Problem 11 - Biology: The Dynamic Science

Question

You have a technique that allows you to measure the amount of DNA in a cell nucleus. You establish the amount of DNA in a sperm cell of an organism as your baseline. Which multiple of this amount would you expect to find in a nucleus of this organism at \(\mathrm{G}_{2}\) of premeiotic interphase? At telophase I of meiosis? During interkinesis? At telophase II of meiosis?

Step-by-Step Solution

Answer

In G2 of premeiotic interphase, you would expect to find 2x the amount of DNA as in a sperm cell. At telophase I of meiosis and during interkinesis, it would still be 2x because no new DNA replication occurs. At telophase II of meiosis, the amount would return to 1x because sister chromatids have been separated, leaving a haploid amount of DNA.

Step 1: Understanding Cell Cycle Phases

First, understand the stages of the cell cycle and the changes in DNA content. A sperm cell is a haploid cell, meaning it has one set of chromosomes. G2 phase is the phase in the cell cycle after DNA has been replicated but before mitosis begins. In telophase I of meiosis and telophase II of meiosis, cells have undergone divisions that reduce the DNA content.

Step 2: DNA content at G2 phase

In the G2 phase of premeiotic interphase, DNA has been replicated. This means the cell has twice the amount of DNA relative to a sperm cell because every chromosome has been duplicated. Therefore, we would expect to find a multiple of 2.

Step 3: DNA content at telophase I of meiosis

At telophase I of meiosis, the cell divides for the first time, separating homologous chromosomes but not sister chromatids. As a result, each new cell still has the same amount of DNA as the original cell had in G2 (if they were diploid to start), but since we started with a haploid sperm cell and the organism duplicated its DNA for meiosis, we have a multiple of 2.

Step 4: DNA content during interkinesis

Interkinesis is the short rest period between meiosis I and meiosis II. There is no further DNA replication here, meaning the DNA content remains the same as it was at the end of telophase I, which is a multiple of 2.

Step 5: DNA content at telophase II of meiosis

During telophase II of meiosis, sister chromatids have been separated into different cells. This means that each cell has half the amount of DNA as that of the G2 phase of a diploid cell. Since our baseline is a haploid sperm, the amount of DNA at telophase II would be equivalent to the haploid amount, which is a multiple of 1.

Key Concepts

G2 Phase of Interphase

The G2 phase of interphase is a crucial stage in a cell's life cycle where it prepares for the process of cell division. During this phase, several important events take place, including the complete replication of the cell's DNA. Since every chromosome has been duplicated, the amount of DNA doubles.

Consider a haploid sperm cell with a single set of chromosomes—the baseline for DNA content. At the completion of the G2 phase, a cell derived from such a sperm cell after DNA replication would contain twice the DNA of the baseline, making it a multiple of 2. It's essential to note that the cell at this point has not yet undergone the physical process of division, which means the doubled DNA is still within a single nucleus.

Telophase I of Meiosis

Telophase I marks the end of the first meiotic division, which is characterized by several significant events. Notably, during this phase, homologous chromosomes (which consist of two sister chromatids each) are separated into two daughter cells, but the sister chromatids remain attached.

Given that meiosis is a specialized form of cell division that results in the production of gametes, the DNA content in each cell at telophase I would be the same as it was after the G2 phase preceding meiosis. Therefore, if the organism duplicated its DNA and entered meiosis as a haploid – like the sperm cell – the DNA content in each of the two cells produced by telophase I would still represent a multiple of 2. This is because the cell has yet to undergo the second meiotic division, which will split the sister chromatids.

Interkinesis

Following the completion of meiosis I, cells enter a transition stage known as interkinesis. This period is akin to an intermission between the two divisions of meiosis. It's important to highlight that, unlike in the interphase prior to mitosis, no DNA replication occurs during interkinesis.

As a result, the quantity of DNA in each cell remains consistent with the amount present at the end of telophase I. This means that the cells in interkinesis would maintain a multiple of 2 compared to the baseline DNA content of a haploid sperm cell. This stage is brief, and its primary function is to prepare the cells for the impending second meiotic division.

Telophase II of Meiosis

Telophase II signifies the conclusion of the second and final division in meiosis. In this phase, the sister chromatids that were previously conjoined are now pulled apart and segregated into separate daughter cells. Each of these new cells has a single set of chromosomes, similar to the original haploid condition of the sperm cell baseline.

Therefore, when we assess the DNA content at telophase II in comparison to our baseline haploid sperm cell, we find that each of the resulting cells has a multiple of 1. This is because they contain a single set of chromosomes, reflecting the haploid state that's characteristic of gametes. Consequently, telophase II culminates in cells that are ready to embark on their journey as independent gametes, potentially contributing to the creation of a new organism upon fertilization.