Replication and division of nuclei and cells — practice questions

During the S phase (synthesis phase) of interphase, the cell undertakes DNA replication. This is when each chromosome, which initially consists of a single DNA molecule, is duplicated to form two identical sister chromatids. These sister chromatids remain joined at the centromere.

The critical importance of this process is to ensure that when the cell undergoes mitosis, each of the two resulting daughter cells receives a complete and identical set of genetic information. Without accurate DNA replication in the S phase, daughter cells would either lack essential genetic material or have an incorrect chromosome number, leading to abnormal cell function or death, making proper growth and repair impossible.

The mitotic index is a measure of the proportion of cells in a tissue that are undergoing mitosis. It is an indicator of the rate of cell proliferation.

Step 1: State the formula The formula for the mitotic index is: $\text{Mitotic Index} = \frac{\text{Number of cells in mitosis}}{\text{Total number of cells}} \times 100\%$

Step 2: Identify the given values

• Number of cells in mitosis = 35
• Total number of cells = 250

Step 3: Substitute the values into the formula $\text{Mitotic Index} = \frac{35}{250} \times 100\%$

Step 4: Calculate the result $\text{Mitotic Index} = 0.14 \times 100\%$ $\text{Mitotic Index} = 14\%$

Final Answer: The mitotic index for this onion root tip tissue is 14%.