WHAT IS A GENOME?
There are 6 billion base pairs of DNA in human cells. The entire 6 billion base pair collection of DNA is the genome. During cell division, the process by which one cell divides to form two new cells, the genomic DNA is copied so that each of the two new cells contains the entire genome. The genome is very long—two meters if stretched end to end—but is tightly packaged to fit into a cell. The genome is contained on 46 chromosomes, 23 of which are inherited from the mother and 23 from the father (Figure 1). Two of the cell types that do not contain the entire genome are the egg and the sperm, which each contain only half the genome or 23 chromosomes.
Our genome contains about 20,000 genes that code for proteins, which are the molecules that carry out many of our biological functions, or in other words, do much of the “work” in our cells (Figure 2). This work consists of processes like digesting our food, storing energy, or copying DNA to generate a new cell. For the most part, each of the various cell types in the body contains the same 46 chromosomes and generally has the same DNA. Cell types differ according to which genes are active, and thus, which proteins are present. For example, our immune cells make proteins called antibodies that bind pathogens and help clear them from our body, retinal cells make proteins that capture light and help us to see, and beta cells in our pancreas make the insulin protein that regulates glucose absorption by cells throughout the body. The regulatory framework that governs which proteins are expressed and at what levels in various cell types is contained in the DNA.
Figure 1. Our DNA is made of four “letters” or (top) that are arranged in a specific order (middle). The bases are paired (middle), and the 6 billion bases are packaged into 46 chromosomes (bottom), 23 from our father and 23 from our mother. Bottom image attribution: By National Cancer Institute [Public domain], via Wikimedia Commons.
Figure 2. Our DNA encodes the 20,000 genes that make proteins that carry out many of the biochemical activities in our various types of cells. The proteins are not made directly from DNA. Rather our DNA makes a similar copy of itself called RNA, which in turns gets “translated” into forming proteins. Different proteins are expressed in distinct cell types to give cells their unique identity.
Proteins are not made directly from genes but rather through a derived close copy of DNA called “messenger” RNA; the production of this type of RNA is an intermediate step in the production of proteins from genes. In addition to protein-encoding genes, there are probably another 20,000 or more genes that encode other types of RNAs that are not made into proteins. The function of most of these RNAs is not known; many of those with known function appear to have a regulatory role in turning other genes on and off.
Only about 2%–3% of our genome is accounted for by the ~40,000 genes that encode proteins and RNA. It is likely that 10% or more of our genome helps control the expression of genes (i.e., turns them on or off) in the various cell types. The purpose of most of the genome is not known, however, and may not have any direct function.