DNA and the genome
Every inherited trait — eye colour, blood group, lactose tolerance, predisposition to disease — is encoded in DNA. Understanding the structure of DNA is the foundation for genetics, genetic engineering and modern medicine.
What DNA is
DNA (deoxyribonucleic acid) is the chemical that makes up the chromosomes in the nucleus of every cell. It is a polymer made of two strands twisted into a double helix.
A gene is a short section of DNA that codes for a particular sequence of amino acids to make a specific protein. The sequence of bases in the gene is the genetic code.
A genome is the entire genetic material of an organism. The human genome has roughly 20,000 genes spread across 23 pairs of chromosomes.
Chromosomes
- Found in the nucleus (in eukaryotes) — coiled lengths of DNA wound around proteins.
- Humans have 23 pairs = 46 in total.
- One pair (the sex chromosomes, XX or XY) determines biological sex.
The Human Genome Project (HGP)
The HGP (1990–2003) was an international project to read the entire base sequence of human DNA.
Why is this important? Three big uses regularly examined:
- Search for genes linked to disease. Identifying mutations that cause inherited disorders (e.g. cystic fibrosis, breast cancer risk via BRCA1/2) supports genetic testing and counselling.
- Understanding inherited disorders. Knowing the gene and protein affected helps researchers design targeted treatments and gene therapies.
- Tracing human migration patterns from the past. Comparing genomes across populations reveals how humans spread out of Africa.
Other uses (good extension answers): personalised medicine (matching drugs to a patient's genome), forensic science (DNA profiling) and paternity testing.
Concerns
- Privacy of genetic information (insurers, employers).
- Discrimination based on genetic risk.
- Psychological impact of knowing you carry a high-risk allele.
- Cost and access — not everyone can afford genome testing.
DNA structure (preview, full detail in B6.3 HT)
DNA is a polymer made of repeating units called nucleotides, each containing a sugar, a phosphate and one of four bases (A, T, C, G). The two strands are held together by base pairing: A always pairs with T, and C always pairs with G. This complementary pairing is the key to DNA replication and protein synthesis.
Mitochondrial DNA (extension)
Mitochondria contain a tiny separate genome inherited only from the mother — used in evolutionary studies and to track maternal lineages.
⚠Common mistakes
- Saying DNA is a single strand. It's a double helix.
- Confusing "gene", "DNA" and "chromosome". A gene is a section of DNA; a chromosome is a long coiled molecule of DNA containing many genes.
- Saying the genome is "all our genes". It's all the DNA — including the non-coding regions, which actually make up most of it.
- Saying every cell has a different genome. All body cells have the same genome (with rare mutations); they only express different genes in different tissues.
Links
Foundation for B6.3 (HT — full DNA structure and protein synthesis), B6.4 (inheritance), B6.5 (mutation as source of variation), and B6.7 (genetic engineering).
AI-generated · claude-opus-4-7 · v3-deep-biology