In this lesson, we will discuss the fundamental characteristic of life: reproduction. Reproduction occurs at various levels of organization. For example, parts of a cell, like chromosomes, create new chromosomes, cells produce new cells, and individuals produce offspring similar to themselves. Rudolf Virchow, a key figure in biology, proposed the important principle that all cells come from other cells. This principle emphasizes that the continuation of life, including all forms of reproduction, relies on the reproduction of cells. We refer to this cellular reproduction as cell division, which is a crucial part of the cell’s life cycle, known as the cell cycle.

Cell Cycle

The cell cycle is the series of events that a cell goes through from the moment it is produced until it completes mitosis and creates new cells. It has two main phases: interphase and the mitotic phase (M phase). The mitotic phase is short and alternates with the longer interphase, during which the cell prepares for division.

During interphase, the cell is very active metabolically and performs various functions. Interphase is divided into three stages: G1 (first gap), S (synthesis), and G2 (second gap).

• G1 Phase: After being formed, the cell begins its cycle in the G1 phase. Here, it increases its protein supply, grows in size, and produces more organelles, such as mitochondria and ribosomes. It also synthesizes enzymes needed for the next phase.
• S Phase: In this phase, the cell duplicates its chromosomes, resulting in each chromosome having two sister chromatids.
• G2 Phase: During the G2 phase, the cell prepares proteins essential for mitosis, particularly for the formation of spindle fibers.

After the G2 phase, the cell enters the M phase, characterized by mitosis, where it divides into two daughter cells. If a cell stops dividing temporarily or permanently, it enters a resting state called the G0 phase.

Mitosis

In the 1880s, German biologist Walther Flemming discovered that during cell division, the nucleus goes through a series of changes known as mitosis. Mitosis is a type of cell division that results in two daughter cells, each containing the same number of chromosomes as the parent cell. This process occurs only in eukaryotic cells, specifically in somatic cells of multicellular organisms. Prokaryotic cells undergo a different process called binary fission, which is similar but not the same as mitosis.

Phases of Mitosis

Mitosis is a complex and regulated process that consists of two main phases: the division of the nucleus (karyokinesis) and the division of the cytoplasm (cytokinesis).

Karyokinesis is further divided into four phases: prophase, metaphase, anaphase, and telophase.

• Prophase: At the start of prophase, the genetic material in the nucleus, which is usually in a loose, thread-like form called chromatin, condenses into structured chromosomes. Each chromosome consists of two sister chromatids held together at a centromere, which has a protein structure called a kinetochore where spindle fibers attach.

Near the nucleus, there are two centrioles that form a structure called the centrosome. Each centriole duplicates, resulting in two centrosomes that move to opposite ends of the cell. They create microtubules from tubulin proteins in the cytoplasm, forming spindle fibers, collectively known as the mitotic spindle. During this phase, the nucleolus and nuclear envelope break down, allowing spindle fibers to occupy the central space.

In plant cells, which have large vacuoles and lack centrioles, the nucleus must move to the center of the cell before prophase begins, and spindle fibers form from tubulin proteins on the nuclear envelope’s surface.

Metaphase

During metaphase, the spindle fibers reach a sufficient length, allowing some fibers, known as kinetochore fibers, to attach to the kinetochores on the chromosomes. Each chromosome is connected by two kinetochore fibers from opposite poles, and they align along the cell’s equator, forming a structure called the metaphase plate. Other spindle fibers, called non-kinetochore fibers, connect with each other from opposite centrosomes.

Anaphase

In anaphase, when a kinetochore spindle fiber attaches to a chromosome’s kinetochore, it begins to pull the chromosome toward its originating centrosome. This pulling force separates the sister chromatids, turning them into sister chromosomes that move apart toward opposite poles of the cell. Meanwhile, the non-kinetochore fibers continue to elongate. By the end of anaphase, the cell has successfully separated the identical copies of chromosomes into two groups at opposite ends.

Telophase

Telophase is essentially the reverse of prophase. A new nuclear envelope forms around each set of separated chromosomes, which then unfold back into chromatin. Nuclear division is now complete, but the cell still needs to finish dividing.

Cytokinesis

Cytokinesis is the process that divides the cytoplasm. In animal cells, it occurs through a mechanism called cleavage. A cleavage furrow forms where the metaphase plate was located, deepening until the parent cell pinches into two daughter cells. In plant cells, cytokinesis happens differently. Vesicles from the Golgi apparatus move to the center of the cell and fuse to create a membrane-bound structure called the cell plate. This plate expands as more vesicles join it, and eventually, the membranes of the cell plate fuse with the plasma membrane, integrating with the parental cell wall.

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