Scientists Say: Genome

Genome (noun, “JEE-nome”)

A genome is an organism’s complete set of genetic instructions.

For most living things, genetic instructions come in the form of DNA. This molecule contains the instructions to build and maintain living things. In most living things, DNA is organized into structures called chromosomes. Some of the DNA in a chromosome consists of genes. These are segments of DNA that carry instructions for making an organism’s proteins (and a few other molecules).

But genes only make up a fraction of an organism’s genome. Most of the genome is made of non-coding DNA. Non-coding DNA doesn’t contain genes. Scientists once called it “junk DNA.” That’s because it was believed to have no function. But it turns out that non-coding DNA helps the organism in other ways, such as allowing a cell to use and read the code in its genes.

Explainer: What are genes?

Most human cells carry one entire genome — 46 chromosomes arranged in 23 pairs. But exceptions exist. Red blood cells are one example. Once mature, red blood cells lack DNA altogether. Egg and sperm cells also do not contain a full genome. These cells carry half a genome — in humans, that’s 23 chromosomes each. When an egg cell and a sperm cell fuse, they create one new cell with a full genome of 46 chromosomes. A complete human genome contains about 25,000 genes and many non-coding regions.

Some viruses use RNA as their genetic material instead of DNA. Viruses are not alive in the same way as a cell. However, we still refer to their complete RNA instructions as a genome.

Genetic material — both DNA and RNA — is made of units called nucleotides. They are represented by the letters A, T, G and C (RNA replaces T with U). DNA’s nucleotides occur in pairs, called base pairs. A human genome consists of about 3 billion of these base pairs. Combinations of these base pairs provide all the instructions for building a living thing.

Genome size varies from one species to another. For example, the Japanese canopy plant’s (Paris japonica) genome is about 50 times bigger than a human’s. A complete set of this plant’s DNA consists of 149 billion base pairs. If we stretched out the DNA from one cell, it would extend about 100 meters (330 feet). In contrast, a human genome would extend only about 2 meters (6.5 feet).

On the other hand, the single-celled green algae called Ostreococcus tauri has a very short genome. It contains only about 12.6 million base pairs. Its DNA — if stretched out — would extend only about 8 millimeters (0.3 inch).

In a sentence

Orange-colored cats get their distinctive look from a DNA deletion in their genome.

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