BIOLOGY Lecture Notes - Golden Pages 2003

3. CELL DIVISION

The Cell Life Cycle

Beginning with The process of division, all Cells undergo the so-called "Cell life cycle," which culminates either in division yielding a new cell or in cell death. The duration of this cycle is species-specific and ranges from a few hours to decades. Throughout their life cycle, cells pass through distinct phases of varying duration. This timing depends on The Cell type and is characterized by strictly specific metabolic processes. These phases are designated as G₁, S, and G₂.

Interphase is one of the periods of the Cell Cycle. During this time, the cell grows, differentiates, Functions, and prepares for division. The first period of interphase is the pre-synthetic, or G phase (from the English "gap"). Throughout this period, the Genetic information encoded in the DNA is at its peak functional activity, directing the Synthesis of specific RNAs and Proteins. This is the longest period. The nuclei of such cells contain a diploid set of Chromosomes, with each chromosome represented by a single DNA molecule. The genetic formula of such a cell is 2n2c, where n represents the haploid set of chromosomes, and c denotes the DNA content. During the subsequent synthetic (S) phase, METABOLISM/36.html">DNA Replication takes place. The features of DNA replication in eukaryotes are associated with the large size of the DNA molecule, the presence of nucleosomes, and nuclear matrix structures. Replication may occur at different times (early, mid, or late S phase). Such asynchrony is observed both within a single chromosome and across different chromosomes.

During DNA replication, the nuclear matrix (specifically, the structural Proteins of the nuclear matrix) plays a crucial role. In addition to DNA replication, the S phase involves the intensive synthesis and transport of histone proteins into The Nucleus, which are essential for packaging the newly synthesized DNA into nucleosomes.

Various Histones undergo modifications during interphase: as cells progress through the individual Phases of the cycle, H₁ histones become phosphorylated. This alters The properties of DNA throughout the mitotic cycle, particularly its accessibility to enzymatic attack by DNases.

In the post-synthetic G₂ phase, cells prepare for mitosis. During this stage, the Cytoskeleton gradually breaks down, and chromosome Condensation (compaction) begins. The synthesis of proteins, Lipids, and CARBOHYDRATES intensifies, and new cell Organelles are formed. The cell increases significantly in size. The G₂ phase transitions into prophase, which is the precise moment in the cell cycle when chromosomes first become visible under a Light Microscope.

Cell Division is the increase in cell number resulting from the reproduction and division of a parent cell. According to a core tenet of Cell Theory, "cells arise only from pre-existing cells," which rules out the "spontaneous generation" of cells or their formation from non-cellular "living matter." Three types of cell division are known: mitosis, amitosis, and Meiosis. Mitosis and amitosis are characteristic of somatic cells (body cells) in all eukaryotes. Meiosis occurs during The formation of Germ Cells in animals. Some cells divide frequently and continuously (such as epithelial cells, Cytology/practical/86.html">Red Bone Marrow cells, and periosteal cells), whereas other cells, once formed, never divide again and survive for the entire lifespan of the Organism (such as nerve and Muscle cells). The higher the degree of cell specialization, the lower its capacity for division.