Comprehensive Notes on the Cell Cycle, Interphase, and Cancer

Fundamental Overview of the Cell Cycle

The cell cycle is the sequence of events which involves growth of a newly formed cell, replication of its genome, and ultimate division into two daughter cells. It consists of two main phases: interphase and the mitotic phase. Interphase is the period in the cell cycle between two consecutive divisions, during which the cell grows and prepares for division.

Stages of Interphase

Interphase is divided into three distinct sub-phases: $G_1$ (Gap 1), $S$ (Synthesis), and $G_2$ (Gap 2).

The $G_1$ phase starts with the production of a new cell. During this phase, the cell grows in size, increases the number of its organelles, and forms needed proteins and other substances. The cell prepares itself for the next phase of the cell cycle ($S$ phase) by forming enzymes and nucleotides of DNA required for the replication process.

The $S$ phase is the synthesis phase of DNA. The cell replicates its entire genetic material to form its two copies. Consequently, each chromosome has two chromatids during cell division, one for each of the daughter cells. These chromatids are attached with one centromere and are called sister chromatids.

During the $G_2$ phase, the cell prepares for the mitotic phase. The cell forms proteins, especially those required for the formation of spindle fibres. Additionally, the cell accumulates energy to carry out the division process, and more mitochondria are formed.

Non-Dividing Phase ($G_0$ Phase)

During $G_1$, a cell may exit the cell cycle and enter the $G_0$ phase. In the $G_0$ phase, the cell stops dividing. The duration and permanence of the $G_0$ phase depend on the specific cell type:

Some cells, such as neurons, never divide again once mature and remain in the $G_0$ phase forever.

Cells of the liver, kidneys, and related organs enter the $G_0$ phase temporarily. When needed, they re-enter the cell cycle and start to divide again.

Many epithelial cells divide continuously throughout their life cycle and never enter the $G_0$ phase.

Mitotic Phase ($M$ Phase)

During the mitotic phase ($M$ phase), the cell divides into daughter cells, distributing its replicated genetic material and cell contents.

Cancer and Tumor Development

Cancer is a disease of the cell cycle. Unlike normal cells of the body, cancer cells do not have a proper functioning cell cycle control system and therefore divide excessively. This excessive growth can result in an abnormal mass of cells called a tumour. Not all tumours are cancerous; a benign tumour is an abnormal mass of essentially normal cells that always remain at their original site.