genome.

Importance of the S Phase

Following G1, the S phase marks the period where DNA replication occurs. A somatic cell's DNA content would double during this stage, preparing the cell for division in the subsequent phases of mitosis.

Mitosis: The Peak of DNA Content

Finally, during the M phase, particularly at metaphase, the DNA, which was duplicated in the S phase, aligns along the metaphase plate of the cell, resulting in a doubled DNA content compared to the G1 phase. Clear understanding of these phases is fundamental in analyzing and predicting the DNA content at different stages of the cell cycle.

Metaphase DNA Duplication

DNA duplication is a pivotal event in cell division, ensuring that each daughter cell inherits a complete set of genetic information. This duplication occurs during the S phase of the cell cycle, which precedes metaphase.

At metaphase, chromosomes are most condensed and aligned at the cell's equatorial plate, ready to be separated into daughter cells. It's at this stage that the duplication becomes evident. The fact that the DNA content at metaphase is double that of the G1 phase reflects the completion of the replication process.

Understanding this duplication is critical, as errors during this phase can lead to chromosomal abnormalities. In summary, the nuclear DNA content measurement in a somatic cell at metaphase, which, as calculated before, is 0.72 picograms, serves as a benchmark for the successful completion of DNA replication.