9 Mitosis, Meiosis and Gene Aberration

• First multicellular organisms appeared about 600-800 million years ago
• Chromosomal aberration and changes in chromosome number may occur
• Gene mutation


  • Substitution, addition, deletion changing the amino acid sequence including frameshift
• Mitotic cell cycle


  • Significance including growth, repair, asexual reproduction, and the need for tight regulation
• Mitosis

  Daughter nuclei possess the same type and number of chromosomes as the parent nuclei
• The first single cellular organisms appeared about 3.5 billion years ago, roughly about a billion years after Earth was formed
• Mitosis and Meiosis


  Bring about genetic stability and genetic variation in organisms respectively
• Meiotic cell cycle


  • Significance including how meiosis and random fertilisation can lead to variation
• Mitotic cell cycle


  Events that occur and main stages of mitosis including behaviour of chromosomes, nuclear envelope, cell membrane, and centrioles
• Nuclear division


  Stage of the cell cycle where genetic information stored in the form of DNA on chromosomes is passed on to daughter cells
• Meiosis

  Daughter nuclei (haploid) end up with half the number of chromosomes as the parent cell (diploid), may have different allelic combinations due to allelic exchanges
• Chromosomal aberration

  • Numerical aberration (aneuploidy) and structural aberration (translocation, duplication, inversion, deletion)
• Meiotic cell cycle


  Events that occur and main stages of meiosis including behaviour of chromosomes, nuclear envelope, cell membrane, and centrioles
• Cell cycle


  A systemic and precise process required as life progressed from single-cellularity to multi-cellularity
• Down syndrome arises due to the presence of an additional copy of chromosome 21
• Chromosomal Aberrations: Variation in chromosomal structure, Variation in chromosomal number
• Interphase - S phase


  Semi-conservative DNA replication occurs
• Significance of Meiosis
• Cytokinesis

  Division of cytoplasmic contents into 2 daughter cells
• Meiosis

  Interphase, Prophase I, Prophase I (late), Metaphase I, Anaphase I, Telophase I, Meiosis II, Cytokinesis after meiosis II
• The cell cycle consists of 3 main stages: Interphase, Nuclear division (either mitosis or meiosis), Cytokinesis
• Cells do not normally divide continuously; a certain period elapses between two divisions
• Interphase

  Stage of preparation for mitosis/meiosis, non-dividing phase of the cell, longest phase of the cell cycle (90% of cell cycle)
• Keywords
• Significance of mitosis: Growth, repair, asexual reproduction, need to regulate tightly
• Interphase - G1 phase


  Intensive cellular synthesis: Organelle synthesis, RNA synthesis, Protein synthesis
• Cell Cycle


  Sequence of events which occurs between the formation of a cell and its division into daughter cells
• Mitosis

  Early Prophase, Late Prophase, Metaphase, Anaphase, Telophase
• Factors influencing cell division include age, temperature, and mechanical stimuli
• S (Synthesis phase)

  Semi-conservative DNA replication occurs: DNA replicates → amount of DNA in the cell doubles
• By the end of interphase


  • Organelles duplicated, DNA duplicated, Nucleus is still bound by nuclear envelope, Nucleolus present, Single centrosome replicated to form two centrosomes, each containing a pair of centrioles in animals
• Importance of Cell-cycle checkpoints


  • Provide sufficient time for cellular activities to occur within a phase, Enable a cell to ensure that important processes, such as DNA replication, are completed properly, Prevent the transmission of mutations to daughter cells, which may lead to cancer
• Nuclear division includes Mitosis & Meiosis, while Cytokinesis is the division of the cytoplasm to form 2 separate daughter cells immediately after mitosis, meiosis I or meiosis II
• G1 (Gap phase 1)


  Intensive cellular synthesis: Organelle synthesis, RNA synthesis, Protein synthesis
• Cell division occurs in 2 main steps: Nuclear division & Cytokinesis (cytoplasmic division)
• Cell division = Nuclear division + Cytokinesis
• Plasma membrane is the longest part of the cell cycle
• Cell cycle checkpoints halt cell cycle progression until specific conditions exist
• Factors influencing cell division


  • Surface area to volume ratio, Nucleus to cytoplasmic ratio
• G2 (Gap phase 2)


  Intensive cellular synthesis (in preparation for mitosis): Organelle synthesis, Synthesis of spindle proteins, Microtubules begin to form