Explain the cell cycle and mitosis, and the relationship between mutation, the cell cycle, and uncontrolled cell growth that can result in cancer (NGSSS SC.912.L.16.5; Reporting Category 1, Molecular and Cellular Biology).

What this topic is asking

The NGSSS benchmark SC.912.L.16.5 asks you to explain the cell cycle and the relationship between mutation, the cell cycle, and uncontrolled cell growth that can result in cancer. For the Florida Biology 1 EOC you need to know the orderly stages a cell passes through to divide, what mitosis produces, how the cycle is regulated, and why a mutation in the controlling genes can cause cancer. The most-tested idea is the chain from a faulty gene to a loss of control to uncontrolled division.

The cell cycle

• Interphase is the longest part of the cycle. The cell grows, carries out its normal work, and, in the S (synthesis) phase, replicates its DNA so each chromosome becomes two identical copies (sister chromatids) joined together. Without this DNA copying, the two daughter cells could not each get a full set.
• The mitotic phase is when division actually happens: mitosis (nuclear division) followed by cytokinesis (cytoplasm division).

Mitosis, phase by phase

Mitosis divides one nucleus into two identical nuclei. The four phases, in order:

1. Prophase. The duplicated chromosomes condense and become visible; the nuclear envelope breaks down; spindle fibers form.
2. Metaphase. The chromosomes line up single file across the middle (the metaphase plate).
3. Anaphase. The sister chromatids are pulled apart to opposite ends of the cell.
4. Telophase. A nuclear envelope reforms around each set, giving two nuclei.

Cytokinesis then splits the cytoplasm, producing two daughter cells. A useful memory aid for the order is PMAT (prophase, metaphase, anaphase, telophase).

How the cycle is regulated

The cell cycle does not run unchecked. Checkpoints monitor whether conditions are right before the cell moves to the next stage: Is the DNA fully copied? Is it undamaged? Are the chromosomes attached correctly? These checkpoints are controlled by genes that produce regulatory proteins, some that promote division when it is needed and some that stop division (apply the brakes) when it is not. This regulation keeps the right number of cells in the right places.

Mutation, the cell cycle, and cancer

This is the heart of the benchmark, and the EOC tests it as a cause-and-effect chain:

mutation in cell-cycle genes leads to loss of regulation leads to uncontrolled cell division leads to a tumor and possibly cancer.

Mutations that lead to cancer can be inherited or caused by environmental factors such as ultraviolet light, tobacco smoke, or certain chemicals (carcinogens), all of which can damage DNA. The damage matters because it strikes the very genes that keep division in check.

Try this

Q1. State what happens to the DNA during interphase and why it is necessary before mitosis. [2]

• Cue. The DNA is replicated (copied) so that each of the two daughter cells can receive a complete, identical set of chromosomes.

Q2. Explain how a mutation can lead to cancer. [2]

• Cue. A mutation in the genes that regulate the cell cycle can remove the controls on division, so the cell divides in an uncontrolled way, forming a tumor that may become cancer.