Genome and Genes

DNA at different scales

DNA is the molecule that stores genetic information. In a eukaryotic cell, long DNA molecules are packaged into chromosomes, mainly inside the nucleus. Each chromosome contains many smaller functional sections called genes.

This gives a useful order from largest collection to smaller section:

genome → chromosomes → DNA molecules → genes

These are not four different genetic substances. The chromosomes are made from packaged DNA, while genes are particular sections of that DNA.

The genome is the complete DNA collection

An organism's genome is its entire DNA. Imagine all the DNA instructions belonging to that organism collected together: that complete collection is its genome.

The genome therefore includes all the organism's genes, but it is not simply another word for “all the genes”. Genes occupy particular sections of DNA, and the genome also contains DNA outside protein-coding genes. In humans and other eukaryotes, most DNA is in the nucleus, with a small amount also present in mitochondria.

The word can be applied to any organism, not only humans. A human, oak tree and bacterium each has a genome, although their genomes differ in size and organisation.

A gene is one section of DNA

A gene is a section of a DNA molecule that codes for a specific protein. The order of bases along that section stores the information needed to determine the protein. Different genes have different base sequences and can therefore code for different proteins.

Proteins carry out many roles. Some form structures, while others act as enzymes and control chemical reactions. This connects genetic information with an organism's cells and characteristics: a gene's information is used to produce a particular protein, and that protein performs a particular role.

At this stage, the essential relationship is:

one gene → instructions for a specific protein

The detailed steps by which cells use DNA to make proteins are studied separately as protein synthesis.

Genome and gene are not interchangeable

A gene is only one selected section of DNA. A genome is the organism's complete set of DNA and contains many genes. It is similar to the difference between a complete instruction collection and one individual instruction within it.

For example, pointing to one protein-coding section of a chromosome identifies a gene. Referring to all the DNA belonging to the organism identifies its genome.

Core recall

• Genome: the entire DNA of an organism.
• Gene: a section of a DNA molecule that codes for a specific protein.

Organisation

Genome → chromosomes → DNA → genes

Chromosomes are packaged DNA molecules. Many genes occur along their DNA.

Essential distinction

The genome contains all of an organism's DNA, including many genes and DNA outside protein-coding genes. A gene is just one specific section of DNA.

Link to proteins

The base sequence within a gene carries the information for a specific protein. Different genes can therefore code for different proteins with different roles.