General Biology

• The cell membrane is a selectively permeable barrier that separates the cell from its environment.
• Mitosis is the process by which a cell replicates its chromosomes and then segregates them, producing two identical nuclei in preparation for cell division.
• Mitosis was first discovered in plant cells by Strasburger in 1875 and later on, W. Flemming in 1879 discovered it in animal cells.
• The term “Mitosis” was coined by Flemming.
• There are three main reasons for Mitosis: cell or organism growth, replacement or repair of damaged cells, and asexual reproduction.
• A typical cell goes through a process of growth, development and reproduction called the cell cycle.
• Interphase, the period preceding mitosis, is the longest phase of the cell cycle and has three distinct sub-stages: G1 phase, S phase, and G2 phase.
• During the G1 phase, cells grow, produce RNA and synthesize proteins.
• The S phase, also known as the synthesis phase, is where DNA replication occurs.
• During the G2 phase, cells continue growing and preparing for mitosis and the cell checks that DNA replicated without errors.
• Nucleus becomes spherical and cytoplasm becomes more viscous during Prophase.
• The chromatin slowly condenses into well-defined chromosomes during Prophase.
• Each chromosome appears as two sister chromatids joined at the centromere during Prophase.
• The microtubules begins to form outside the nucleus during Prophase.
• Nuclear envelop breaks down into membrane vesicles and the chromosomes set free into the cytoplasm during Prometaphase.
• Chromosomes are attached to spindle microtubules through kinetochores during Prometaphase.
• Nucleolus disappears during Prometaphase.
• Kinetochore microtubules align the chromosomes in one plane to form equatorial plate or metaphase plate during Metaphase.
• Smaller chromosomes remain towards the center while larger ones occupy the periphery during Metaphase.
• The shortest of all stages of mitosis, Anaphase, occurs during Anaphase.
• Chromosomes splits simultaneously at the centromeres so that the sister chromatids separate during Anaphase.
• The separated sister chromatids move towards opposite poles during Anaphase.
• Telophase, considered as the reverse phase of prophase, occurs during Telophase.
• Daughter chromosomes arrive at the poles during Telophase.
• Kinetochore microtubules disappear during Telophase.
• Chromosomes uncoil into chromatin during Telophase.
• Nucleolus re-appears during Telophase.
• Cytokinesis, the division of the Cytoplasm creating two daughter cells, occurs during Cytokinesis.
• G0 Phase, a “resting” phase where the cell exits the cell cycle and stops dividing, occurs during G0 Phase.
• Cytokinesis II results in four cells that are genetically different haploid daughter cells, each containing only one set of chromosomes.
• Meiosis is the process in which a single cell divides twice to form four haploid daughter cells, which are the gametes, sperm in males and egg in females.
• The process of meiosis is divided into two stages, each stage is subdivided into several phases.
• Meiosis I includes Prophase I, Metaphase I, Anaphase I, Telophase I, and Cytokinesis I.
• Meiosis II includes Prophase II, Metaphase II, Anaphase II, Telophase II, and Cytokinesis II.
• Diploid refers to a cell that contains both sets of homologous chromosomes.
• Haploid refers to a cell that contains only a single set of chromosomes.
• Homologous refers to chromosomes that each have a corresponding chromosome from the opposite-sex parent.
• Homologous chromosomes are similar in shape, position, and information, but not identical to each other.
• Synapsis is the pairing of homologous chromosomes that occurs during meiosis.
• Tetrad is a group of two homologous chromosomes, each consisting of four chromatids.