DNA, Punnett squares, and how the instructions of life are written, copied, and passed on.
Suggested pace: 5 weeks. Mastery looks like:
DNA is the most famous molecule in science, and it earns the fame. Shaped as a double helix — two strands twisted like a spiral staircase — it stores instructions in a four-letter alphabet of bases: adenine, thymine, guanine, and cytosine. The two strands pair by a strict rule: A always bonds with T, and G always with C. That rule is the secret of copying. Unzip the helix, and each strand serves as a template to rebuild its partner — which is exactly what happens before every cell division, so that each daughter cell receives a complete set of instructions.
A gene is a stretch of DNA that carries the recipe for one protein. The cell reads it in two steps. Transcription, in the nucleus, copies the gene into a portable molecule called messenger RNA. Translation, at the ribosome, reads the mRNA three letters at a time — each triplet, or codon, specifying one amino acid — and chains the amino acids into a protein. DNA to RNA to protein: biologists call this the central dogma, and it is the production line behind every trait your student can see, from eye color to enzyme function.
Mitosis and meiosis are both cell division, with opposite purposes. Mitosis produces two genetically identical daughter cells with a full set of chromosomes — the division behind growth, repair, and replacement. Meiosis produces four genetically different cells, each with half the chromosome number: the gametes — sperm and egg. The halving matters because fertilization will join two gametes and restore the full count. The differences matter because meiosis shuffles the deck — crossing over and independent assortment guarantee that no two gametes are alike, which is why siblings share parents but not faces.
An Austrian monk counting pea plants discovered that inheritance follows rules. Genes come in versions called alleles; an individual carries two, one from each parent. A dominant allele masks a recessive one, so a plant carrying Tt shows the tall phenotype while hiding the short allele in its genotype. The Punnett square turns this into arithmetic your student can verify: cross two Tt parents and the offspring probabilities are 1 TT : 2 Tt : 1 tt — a 3:1 ratio of tall to short, with a 25% chance of the recessive trait appearing. Genetics is one of the few places in biology where a student can predict the future and then check the answer.
A mutation is any change in the DNA sequence — a swapped letter, a deletion, a duplication. Most are neutral; some are harmful, including those behind genetic disorders such as cystic fibrosis and sickle cell disease; a few change a protein in ways that turn out useful. Mutation is the original source of all new alleles, and together with the shuffling of meiosis it explains the variation visible in every family, every field, and every species. With this unit complete, your student can read the next one — classification and the diversity of life — as the story of that variation written large.
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GENO — a robot you can actually TALK to — has studied this entire unit and is available day or night, in 32 languages, at no cost. Ask him to re-explain any idea on this page, quiz you out loud, or go deeper than the lesson goes.