Chromosomes, Genes & DNA
Describe the relationship between chromosomes, genes, and DNA in heredity, including the double helix structure of DNA and the historical roles of Watson, Crick, Franklin, and Wilkins
What a learner can do afterwards
- Explains the hierarchy: DNA → gene → chromosome → nucleus → cell
- Describes what a gene is and what it codes for
- States that human body cells contain 46 chromosomes in 23 pairs
- Describes the contributions of Watson, Crick, Franklin, and Wilkins to the discovery of DNA structure
1 · Read
Almost every cell in your body has a nucleus, and packed inside it is DNA, a very long molecule that carries the instructions for building you and keeping you alive. There is a lot of it. If the DNA from one cell were stretched out end to end, it would reach about two metres. To fit inside a tiny nucleus, DNA coils up tightly into structures called chromosomes.
A gene is a short section of DNA. Each gene carries the instructions for one particular trait, such as eye colour or blood type. Thousands of genes sit one after another along a single strand of DNA, and one whole strand coiled up makes one chromosome. So the size order is clear: a gene is the smallest piece, then the whole DNA molecule, then the chromosome.
Nearly all of your body cells carry the same full set: 46 chromosomes arranged in 23 pairs. One chromosome of each pair came from your mother and one from your father, whether the cell is a skin cell, a nerve cell, or a muscle cell. Mature red blood cells are the odd ones out. They have no nucleus, so they carry no chromosomes at all.
DNA's shape is a double helix, which looks like a ladder twisted into a spiral. The sides of the ladder are made of sugar and phosphate, and each rung is a pair of chemical letters: A always joins with T, and G always joins with C. Working out this shape was a famous scientific race. In the 1950s, Rosalind Franklin, a researcher in Maurice Wilkins' laboratory, took X-ray pictures of DNA that revealed the twist. James Watson and Francis Crick used her data to build the first correct model of DNA. Watson, Crick, and Wilkins received the 1962 Nobel Prize in Medicine for the discovery. Franklin had died by then, and Nobel Prizes are never given after a scientist's death, so she missed the credit her pictures had earned.
Need a trick for remembering the order? Picture the nucleus as a library. Each chromosome is a book in it, the book is written in DNA, and a gene is a single instruction in the book, like one recipe in a cookbook.
A gene is a short section of DNA with instructions for one trait. DNA coils up into chromosomes, which sit inside the nucleus, and human body cells hold 46 chromosomes in 23 pairs. Rosalind Franklin's X-ray pictures revealed DNA's twisted ladder shape.
Adapted from OpenStax Biology (CC-BY 4.0), openstax.org · license
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