Biology — Cell Division and DNA
Every human starts as ONE cell. Nine months later, we are 3 trillion cells. A lifetime later, we've made quadrillions. This all happens through CELL DIVISION — one of nature's most elegant processes. Paired with DNA replication, it is how life perpetuates. Today, the molecular basics every HS Bio student must know.
DNA — The Blueprint
DNA (deoxyribonucleic acid) is a DOUBLE HELIX — two strands twisted together, discovered by Watson, Crick, Franklin, and Wilkins in 1953.\n\nStructure:\n- **Sugar-phosphate backbone** (the rails)\n- **Base pairs** (the rungs): A-T and C-G only\n\nSequence of bases = GENETIC INFORMATION. About 3 BILLION base pairs in each human cell. Each gene = a stretch of DNA coding for a protein.
DNA Replication
Before a cell divides, it must COPY its DNA exactly.\n\n1. **Helicase** unwinds the double helix\n2. **DNA polymerase** reads each strand as a template\n3. Adds complementary bases: A pairs with T, C pairs with G\n4. Result: TWO identical double helices\n\nThis is called SEMI-CONSERVATIVE replication (each new helix has one old and one new strand).
Mitosis — Copying Yourself
MITOSIS = cell division for GROWTH and REPAIR. One cell becomes two IDENTICAL daughter cells.\n\nPhases (IPMAT):\n- **Interphase** — DNA replicates (not technically mitosis but essential prep)\n- **Prophase** — chromosomes condense\n- **Metaphase** — chromosomes align at the center\n- **Anaphase** — chromosomes separate, pulled to opposite sides\n- **Telophase** — two new nuclei form\n- **Cytokinesis** — cell splits in two\n\nFrom 1 cell → 2 cells with identical 46 chromosomes each.
In DNA, which bases pair together?
Meiosis — Making Sex Cells
MEIOSIS = special cell division for making sperm and eggs. One cell becomes FOUR GENETICALLY UNIQUE cells with HALF the chromosomes (23 instead of 46).\n\nTwo rounds of division:\n- Meiosis I — homologous chromosomes separate\n- Meiosis II — chromatids separate\n\nGenetic variation from:\n- **Crossing over** — chromosomes swap sections\n- **Independent assortment** — random combinations\n- **Random fertilization** — any sperm + any egg\n\nThis is why siblings look different despite same parents.
Mitosis vs. Meiosis
| Feature | Mitosis | Meiosis |\n|---|---|---|\n| Purpose | Growth, repair | Sex cells |\n| Divisions | 1 | 2 |\n| Daughters | 2 identical | 4 unique |\n| Chromosome # | Same (46) | Halved (23) |\n| Genetic variation | No | Yes |\n\nBoth essential; very different.
Meiosis produces how many DAUGHTER cells from ONE original?
Genetics Basics
GENES come in pairs (one from each parent). Each gene has variations called ALLELES.\n\n- **Homozygous** — two of same allele (TT or tt)\n- **Heterozygous** — two different alleles (Tt)\n- **Dominant** (T) — shows when present\n- **Recessive** (t) — hidden unless both alleles match\n\nPUNNETT SQUARES predict offspring genetics. Foundation for Mendelian genetics.
Mutations
Sometimes DNA replication makes a MISTAKE — a MUTATION:\n\n- **Point mutation** — one base changed\n- **Insertion/Deletion** — extra or missing base\n- **Frameshift** — changes entire reading\n\nMost mutations are neutral. Some cause disease (sickle cell, cystic fibrosis). Some confer advantages and drive EVOLUTION. Mutations are the raw material of all genetic diversity.
Punnett Square Practice
Parent 1: Tt (heterozygous tall). Parent 2: tt (recessive short).\n\n1. Draw 2×2 grid.\n2. Label rows with Parent 1's alleles, columns with Parent 2's.\n3. Fill each box.\n4. What % offspring are tall (have at least one T)?\n5. What % short (tt)?\n\nAnswer: 50% tall, 50% short.
Build a DNA Model
1. Use candy (Twizzlers for backbone, gummies or marshmallows for bases).\n2. Build a DNA segment with the sequence ATCGAT.\n3. Build its complementary strand (TAGCTA).\n4. Twist into a helix.\n5. Take photos. Edible DNA = memorable biology.
Genetic VARIATION among offspring comes mostly from:
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