Use a model of the cell cycle to explain how cell division and differentiation support growth, maintenance, and repair, and how a loss of control leads to cancer (Ohio's Learning Standards for Science, Biology, B.C.1).

What this topic is asking

Ohio standard B.C.1 states that "cell division and differentiation are the basis for growth, maintenance and repair of all multicellular organisms." Ohio's Biology EOC turns this into items on the cell cycle, the phases of mitosis, what mitosis is for, and what goes wrong in cancer. Because the standards ask you to use models, the exam often gives a diagram of the cell cycle or a sequence of dividing cells and asks you to label, order, or reason from it.

The cell cycle

The cell cycle is the full life of a dividing cell, and it has two broad parts.

• Interphase. The longest part. The cell grows, carries out its normal jobs, and copies its DNA so that each new cell will get a complete set. By the end of interphase every chromosome is duplicated into two identical sister chromatids.
• Mitosis and cytokinesis. The cell divides its copied DNA evenly into two nuclei (mitosis) and then splits the cytoplasm into two cells (cytokinesis).

Ohio standard B.C.1 stresses that this division supports growth (more cells), maintenance, and repair (replacing damaged cells, such as healing a cut). In single-celled and some multicellular organisms, mitosis is also a form of asexual reproduction.

The phases of mitosis (PMAT)

Mitosis is the part where the duplicated chromosomes are separated. The EOC expects the four phases in order.

• Prophase. The chromosomes condense into visible structures and the nuclear membrane begins to break down.
• Metaphase. The chromosomes line up along the middle (the metaphase plate).
• Anaphase. The sister chromatids separate and are pulled to opposite ends of the cell.
• Telophase. Two new nuclear membranes form, one around each set of chromosomes, giving two nuclei.

Cytokinesis then divides the cytoplasm, finishing with two genetically identical daughter cells, each with the same chromosome number as the parent.

Differentiation

Making identical copies is only half the story. After cells divide, they differentiate: they switch on different genes and become specialized cell types (a muscle cell, a nerve cell, a red blood cell). Every cell in your body has the same DNA, but each type uses a different part of it, which is why a nerve cell and a skin cell look and act differently. Differentiation is how one fertilized egg builds a body with many tissues, the link Ohio standard B.C.4 makes between cells and the whole organism.

When control is lost: cancer

The cell cycle is normally regulated by genes that act as checkpoints, telling a cell when to divide and when to stop. A mutation in those regulatory genes can disable the controls. When that happens:

1. the checkpoints fail, so the cell divides when it should not,
2. the cell divides in an uncontrolled way,
3. the dividing cells pile up into an abnormal mass called a tumor,
4. a tumor that grows into and spreads to other tissues is cancer.

This chain, mutation, then loss of control, then uncontrolled division, then tumor, then cancer, is exactly what the EOC asks you to describe.

Try this

Q1. List the four phases of mitosis in order and state one event in each. [4]

• Cue. Prophase (chromosomes condense), metaphase (chromosomes line up in the middle), anaphase (sister chromatids separate to opposite ends), telophase (two new nuclei form).

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

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