Genes whose mutation causes an inherited human condition are indicated by red brackets. The Abcd1 gene codes for a protein that imports fatty acids into the peroxisome; mutations in the gene cause demylination of nerves which can result in cognition and movement disorders. Incontinentia pigmenti is a disease of the skin, hair, nails, teeth, and eyes. Hemophilia A is a bleeding disorder caused by mutations in the Factor VIII gene, which codes for a blood-clotting protein. Because males have only a single copy of the X chromosome, most of the conditions shown here affect only males; females that inherit one of these defective genes are often asymptomatic because a functional protein is made from their other X chromosome. (Courtesy of Alex Williams, obtained from the University of California, Genome Browser, http://genome.ucsc.edu) organism. Yet the cells in our body do this automatically, thousands of times a second.
The problems that cells face in decoding genomes can be appreciated by considering a tiny portion of the human genome (Figure 6–2). The region illustrated represents less than 1/2000th of our genome and includes at least 48 genes that encode proteins and 6 genes for noncoding RNAs. When we consider the entire human genome, we can only marvel at the capacity of our cells to rapidly and accurately handle such large amounts of information.
In this chapter, we explain how cells decode and use the information in their genomes. Much has been learned about how the genetic instructions written in an alphabet of just four “letters”—the four different nucleotides in DNA—direct the formation of a bacterium, a fruit fly, or a human. Nevertheless, we still have a great deal to discover about how the information stored in an organism’s genome produces even the simplest unicellular bacterium with 500 genes, let alone how it directs the development of a human with approximately 30,000 genes. An enormous amount of ignorance remains; many fascinating challenges therefore await the next generation of cell biologists.
Transcription and translation are the means by which cells read out, or express, the genetic instructions in their genes. Because many identical RNA copies can be made from the same gene, and each RNA molecule can direct the synthesis of many identical protein molecules, cells can synthesize a large amount of protein from a gene when necessary. But genes can be transcribed and translated with different efficiencies, allowing the cell to make vast quantities of some proteins and tiny amounts of others (Figure 6–3). Moreover, as we see in the next chapter,
Figure 6–3 Genes can be expressed with different efficiencies. In this example, gene A is transcribed much more efficiently than gene B and each RNA molecule that it produces is also translated more frequently. This causes the amount of protein A in the cell to be much greater than that of protein B.
a cell can change (or regulate) the expression of each of its genes according to its needs—most commonly by controlling the production of its RNA.
The first step a cell takes in reading out a needed part of its genetic instructions is to copy a particular portion of its DNA nucleotide sequence—a gene—into an RNA nucleotide sequence (Figure 6–4). The information in RNA, although copied into another chemical form, is still written in essentially the same language as it is in DNA—the language of a nucleotide sequence. Hence the name given to producing RNA molecules on DNA is transcription.