IGCSE Biology / Chromosomes, genes and DNA
How the nucleus, chromosome, gene and DNA fit together revision notes
Edexcel International GCSE Biology (4BI1), chapter 16: Chromosomes, genes and DNA, specification point 3.14, 3.15. Free lesson.
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Genetic material is organised from the cell and nucleus down to chromosomes, genes and DNA. Knowing how these fit together, and using gene and allele correctly, helps you give precise answers in genetics.
How genetic material is organised
Each structure fits inside the one before it. Almost every body cell contains a complete copy of your genetic material. It is kept in the nucleus, which contains the chromosomes. A human body cell has 46 chromosomes, arranged in 23 pairs. In order of size: cell, nucleus, chromosome, gene, DNA. Each chromosome is a single, very long DNA molecule coiled tightly around proteins. There are thousands of genes on each chromosome.
What a chromosome, gene and DNA are
A chromosome is a long DNA molecule carrying many genes. A gene is a section of DNA that codes for a particular protein, or a particular sequence of amino acids. DNA, or deoxyribonucleic acid, is the molecule itself. It has a double helix shape, made from two strands. A gene codes for a protein. The protein then produces a characteristic. Saying that a gene codes for a characteristic is not precise enough for a definition.
How alleles and the genome fit in
An allele is a different version of the same gene. For example, brown-eye and blue-eye alleles are two versions of the eye colour gene. They sit at the same place on the two chromosomes of a pair. The genome is all the genetic material of an organism, including every gene on every chromosome. Use “allele for brown eyes” and “allele for blue eyes”. These are versions of one eye colour gene, not separate genes.
Use the correct word in your answer
Gene and allele are often mixed up, but they do not mean the same thing. A gene is a section of DNA. An allele is one particular version of that gene, so using the right word makes your meaning clear. A gene codes for a protein. A protein produces the characteristic. An allele is a version of a gene.