Cell Cycle by Mohammed Walid Elċanai PDF

## Cell Cycle **Seek your dream and you will reach it.** **Mohammed Walid Elčanai** * **Cell cycle** is a series of events within a cell that prepare it for dividing into two daughter cells. This process occurs by division of pre-existing cells. * **Multiplication of cells** is essential for e...

## Cell Cycle **Seek your dream and you will reach it.** **Mohammed Walid Elčanai** * **Cell cycle** is a series of events within a cell that prepare it for dividing into two daughter cells. This process occurs by division of pre-existing cells. * **Multiplication of cells** is essential for embryonic development and is necessary after birth for growth and replacement of dead cells. * **Mitosis** is a type of cell division where daughter cells have chromosomes identical in number and genetic content to those in the mother cell. * **Meiosis** consists of two successive divisions called first and second and results in a reduction of the number of chromosomes to half the normal number. The genetic information in various gametes produced is not identical. **Cell cycle is divided into two major events:** **Interphase:** A long period of time where the cell: 1. **Increases in size** 2. **Replicates its genetic material** **Mitosis:** A shorter period of time where the cell divides its nucleus first, then the cytoplasm, generating two daughter cells. ### Interphase Interphase is subdivided into **three phases:** 1. **G1 (Gap) phase**: When the synthesis of macromolecules essential for DNA duplication occurs. * The cells synthesize RNA, regulatory proteins, and enzymes essential for DNA replication. * The cell volume is restored to normal. * The nucleoli are reestablished. * The centrioles begin to duplicate themselves, and this is completed in G2 phase. 2. **S (Synthetic) phase**: When DNA is duplicated. * Autosomal cells contain the diploid (2n) amount of DNA before the S phase, and this is 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. 3. **G2 (gap) phase**: The cells undergo preparation for mitosis. * This is the period between the end of DNA synthesis and the beginning of mitosis. * Synthesized RNA and proteins essential for cell division are produced. * 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 nucleus division called **karyokinesis**, followed by division of the cytoplasm called **cytokinesis**. * Mitosis is subdivided into **four stages:** * Prophase * Metaphase * Anaphase * Telophase ### Prophase 1. The centrosome divides into two regions, each containing a pair of **centrioles**, which migrate away from each other towards opposing poles of the cell. 2. **Chromosomes** condense and become visible microscopically. 3. The chromosomes are scattered randomly throughout the cytoplasm. 4. Each chromosome consists of two parallel sister chromatids joined together at one point (the **centromere**). 5. The **nucleolus and nuclear envelope disappear**. ### Metaphase * Chromosomes become maximally condensed and line up at the **equator of the mitotic spindle** (Metaphase plate configuration). ### Anaphase 1. Anaphase begins when **sister chromatids** are pulled apart and migrate towards the opposite poles. 2. In the late stage of this phase, a **cleavage furrow** begins to form at the plasma membrane during **cytokinesis**. ### Telophase 1. **Complete cytokinesis** (Division of cytoplasm). 2. **Disappearance** of spindle fibers. 3. **Reconstitution** of nucleus and nuclear envelope. 4. **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 genome. ### Meiosis * Meiosis 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). * Meiosis begins at the conclusion of interphase and it produces germ cells, the ova and sperm. **This process has two crucial results:** 1. **Reduction** in the number of chromosomes from diploid (2n) to haploid (1n). 2. **Recombination** of genes ensures genetic **variability and diversity**. ### 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 DNA is doubled to 4n, but the chromosome number remains at 2n (46 chromosomes). ### Prophase I: 1. **Homologous pairs** of chromosomes approximate each other, lining up and making synapses to form a **tetrad**. 2. **Chiasmata** (crossing over sites) are formed and random exchange of genetic material occurs between homologous chromosomes. ### Metaphase I: 1. **Homologous pairs** of chromosomes are lined up in the **equatorial plate**. 2. Each chromosome is composed of **two chromatids**. ### Anaphase I: * Homologous pairs of chromosomes migrate away from each other going to opposing poles. ### Telophase I: 1. Similar to telophase of mitosis. 2. Nuclei are reformed and cytokinesis occurs. 3. This gives rise to two daughter cells. 4. Each cell possesses 23 chromosomes (haploid number). 5. Each chromosome is composed of **two chromatids**. ### 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 line up in the equator, and the kinetochores attach to spindle fibers, followed by chromatids migrating to opposing poles, and cytokinesis divides each cell into two cells. * Each cell contains haploid amount of DNA and a haploid chromosome number. * This results in a total of 4 cells from the original diploid germ cell. * Each gamete contains its own unique genetic content. ## Additional Notes * During the **anaphase** stage of mitosis or meiosis, the cell is thought to have lost part of its content, however, this is not the case. The separation of chromosomes during this stage ensures cell division and the production of more cells. * For the same reason that cells divide, we are also able to learn and grow and experience new things in life. Even though we may experience some setbacks, we can learn from them, grow stronger, and find new ways to move forward in life.