Cell Cycle & Nucleus Lecture Notes PDF

Nucleus and cell cycle: M.Sc. Elaf Qussay Department: Basic Sciences College of Dentistry Al maaqal University Nucleus  Nucleus it is the control center of the cell that control the metaboli...

Nucleus and cell cycle: M.Sc. Elaf Qussay Department: Basic Sciences College of Dentistry Al maaqal University Nucleus  Nucleus it is the control center of the cell that control the metabolism and reproduction, it contains DNA (genetic material) that found on the chromosomes. The nucleus is the largest organelle, dark and round, surrounded by a double membrane called the nuclear envelope. Nuclear pore is a large complex of proteins that allows small molecules and ions to freely pass, or diffuse, into or out of the nucleus. The main functions of the nucleus include: 1. Protein synthesis, cell division, and differentiation. 2. Control the synthesis of enzymes involved in cellular metabolism 3. Controlling hereditary traits of the organism.  Chromosomes are thread-like structures made up of strands of DNA and the proteins. Cell cycle and Cell division :  Cell cycle or cell division is a very important process in all living organism.  The cell cycle represents a self-regulated sequence of events that controls cell growth and cell division.  The goal of the cell cycle is to produce two daughter cells, each containing chromosomes identical to those of the parent cell.  The cell cycle in eukaryotic cells divided into two main type : 1. Mitosis (Asexual reproduction cycle): Is the process in which the nucleus of a cell divides to create two new nuclei, each containing an identical copy of DNA (somatic cells). 2.Meiosis (Sexual reproduction cycle): Is the process by which certain sex cells are created. In the male, the body uses meiosis to create sperm cells in the female, it uses meiosis to create egg cells. Phases of the Cell Cycle  The cell cycle incorporates two principal phases: the interphase, and the M phase (mitosis). A) Interphase :The cell grows at a steady rate throughout interphase; It is the longest part of the cell cycle involving three sub-phases G1 (gap1) phase, S (synthesis) phase, and G2 (gap 2) phase. Same cell (Nerve and muscle cell) typically do not complete the cell cycle and are permanently arrested in G1. These cells are said to have entered a G0 stage During interphase, the cell grows, accumulating nutrients needed for mitosis, and replicates its DNA and some of its organelles. Phases Events (G1) phase During which the cell is metabolically active and continuously grows but does not replicate its DNA (S)Phase During which DNA replication takes place. (G2) phase During which cell growth continues and proteins are synthesized in preparation for mitosis.  Cell Cycle Checkpoints The restriction checkpoint (or “point of no return”) is the most important ´checkpoint in the cell cycle. At this checkpoint, the cell self- evaluates its own replicative potential before deciding to either enter the S phase and the next round of cell division or to retire and leave the cell cycle.  Malfunction of any of the checkpoints at the G1, S, and G2 phases of the cell cycle and the checkpoint at M phase may lead to a mitotic catastrophe.  Mitotic catastrophe is defined as the failure to arrest the cell cycle before or at mitosis, resulting in abnormal chromosome separation.  Under normal conditions, death in these cells will occur by activation of the apoptotic cycle.  Cells that fail to execute the apoptotic cycle are likely to divide asymmetrically in the next round of cell division.  This leads to the generation of aneuploid cells (cells containing abnormal chromosome numbers).  Thus, a mitotic catastrophe may be regarded as one of the mechanisms contributing to oncogenesis (tumor cell development).  B) Mitosis (M) phase: Mitosis nearly always includes both karyokinesis (division of the nucleus) and cytokinesis (division of the cell) and lasts about 1 hour. Mitosis takes place in several stages described in more detail below. Separation of two identical daughter cells concludes the M phase.  Phases of Mitosis 1. Prophase: this is the first step in mitosis  The replicated chromatin condenses and become visible as chromosomes.  Each chromosome can be seen to consist of two chromatids.  The sister chromatids are held together by the ring of proteins at the centromere. In late prophase, the nuclear envelope begins to disintegrate, and the nucleolus completely disappears.  2.Metaphase: In this phase, Each chromatid is attached to a spindle fiber, centromeres of duplicated chromosomes are aligned at the equator.  3.Anaphase Its shortest stage of mitosis. the centromeres divide, and the sister chromatids of each chromosome are pulled apart and move to the opposite ends of the cell, pulled by spindle fibers attached to the kinetochore regions. Kinetochore: protein structure disc-shaped, found at the centromere of a chromatid which microtubules attach to during cell division.  4. Telophase: During telophase, the chromosomes become indistinct chromatin again. The spindle disappears , the nuclear envelope and nucleolus reaper, daughter cells are forming. Cytokines  It is the process by which the cytoplasm of the cell splits to form two independent cells. The separation at the cleavage furrow is achieved by a contractile ring consisting of a very thin array of actin filaments positioned around the perimeter of the cell. many granular matrix formed by the Golgi body and endoplasmic reticulum accumulates in the equatorial region. These granular matrix form cell plate. Cytokinesis is the shortest phase of the cell cycle. Thank you