Mitosis and Meiosis: Cell Division Processes

• Mitosis is cell division resulting in two genetically identical diploid (2n) daughter cells with the same number of chromosomes as the parent cell
• Mitosis occurs in somatic (body) cells for growth, repair, and maintenance

Stages of Mitosis

• Interphase: Cell grows, DNA replicates, and organelles prepare for division

• Prophase: Chromatin condenses into chromosomes, the nuclear membrane breaks down, and spindle fibers form

• Metaphase: Chromosomes align at the cell's equator (metaphase plate)

• Anaphase: Sister chromatids separate and move to opposite poles

• Telophase: Nuclear membranes reform, chromosomes de-condense

• Cytokinesis: The cytoplasm divides, forming two daughter cells

• Result is 2 identical diploid cells (2n → 2n)

• Meiosis produces gametes (sperm & egg cells)

• Meiosis reduces the chromosome number by half, creating four genetically unique haploid cells (n)

• Meiosis occurs in two stages: Meiosis I and Meiosis II

Meiosis I (Reduction Division, 2n → n)

• Prophase I: Chromosomes condense, homologous chromosomes pair up (forming tetrads), and crossing over occurs creating genetic variation
• Metaphase I: Homologous pairs align at the equator
• Anaphase I: Homologous chromosomes separate and move to opposite poles; however, sister chromatids remain attached
• Telophase I & Cytokinesis: Two haploid (n) cells form, but chromosomes still consist of sister chromatids

Meiosis II (Similar to Mitosis, n → n)

• Prophase II: Chromosomes condense, spindle fibers reform

• Metaphase II: Chromosomes align at the equator

• Anaphase II: Sister chromatids separate and move to opposite poles

• Telophase II & Cytokinesis: Four genetically unique haploid (n) gametes are produced

• Result is 4 haploid gametes (2n → n, n → n)

• Natural selection is the process by which traits that enhance survival and reproduction become more common in a population

Types of Natural Selection

• Directional Selection: Favors one extreme phenotype
  • Example: Giraffes evolving longer necks to reach higher leaves
• Stabilizing Selection: Favors the intermediate phenotype and selects against extremes
  • Example: Human infant birth weight; very low or very high weights have lower survival chances
• Disruptive Selection: Favors both extreme phenotypes and selects against the intermediate
  • Example: Light and dark-colored moths surviving better than medium-colored moths due to camouflage

Mitosis produces two identical diploid cells for growth and repair, while meiosis generates four unique haploid gametes for sexual reproduction. Mitosis involves stages like prophase, metaphase, anaphase, and telophase. Meiosis includes two divisions to reduce chromosome number.