What's a Genome?

A genome is all of a living thing's genetic material. It is the entire set of hereditary instructions for building, running, and maintaining an organism, and passing life on to the next generation. The whole shebang.

In most living things, the genome is made of a chemical called DNA. The genome contains genes, which are packaged in chromosomes and affect specific characteristics of the organism.

Imagine these relationships as a set of Chinese boxes nested one inside the other. The largest box represents the genome. Inside it, a smaller box represents the chromosomes. Inside that is a box representing genes, and inside that, finally, is the smallest box, the DNA.

In short, the genome is divided into chromosomes, chromosomes contain genes, and genes are made of DNA.
The word " genome " was coined in about 1930, even though scientists didn't know then what the genome was made of. They only knew that the genome was important enough, whatever it was, to have a name.

Each one of earth's species has its own distinctive genome: the dog genome, the wheat genome, the genomes of the cow, cold virus, bok choy, Escherichia coli (a bacterium that lives in the human gut and in animal intestines), and so on.

So genomes belong to species, but they also belong to individuals. Every giraffe on the African savanna has a unique genome, as does every elephant, acacia tree, and ostrich. Unless you are an identical twin, your genome is different from that of every other person on earth—in fact, it is different from that of every other person who has ever lived.

Though unique, your genome is still recognizably a human genome. The difference is simply a matter of degree: The genome differences between two people are much smaller than the genome differences between people and our closest relatives, the chimpanzees.

Genomes are found in cells, the microscopic structures that make up all organisms. With a few exceptions, each of your body's trillions of cells contains a copy of your genome: the cells in your muscles, the cells in your brain, the cells in your blood, and so on.

Imagine all the trillions of genomes in your body, in other people's bodies, in cedar and apple trees, in walruses, forest mushrooms, and migrating birds: The whole world is full of genomes.

But if the genome is a commonplace thing, it is also quite powerful. A genome is information that affects every aspect of our behavior and physiology. Cooking dinner, digesting your food, talking, singing, sleeping—your genome has a hand in all these things.

A genome alone can't make a person, because we are also influenced by where we live, the human culture that surrounds us, and hundreds of other aspects of our environment. But the fact remains that you can't make a person without a genome.

Studying the human genome, therefore, is likely to give us insights into why some people die of heart disease and others die of cancer, why some people are comfortable schmoozing with a crowd of strangers and others are paralyzed by shyness, why some people have trouble keeping weight on while others have trouble keeping it off, and so on.

It's not difficult for scientists to get their hands on the human genome. They draw a bit of blood from people who volunteer to have their genomes studied, then use some simple laboratory procedures to break open the cells in the blood sample and extract the DNA. The body is constantly making more blood cells, so blood is a renewable genome resource.

Each human being contains a slightly different version of the human genome, but all human genomes are similar enough that we can learn about the human genome in general by studying the genomes of one or a few individual people.

Studying the genome can mean many different things. You can study a very small part of the genome or the genome as a whole. You can study the sequence of a gene, the function of a gene, the parts of the genome that regulate genes, or the DNA outside of genes. You can observe where genes are located in the genome, or investigate how different genes work together.

The next three chapters discuss several ways how scientists are studying the genome as a whole: sequencing genomes, mapping genomes, and studying the variation within genomes. Some of what we learn from this work might help doctors prevent and treat diseases better. Some of it might simply celebrate the variety that is the human species, or our unity with other forms of life. Some of it might open up possibilities we haven't even thought of yet.

A chromosome is a package containing a chunk of a genome—that is, it contains some of an organism's genes. The important word here is "package": chromosomes help a cell to keep a large amount of genetic information neat, organized, and compact.

Chromosomes are made of DNA and protein. Most living things have chromosomes that are linear, like bits of fat thread, and are kept in the nucleus, a sphere-shaped sac within the cell.

Source: http://www.genomenewsnetwork.org/resources/whats_a_genome/Chp1_1_1.shtml#genome1