Biology: Cell Cycle and Meiosis

Cell Cycle

• The cell cycle is a series of events that cells undergo for growth and division.
• It consists mainly of two phases: Interphase (90% of the cycle) and M Phase (Mitosis or Meiosis).
• Interphase is further divided into G1, S, and G2 phases.
• G1: Cell growth and preparation for DNA replication.
• S: DNA replication occurs.
• G2: Cell prepares for division, organelles replicate.
• M Phase: Cell division, followed by Cytokinesis (cytoplasm division).

Meiosis

• Meiosis produces four genetically different haploid gametes (sperm/egg).
• It involves two divisions: Meiosis I and Meiosis II.
• Meiosis I: Homologous chromosomes separate.
• Prophase I: Crossing over (genetic variation) occurs.
• Metaphase I: Homologous pairs line up randomly (independent assortment).
• Anaphase I: Homologous chromosomes separate.
• Telophase I and Cytokinesis: Two haploid cells form.
• Meiosis II: Sister chromatids separate.
• Prophase II: Chromosomes condense.
• Metaphase II: Chromosomes align at the equator.
• Anaphase II: Sister chromatids separate.
• Telophase II and Cytokinesis: Four haploid daughter cells form.
• Meiosis increases genetic variation through crossing over and independent assortment.

Mitosis

• Mitosis produces two identical diploid daughter cells.
• It is crucial for growth and repair of tissues.
• The main stages are Prophase, Metaphase, Anaphase, and Telophase (PMAT).
• Prophase: Chromosomes condense, spindle fibers form.
• Metaphase: Chromosomes align at the cell's equator.
• Anaphase: Sister chromatids separate.
• Telophase: Nuclear membranes reform, chromosomes decondense.
• Cytokinesis follows to split the cytoplasm and form two identical cells.
• Mitosis is essential for asexual reproduction in some organisms.

Cell Transport

• Cell transport refers to the movement of substances across the cell membrane.
• Passive transport doesn't require energy.
• Diffusion: Movement of molecules from high to low concentration.
• Osmosis: Water diffusion across a membrane.
• Facilitated diffusion: Transport proteins assist molecule movement.
• Active transport needs energy (ATP).
• Active transport moves substances against their concentration gradient (low to high).
• The sodium-potassium pump is an example of active transport.
• Bulk transport involves endocytosis (cell taking in material) – phagocytosis (solids) and pinocytosis (liquids), and exocytosis (cell expelling material).

Mitosis vs. Meiosis Comparison

• Mitosis produces identical diploid cells for growth and repair; meiosis produces different haploid gametes.
• Mitosis involves a single cell division; meiosis involves two cell divisions.
• Mitosis creates two daughter cells; meiosis creates four.
• Mitosis maintains chromosome number; meiosis reduces the chromosome number.
• Mitosis doesn't introduce genetic variation; meiosis creates unique combinations through crossing over and independent assortment.