The Cell Cycle, Mitosis, and Mutations

Study Tip: When identifying mitosis under a microscope, look for the position of the chromosomes. If they are in a single line in the middle, you are looking at Metaphase!

Cytokinesis: Dividing the Cytosol

While mitosis handles the genetic material, cytokinesis handles the rest of the cell. This stage divides the cytosol to complete the formation of two distinct daughter cells. The method of division depends on the type of organism:

• In Animals: A cleavage furrow forms, where the plasma membrane appears to pinch inward at the equator.
• In Plants: Because of the rigid cell wall, a cell plate forms at the equator to divide the two new cells.

Mutations and the Breakdown of Control

The cell cycle is governed by specific genes that act as "brakes" and "accelerators." When these genes are damaged, mutations occur, which can lead to uncontrolled cell growth.

The Drivers of Mutation: UV Light

One of the most common external causes of DNA damage is UV light from the sun. UV light is a form of electromagnetic radiation; the shorter the wavelength, the higher the energy of the wave.

There are three main types of UV light:

• UVA (400-320nm): Penetrates deep into skin layers and can cause cancer.
• UVB (320-290nm): The primary cause of sunburns.
• UVC (290-200nm): The most energetic, but it is mostly absorbed by the atmosphere's ozone layer.

Proto-oncogenes vs. Oncogenes

In a healthy cell, proto-oncogenes are responsible for regulating the cell cycle, ensuring cells divide only when necessary. However, if a mutation occurs, these can transform into oncogenes. Oncogenes are "super active" versions of the gene that drive the cell to divide uncontrollably.

Tumor Suppressors

Conversely, cells have tumor suppressors that act as checkpoints to stop division if something is wrong. A mutation can render these proteins non-functional, effectively "removing the brakes" from the cell cycle.

Tumors and Malignancy

When mutations accumulate, abnormal masses of cells called tumors can form. These are categorized based on their behavior:

• Benign Tumor: A mass of abnormal cells that remains localized to its original site and does not spread.
• Malignant Tumor: A mass of abnormal cells whose growth cannot be controlled by regular mechanisms. These cells have the ability to invade surrounding tissues and spread to other parts of the body, a state known as malignancy.

Malignancy can be caused by environmental factors like UV light, but it can also be genetically predisposed, meaning an individual may inherit mutations in their cell cycle genes from their parents. A notable example is melanoma, a type of skin cancer that originates in the melanocyte cells (the cells that produce melanin).

Mathematical Note: Understanding SPF Sun Protection Factor (SPF) measures how much longer you can stay in the sun compared to not wearing sunscreen. You can calculate your protected time using this formula:

$\text{Total Time} = \text{Unprotected Time} \cdot \text{SPF}$

Example: If you normally burn after 20 minutes and apply SPF 30: $\text{Total Time} = 20 \cdot 30 = 600 \text{ minutes (10 hours)}$