The Gene and the Chromosome

The Chromosome, a Long Strand of Condensed DNA

In the nucleus of each cell, DNA is not a disorganized tangle: it is organized into chromosomes, rod-shaped structures, visible under a microscope only during cell division, when DNA condenses tightly. Outside of these moments, DNA remains unwound, in the form of long filaments in the nucleus.

Each species has its own characteristic number of chromosomes: humans have 46 (23 pairs), dogs have 78, a fruit fly has only 8. This number has no connection with the complexity of the organism.

The Gene, a Precise Portion of DNA

Along the length of a chromosome, thousands of genes follow one another. A gene is a precise portion of the DNA molecule, a particular sequence of bases (A, T, G, C), which contains the information needed to build a specific element of the organism, most often a protein.

Each chromosome therefore carries many different genes, a bit like a long string of beads would carry, at precise spots, different instructions.

Information in Two Copies

In humans, chromosomes come in pairs: one chromosome inherited from the mother, one chromosome inherited from the father. Each gene is therefore present in two copies, called alleles, which may be identical or slightly different from one another.

Remember the Hierarchy

• The nucleus contains DNA, organized into chromosomes.
• Each chromosome carries many genes.
• Each gene is a portion of DNA coding for a specific piece of information.