Problem 9 - Campbell Biology Concepts & Connections

Question

A micrograph of a dividing cell from a mouse showed 19 chromosomes, each consisting of two sister chromatids. During which of the following stages of cell division could such a picture have been taken? (Explain your answer.) a. prophase of mitosis b. telophase II of meiosis c. prophase I of meiosis d. prophase II of meiosis

Step-by-Step Solution

Answer

The picture was taken during prophase II of meiosis.

Step 1 - Identify Number of Chromosomes

Observe that the micrograph of the dividing cell from a mouse shows 19 chromosomes. In mice, the diploid number of chromosomes is 40. Therefore, the fact that there are 19 chromosomes observed suggests that we are not looking at a normal mitotic cell, as mitosis would show 40 chromosomes.

Step 2 - Consider Two Sister Chromatids

Each chromosome consists of two sister chromatids. This indicates that the chromosomes have undergone replication but not yet separated into individual chromosomes. This typically occurs during the stages of mitosis and meiosis after DNA replication.

Step 3 - Rule Out Mitosis Stages

During mitosis, particularly in prophase, metaphase, anaphase, and telophase, a dividing cell from a mouse should show 40 chromosomes when fully duplicated (each consisting of two sister chromatids). Thus, 19 chromosomes cannot represent any mitosis stage.

Step 4 - Analyze Meiosis Stages

During meiosis, cells go through reductional division to create haploid cells. Specifically, meiosis I results in the halving of chromosome number. During prophase I of meiosis, cells would show 40 chromosomes with sister chromatids (as in mitosis prophase).

Step 5 - Consider Prophase II

In prophase II of meiosis, cells start with haploid chromosome number due to the reduction division in meiosis I. Thus, what was originally a diploid number (40) would be halved to form 20 chromosomes. It’s logical these cells are observed in prophase II with paired sister chromatids, showing a view of 19 chromosomes due to an unusual anomaly.

Step 6: Conclusion - Choose the Correct Stage

Considering the evidence, the stage that matches 19 chromosomes each with two sister chromatids most accurately within meiosis, is prophase II. Prophase II of meiosis is when the cell is readying to divide again with haploid chromosomes each having two sister chromatids.

Key Concepts

meiosis

Meiosis is a special type of cell division that reduces the chromosome number by half. This process is crucial for sexual reproduction and leads to the formation of gametes in animals or spores in plants.

Meiosis involves two consecutive divisions: meiosis I and meiosis II. Each division has several stages that ensure the proper distribution of chromosomes.

In meiosis I, homologous chromosomes (pairs of similar chromosomes from each parent) are separated. This results in two cells with half the original number of chromosomes. These chromosomes still consist of two sister chromatids joined at the centromere.

Meiosis II resembles mitosis, where sister chromatids are finally separated into individual chromosomes. By the end of meiosis II, four haploid cells are produced from the original diploid cell, each containing a unique set of chromosomes.

chromosomes

Chromosomes are long strands of DNA wrapped around proteins called histones. They carry genetic information vital for cell function and are located in the nucleus of eukaryotic cells.

Every species has a specific number of chromosomes. For example, humans have 46, while mice have 40. Chromosomes ensure that DNA is accurately copied and distributed during cell division.

During cell division, chromosomes condense and become visible under a microscope. Each chromosome consists of two identical sister chromatids joined together by a region called the centromere.

When cells divide, these sister chromatids are separated so that each new cell inherits the correct number of chromosomes. This precise distribution is essential to maintain genetic stability.

sister chromatids

Sister chromatids are two identical copies of a single chromosome that are connected by a centromere. They are formed during the DNA replication process before a cell goes through division.

The primary purpose of sister chromatids is to ensure accurate distribution of genetic material to the daughter cells. During cell division, these chromatids are pulled apart, ensuring each new cell receives an exact copy of the genetic information.

In stages like prophase of mitosis or prophase II of meiosis, chromosomes appear as X-shaped structures where the two arms of the X are the sister chromatids. They remain attached to one another until they are separated in later stages, like anaphase.

Understanding sister chromatids is crucial for grasping how genetic information is preserved and passed on during cell division.

prophase II

Prophase II is one of the stages in meiosis II, the second division of meiosis. It follows meiosis I, which reduces the chromosome number by half.

In prophase II, cells prepare for another round of division. Each cell entering prophase II is haploid, containing half the original chromosome number but each chromosome still consists of two sister chromatids.

During this stage:

• The nuclear envelope, which had reformed at the end of meiosis I, begins to disintegrate again.
• Chromosomes condense and become visible under the microscope.
• A new spindle apparatus starts to form to help segregate the sister chromatids in the ensuing division.

Prophase II is critical because it sets the stage for the final separation of sister chromatids in the subsequent phases, ensuring that each resulting cell has a correct and diverse set of chromosomes.