Cell Cycle PDF

## Cell cycle ### **Seek your dream and you will reach it** <br> ### Cell cycle 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 essential...

## Cell cycle ### **Seek your dream and you will reach it** <br> ### Cell cycle 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 essential feature of embryonic development** - **Necessary after birth for growth & replacement of dead cells** ### The cell cycle is divided into two major events: 1. **Interphase**: A long period of time (23 hrs). During Interphase the cell: - Increases its size. - Replicates its genetic material (DNA) 2. **Mitosis**: A shorter period of time (1 hr) - During mitosis the cell divides its nucleus first than cytoplasm giving rise to two daughter cells. ### Mitosis: type of division 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 genetics information in various gametes produced is not identical. ### Karyokinesis: - Is the process of the nucleus division followed by division of the cytoplasm called cytokinesis. ### Interphase: - Interphase are subdivided into three phases. - **G1 (Gap) phase**: When the synthesis of macromolecules essential for DNA duplication. 1. The cell synthesizes RNA, regulatory proteins & enzymes essential to DNA replication. 2. The cell volume is restored to normal. 3. The nucleoli are reestablished. 4. The centrioles begin to duplicate themselves & completed in G2 phase. - **S (Synthetic) phase: when DNA is duplicated.** 1. Autosomal cells contain the diploid (2n) amount of DNA before the S phase & doubled (4n) in preparation for cell division. 2. All of the requisite nucleoproteins (histone) are manufactured and incorporated into DNA, forming chromatin. 3. Cell contains twice the normal of DNA. - **G2 (gap) phase: the cells undergo preparation mitosis.** 1. Its the period between the end of DNA synthesis and the beginning of mitosis. 2. Synthesized RNA and proteins essential to cell division. 3. Stored the energy for mitosis. 4. Tubulin is synthesized for assembly into microtubules required for mitosis. ### Mitosis (M) occurs at the conclusion of the G2 phase and completes the cell cycle. - Mitosis are subdivided into four stages: 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase - **1. Prophase** - The centrosome also divides into 2 regions, each half containing a pair of centrioles, which migrate away from each other to opposing poles of the cell - Chromosomes are condensing and become visible microscopically - The chromosomes scattered randomly throughout the cytoplasm. - Each chromosome consists of 2 parallel sister chromatids joined together at one point (the centromere) - The nucleolus and nuclear envelope disappear - **2. Metaphase:** - The chromosomes become maximally condensed & lined up at the equator of the mitotic spindle (Metaphase plate configuration). - **3. Anaphase:** - Anaphase: begins when sister chromatides pull apart and migrate towards the opposite poles. - In the late of phase a cleavage furrow begins to form at the plasma lemma during cytokinesis. - **Animal cell in anaphase:** During this phase, the spindle fibres contract, pulling the chromatids apart. In animal cells, a small cleavage furrow may appear as cytoplasm division begins. - **4. Telophase:** - Is the terminal phase of mitosis - Complete cytokinesis (division of cytoplasm) - Disappearance of spindle fibers - Reconstitution of nucleus & 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 identical in genome. - Prophase - Metaphase - Anaphase - Telophase ### Meiosis: - Is a special type of cell division resulting in the formation of gametes (spermatozoa or ova) - Whose chromosome number has been reduced from diploid (2n) to haploid (1n) number. - Meiosis begins at the conclusion of interphase & it produces germ cells (the ova & 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 it 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 the S phase of the cell cycle preceding meiosis, the amount of the DNA is doubled to 4n but the chromosome number remains at 2n (46 chromosome) - **1. Prophase I:** 1) Last long time & homologous pairs of chromosomes approximate each other, lining up & make synapse forming tetrade 2) Chiasmata (crossing over site) are formed and random exchange of genetic material occurs between homologous chromosomes. - **2. Metaphase I:** 1) Characterized by homologous pairs of chromosomes are lining up in equatorial plate. 2) Each chromosome composed of two chromatids. - **3. Anaphase I:** - Homologous pairs of chromosomes migrate away from each other going to opposing poles. - **4. Telophase I:** 1) Similar to telophase of mitosis. 2) Nuclei are reformed & cytokinesis occurs, giving rise to two daughter cells. 3) Each cell possesses 23 chromosomes (Haploid number) 4) Each chromosome composed of two chromatids. - Daughter nuclei II - Daughter Nuclei - Interphase - Meiosis I - Homologous chromosomes - Meiosis II ### Meiosis II: - Is not preceded by S phase (without DNA synthesis) - Similar to mitosis and subdivided into prophase II, Metaphase II, Anaphase II, Telophase II - The chromosomes lined up in the equator & the kinatochores attach to spindle fibres, followed by chromatides migrate to opposing poles & cytokinesis divides each cell into two cells. - Each cell contains haploid amount of DNA & haploid chromosome number. - Resulting total 4 cells from original diploid germ cell. - Each gamete contain its own unique genetic content. - "***Keep it in mind during cell division in the separation phase where the chromatids or chromosomes have been separated from each other; you will believe in the first instance that the cell has lost part of it; no, my dear, it has separated to produce a greater number of cells; better cells in terms of growth, activity and gaining a new life according to the surrounding environmental conditions***