Every fact in the three DNA lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Simplifications to be aware of: the lessons use single genes with one dominant and one recessive allele, as in Mendel's peas. Real inheritance is often more complicated, so do not use these ratios to predict the features of real families. 'Half of your DNA comes from each parent' refers to the chromosomes in the nucleus; mitochondria also hold a small amount of DNA. The number of human genes is an estimate that has changed over time (about 20,000; MedlinePlus gives about 19,900 protein-coding genes). The white strands in the strawberry experiment are many DNA molecules clumped together, not a single molecule. Sensitive topic: not every child lives with or knows their biological parents; talk about 'biological parents' and avoid asking pupils about their own families' features.
All sources
[1] Deoxyribonucleic Acid (DNA) Fact Sheet
National Human Genome Research Institute (NIH) · Reliability: High
Type: Government science agency
DNA carries the biological instructions that make each species unique and is passed from parents to offspring. In eukaryotes it is in the nucleus, packed into chromosomes; mitochondria hold a little DNA too. It is built from nucleotides with four bases, A, T, G and C; A always pairs with T and C with G, so one strand determines the other. In 1953 Watson, Crick, Wilkins and Franklin helped reveal the double helix. When a cell divides the strands separate and each is a template for a new matching strand. The human genome has about 3 billion bases and about 20,000 genes on 23 pairs of chromosomes; genes are only about 1% of the sequence.
MedlinePlus Genetics, U.S. National Library of Medicine · Reliability: High
Type: Government health information service
DNA is the hereditary material in humans and almost all other organisms. Most of it is in the nucleus, a little in mitochondria. The paired bases form the rungs of a ladder-like double helix, with sugar and phosphate forming the sides. Human DNA has about 3 billion bases and more than 99% of them are the same in all people. DNA can copy itself, so each new cell gets a copy.
MedlinePlus Genetics, U.S. National Library of Medicine · Reliability: High
Type: Government health information service
A gene is the basic physical and functional unit of heredity, made of DNA; some genes are instructions for proteins, many others control other genes. Human genes range from a few hundred to more than 2 million bases; there are about 19,900 protein-coding genes. Each person has two copies of each gene, one from each parent. Alleles are forms of the same gene with small differences; 'these small differences contribute to each person's unique physical features.'
[4] Chromosome (Talking Glossary of Genomic and Genetic Terms)
National Human Genome Research Institute (NIH) · Reliability: High
Type: Government science agency
Chromosomes are threadlike structures made of protein and a single DNA molecule, found in the nucleus. Humans have 46: 22 numbered pairs and one pair of sex chromosomes; one of each pair comes from each parent. Typically females have XX and males XY.
Alleles are versions of a gene; the dominant allele is written as a capital letter and the recessive as lower case. Homozygous = two identical alleles; heterozygous = two different. Phenotype = observable traits; genotype = the genetic make-up. Each parent passes one allele of each gene to the offspring. In Mendel's peas a Yy × Yy cross gives 25% YY, 50% Yy and 25% yy (1:2:1) and a 3:1 ratio of yellow to green.
National Human Genome Research Institute (NIH) · Reliability: High
Type: Government science agency activity sheet
Mash strawberries in a sealed bag to break open the cells, add a mix of washing-up liquid, salt and water, mash again, filter, then pour cold alcohol gently down the side without stirring; a white cloudy substance, the DNA, forms in the top layer and can be lifted out with a stirrer.
[7] Lab Manual for Biology Part I: DNA Extraction from Strawberries
LOUIS, the Louisiana Library Network (2022) · Reliability: Medium-high
Type: University open lab manual
Detergent breaks up the lipids of the cell membrane and nuclear membrane, releasing the DNA; salt neutralises the charge on DNA's sugar-phosphate backbone and helps the molecules clump together; cold ethanol makes the DNA precipitate out of the watery extract. Cultivated strawberries have eight sets of chromosomes per cell, which gives a large yield of DNA.
Used in: Ages 15–16 (experiment); Ages 15–16 (quiz)