I have answered all the other questions on this sheet but I am stuck on this one:

Why is it fair to assume that the proportion of cells in each phase of the cell cycle represents the proportion of time spent by individual cells?

This is to do with the cell cycle and mitosis.

the time is taking by cells individually for enlargement of cells in G1 phase and replication of DNA in S phase and synthesis of cell organells etc

To answer the question, you need to understand the concept of the cell cycle and its stages. The cell cycle is a series of events that cells go through as they grow and divide. It consists of three main stages: interphase, mitosis, and cytokinesis.

1. Interphase: This is the longest phase of the cell cycle where the cell prepares for division. During interphase, the cell grows, replicates its DNA, and carries out normal cell functions. It is further divided into three subphases: G1, S, and G2.

2. Mitosis: This phase involves the actual division of the cell's nucleus into two identical daughter nuclei. It is divided into four stages: prophase, metaphase, anaphase, and telophase.

3. Cytokinesis: This is the final stage of the cell cycle, where the cytoplasm divides, resulting in the formation of two separate daughter cells.

Now, let's address the given question: Why is it fair to assume that the proportion of cells in each phase of the cell cycle represents the proportion of time spent by individual cells?

It is fair to assume that the proportion of cells in each phase of the cell cycle represents the proportion of time spent by individual cells for a few reasons:

1. Length of the cell cycle stages: Each stage of the cell cycle takes a different amount of time to complete. For example, interphase is the longest stage and can take up to 90% of the total cell cycle time. On the other hand, mitosis is relatively shorter and takes only a fraction of the total cell cycle time. Therefore, if you observe a large proportion of cells in interphase, it suggests that most cells are spending their time in this stage.

2. Phases are distinct and non-overlapping: The cell cycle stages are well-defined and distinct from each other. Once a cell completes a particular stage, it moves onto the next stage. This means that at any given time, a cell will predominantly be in one particular stage. Therefore, observing a higher proportion of cells in a specific phase indicates that individual cells spend more time in that phase.

3. Regulation of cell cycle progression: The transition from one phase to another in the cell cycle is tightly regulated by various molecular signals and checkpoints. This ensures that cells spend an appropriate amount of time in each phase. If a cell undergoes an abnormality or is delayed in a particular phase, it will affect the overall proportion of cells in that phase.

To summarize, by observing the proportion of cells in each phase of the cell cycle, you can infer the proportion of time spent by individual cells in those phases. The longer a particular stage takes, the higher the proportion of cells you would expect to find in that stage at any given time.