Cell Cycle Biology PDF

# Cell cycle ## I. MOHAMMED WALID ELĊANAI * The cell cycle is a series of events within the cell that prepare the cell for dividing into two daughter cells. * Multiplication of cells occurs by division of pre-existing cells. * Multiplication is an essential feature of embryonic development. * I...

# Cell cycle ## I. MOHAMMED WALID ELĊANAI * The cell cycle is a series of events within the cell that prepare the cell for dividing into two daughter cells. * Multiplication of cells occurs by division of pre-existing cells. * Multiplication is an essential feature of embryonic development. * It is necessary after birth for growth and replacement of dead cells. * Mitosis is a type of division where the daughter cells must have chromosomes identical in number and genetic content to those in the mother. * Meiosis consists of two successive divisions called first and second. * In meiosis, the number of chromosomes is reduced to half the normal number. * In meiosis, the genetic information in various gametes produced is not identical. ## The cell cycle is divided into two major events: ### 1-Interphase: A long period of time. * During interphase, the cell: * A. increases its size. * B. replicates its genetic material. ### 2-Mitosis: A shorter period of time. * During mitosis, the cell divides its nucleus first than the cytoplasm giving rise to two daughter cells. ## Cells may cease mitosis permanently (muscle and neurons cells) ## Cells that leave the cell cycle are said to be in the stable phase, the GO (outside) phase or a resting stage. ## INTERPHASE: * Interphase is subdivided into three phases: ### A-G1 (Gap) phase: When the synthesis of macromolecules essential for DNA duplication. * The cells synthesize RNA, regulatory proteins, and enzymes essential to DNA replication. * The cell volume is restored to normal. * The nucleoli are reestablished. * The centrioles begin to duplicate themselves and are completed in the G2 phase. ### B-S (synthetic) phase: When DNA is duplicated. * Autosomal cells contain the diploid (2n) amount of DNA before the S phase and doubled (4n) in preparation for cell division. * All of the requisite nucleoproteins (histone) are manufactured and incorporated into DNA, forming chromatin. * The cell contains twice the normal amount of DNA. ### C-G2 (gap) phase: The cells undergo preparation for mitosis. * It is the period between the end of DNA synthesis and the beginning of mitosis. * Synthesized RNA and proteins essential to cell division. * The energy for mitosis is stored. * Tubulin is synthesized for assembly into microtubules required for mitosis. ## MITOSIS: * Mitosis (M) occurs at the conclusion of the G2 phase and completes the cell cycle. * It is the process of the nucleus division called karyokinesis followed by the division of the cytoplasm called cytokinesis. ## Mitosis is subdivided into four stages: 1. prophase 2. metaphase 3. anaphase 4. telophase. ## 1. Prophase: * The centrosome also divides into two regions, each half containing a pair of centrioles, which migrate away from each other to opposing poles of the cell. * Chromosomes condense and become visible microscopically. * The chromosomes are scattered randomly throughout the cytoplasm. * Each chromosome consists of two parallel sister chromatids joined together at one point (the centromere). * The nucleolus and nuclear envelope disappear. ## 2. Metaphase: * The chromosomes become maximally condensed and lined up at the equator of the mitotic spindle (metaphase plate configuration). ## 3. Anaphase: * Anaphase begins when the sister chromatids are pulled apart and migrate towards the opposite poles. * In the late stage of this phase, a cleavage furrow begins to form at the plasma lemma during cytokinesis. ## Animal Cell in Anaphase * During this phase, the spindle fibers contract, pulling chromatids apart. * In animal cells, a small cleavage furrow may appear as cytoplasm division begins. ## 4. Telophase: * It is the terminal phase of mitosis. * Complete cytokinesis (division of cytoplasm). * Disappearance of spindle fibers. * Reconstitution of the nucleus and nuclear envelope. * Unwinding of chromosomes into chromatin. ## NOTE RESULT: * Each daughter cell possesses a diploid (2n) number of chromosomes. * Each daughter cell resulting from mitosis is identical in its genome. ## MEIOSIS: * It is a special type of cell division resulting in the formation of gametes (spermatozoa or ova). * The chromosome number is reduced from diploid (2n) to haploid (1n) number. * Meiosis begins at the conclusion of interphases, and it produces germ cells: ova and sperms. ## This process has two crucial results: 1. Reduction in the number of chromosomes from diploid (2n) to haploid (1n). 2. Recombination of genes ensuring genetic variability and diversity. ## Meiosis is divided into two separate events. ### Meiosis I (Reductional division): 1. Separate the homologous pairs of chromosomes, thus reducing the number from diploid to haploid. 2. In gametogenesis, where the germ cells are in S phase of the cell cycle preceding meiosis, the amount of DNA is doubled to 4n, but the chromosome number remains at 2n (46 chromosomes). ### 1.Prophase I: * The homologous pairs of chromosomes approximate each other, lining up and making synapses forming tetrades. * Chiasmata (crossing-over sites) are formed, and random exchange of genetic material occurs between homologous chromosomes. ### 2. Metaphase I: * It is characterized by homologous pairs of chromosomes lining up in the equatorial plate. Each chromosome is composed of two chromatids. ### 3. Anaphase I: * Homologous pairs of chromosomes migrate away from each other going to opposing poles. ### 4. Telophase I: * It is similar to telophase of mitosis. * Nuclei are reformed, and cytokinesis occurs, giving rise to two daughter cells. * Each cell possesses 23 chromosomes (haploid number). * Each chromosome is composed of two chromatids. ## MEIOSIS II: * It is not preceded by S phase (without DNA synthesis). * Similar to mitosis, and it is subdivided into prophase II, metaphase II, anaphase II, and telophase II. * The chromosomes line up in the equator, and the kinatochores attach to spindle fibers, followed by chromatids migrating to opposing poles, and cytokinesis divides each cell into two cells. * Each cell contains a haploid amount of DNA and a haploid chromosome number. * It results in a total of four cells from the original diploid germ cell. * Each gamete contains its own unique genetic content.