1From cell to gene
Almost every cell in your body has a nucleus. Inside it are chromosomes, each one a very long, coiled molecule of DNA. A gene is a short section of that DNA.
chromosomesDNAgene
Lesson 28.1 notes
DNA, genes and chromosomes
From cell to gene, the double helix and the scientists who found it, chromosomes in pairs, sex cells and heredity.
2The double helix
DNA is a double helix: two twisted strands joined by pairs of bases. There are four bases: A always pairs with T, and C always pairs with G.
double helixbases
3Genes
A gene is a section of DNA that carries one instruction. The order of its bases is a code. Genes control characteristics such as blood group, and many characteristics depend on several genes.
4Finding the double helix
Rosalind Franklin and Maurice Wilkins made X-ray images of DNA. Photo 51 (1952), from Franklin's lab, showed it was a helix. Using this, James Watson and Francis Crick built the double-helix model in 1953.
5Chromosomes in pairs
Human body cells have 46 chromosomes in 23 pairs. One chromosome of each pair came from the mother and one from the father. Other species have different numbers: a dog has 78.
6Sex cells
Sperm and egg cells are sex cells. Human ones have just 23 chromosomes, one from each pair. At fertilisation they join, so the new cell has 46, half from each parent.
sex cellsfertilisation
7Heredity
Passing genetic information from parents to their offspring is called heredity. Each sex cell gets a different mix of chromosomes, so brothers and sisters are alike but not identical, unless they are identical twins.
heredity
Explore: Chromosome lab
Make an egg and a sperm from two parents, then fertilise the egg. See how a child gets one chromosome of each pair from each parent, and why brothers and sisters differ.
Common mistake: A gene is just a short section of a chromosome
A chromosome is one very long DNA molecule carrying hundreds or thousands of genes. A gene is a short section of it that carries one instruction.
Remember
- Inside the nucleus are chromosomes. Each is a long molecule of DNA, and a gene is a short section of DNA that carries one instruction.
- DNA is a double helix: two strands joined by pairs of bases, A with T and C with G. X-ray images from Franklin and Wilkins helped Watson and Crick build the model in 1953.
- Human body cells have 46 chromosomes in 23 pairs. Sperm and egg cells have 23, so a child gets half its chromosomes from each parent: heredity.
Going further at GCSE