Cell Division, Genetics & Evolution: Mitosis/Meiosis, Punnett Squares & Natural Selection

The second half of GED Life Science connects cellular and molecular biology to heredity and evolution: how cells divide via mitosis and meiosis (L.d.1), how DNA codes for proteins and passes alleles from parents to offspring in Punnett squares and pedigrees (L.e.1), how mutations and environmental epigenetics shape traits (L.e.2), and how genetic variation drives evolution by natural selection and common ancestry on cladograms (L.f.1–L.f.2). Punnett square probability questions and cladogram/natural-selection questions appear on virtually every GED Science test. Mastering the 2×2 Punnett grid and reading branching points on an evolutionary tree guarantees fast, accurate points.

1. Cell Theory, Mitosis, Meiosis & Differentiation (L.d.1): • Cell Theory: All living things are composed of cells; the cell is the basic unit of life; all cells arise from pre-existing cells. • Mitosis vs Meiosis: Mitosis = 1 division → 2 identical diploid (2n) somatic cells (growth & repair). Meiosis = 2 divisions + crossing over → 4 genetically varied haploid (n) gametes (sexual reproduction). • Cell Specialization (Differentiation): Every body cell in an organism contains the EXACT SAME DNA genome; a neuron and a muscle cell differ only because different genes are switched ON (expressed) or OFF! 2. Central Dogma, Mendelian Genetics & Punnett Squares (L.e.1): • Central Dogma: DNA (double helix with bases A–T, C–G in nucleus) is transcribed into mRNA (A–U, C–G), which travels to a ribosome and is translated (in 3-base codons) into a chain of amino acids that folds into a functional PROTEIN. • Alleles, Genotype & Phenotype: - Genotype = the pair of alleles an organism carries: Homozygous Dominant (BB), Heterozygous (Bb, a 'carrier' of recessive b), or Homozygous Recessive (bb). - Phenotype = the observable physical trait expressed (e.g., brown vs blue eyes). Under complete dominance, both BB and Bb show the dominant phenotype; only bb shows the recessive phenotype. 3. Mutations & Epigenetics (L.e.2): • Mutations: Random changes in the DNA base sequence (substitution, insertion, deletion). A mutation in a GAMETE (sperm/egg) is passed to offspring; a mutation in a somatic (skin/lung) cell affects only that individual and is NOT inherited. Mutations can be harmful, neutral, or beneficial. • Epigenetics & Environment: Environmental factors (diet, toxins, stress, sunlight) can attach chemical tags (like methyl groups) to DNA or histones that turn gene expression up or down WITHOUT altering the underlying A-T-C-G DNA sequence. 4. Evidence of Common Ancestry & Natural Selection (L.f.1–L.f.2): • Evidence for Evolution: (a) Fossils in sedimentary rock layers (older layers at the bottom by superposition); (b) Homologous structures—similar bone anatomy inherited from a common ancestor despite different functions (human arm, whale flipper, bat wing), contrasted with analogous structures (bird wing vs insect wing, evolved independently); (c) DNA/protein sequence similarity (fewer base differences = more recent common ancestor). • Cladograms (Phylogenetic Trees): Each branching node represents the most recent common ancestor of the lineages above it. Two species are more closely related if they share a MORE RECENT branching node! • Four Steps of Natural Selection: (1) Inherited genetic variation exists in a population → (2) Environmental selective pressure / competition for limited resources → (3) Differential survival and reproduction of individuals with advantageous heritable traits → (4) Over generations, advantageous alleles increase in frequency (adaptation; reproductive isolation can lead to speciation).