Lesson 1: Cell Cycle

Video Lesson:

Competences

At the end of this lesson, you will be able to:

• Define cell cycle,
•  Outline  the different stages of cell cycle  and
•  Collect  information on cancer

Overview of the Cell Cycle

The cell cycle is a series of stages that cells go through to grow and divide. This process is essential for growth, development, and repair in multicellular organisms. The cell cycle consists of two main phases: Interphase and the Mitotic (M) phase.

Brainstorming Questions

Dear learner! What are the main divisions of a  cell  cycle ?

Before dealing with cell cycle, it is important to refresh your memory about cell structure and function, particularly the nucleus where key processes of cell reproduction takes place.

A cell cycle is a sequence of events that takes place in the parent cell as a means of distributing genetic materials thereby forming daughter cells (Figure below).  There are two main divisions of the cell cycle: Interphase and cell division.

Interphase

Interphase is the period during which the cell grows and prepares for division. It is subdivided into three stages:

1. G1 Phase (Gap 1):
   • The cell grows in size.
   • It synthesizes various enzymes and nutrients needed for DNA replication and cell division.
   • The cell performs its regular metabolic functions.
2. S Phase (Synthesis):
   • DNA replication occurs, resulting in two copies of each chromosome (sister chromatids).
   • The cell continues to grow and produce proteins necessary for chromosome structure.
3. G2 Phase (Gap 2):
   • The cell undergoes final preparations for division.
   • It synthesizes proteins and organelles required for the mitotic phase.
   • The cell checks for DNA damage and repairs any errors.

The Cell Division

Cell Division:

• Nuclear Division (Karyokinesis):
  • Also known as mitosis or the M stage.
  • Involves the division of the cell’s nucleus into two identical nuclei.
• Cytoplasmic Division (Cytokinesis):
  • Follows karyokinesis.
  • Divides the cytoplasm, resulting in two separate daughter cells.

This process ultimately produces two identical daughter cells. Each daughter cell then grows and enters its own cell cycle, repeating the process undergone by the mother cell.

Importance of Controlled Cell Division

Dear learner! What will happen if cell cycle  is not controlled?

The cell cycle and division processes are tightly regulated by an internal controlling system to ensure a regular pattern of growth and replication. If this regulatory system fails, cells may begin to divide excessively, a condition often referred to as “cell madness.” This uncontrolled division can lead to cancer, where cells no longer respond to the normal cell cycle controls and may invade other tissues of the body.

Cell Division and Its Significance

In Grade 9, you learned that a cell is the basic structural and functional unit of life. The ability of cells to reproduce is what distinguishes living organisms from non-living ones. All living organisms start from at least a single cell, such as the unicellular organism amoeba, and multicellular organisms grow by repeated cell division. According to cell theory, new cells arise from pre-existing cells through cell division. This principle, known as the cell doctrine, was postulated by Rudolf Virchow in 1858 and underpins the continuity of life.

The Cell Cycle

Cell division involves a series of precisely timed and regulated stages of growth, DNA replication, and nuclear and cytoplasmic division. This process results in two identical daughter cells from a single parent cell. As cells grow, their cytoplasmic volume increases relative to the cell membrane, leading to a low surface area-to-volume ratio, which impedes material transport and nuclear control. Cell division solves these problems by creating more manageable-sized cells.

Dear learner! What is the importance of cell division to  organisms ?

Reasons for Cell Division

• Unicellular Organisms: Cell division is a means of reproduction, increasing the population of organisms like amoeba.
• Multicellular Organisms: Cell division increases cell numbers for growth, repair of damaged or dead cells, and production of gametes in reproductive organs.

The Process of Cell Division

After the G2 stage of interphase, cell division consists of two main subdivisions:

1. Nuclear Division (Karyokinesis):
   • Separates and distributes duplicated genetic materials to daughter cells.
   • Occurs through mitosis or meiosis.
2. Cytokinesis (Cytoplasmic Division):
   • Separates the cytoplasmic components into the daughter cells.

Next, you will learn about the arrangement and redistribution of duplicated genetic material during mitosis and meiosis.