Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

FROM DNA TO RNA
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Figure 6–2 Schematic depiction of a small portion of the human X chromosome. As summarized in the key, the known
protein-coding genes (starting with Abcd1 and ending with F8) are shown in dark gray, with coding regions (exons) indicated by
bars that extend above and below the central line. Noncoding RNAs with known functions are indicated by purple diamonds.
Yellow triangles indicate positions within protein-coding regions where the Neanderthal genome sequences codes for a different
amino acid than the human genome. The stretch of yellow triangles in the Txtl1 gene appear to have been positively selected
for since the divergence of Homo sapiens from Neanderthals some 200,000 years ago. Note that most of the proteins are
identical between us and our extinct relative. The blue histogram indicates the extent to which portions of the human genome
are conserved with other vertebrate species. It is likely that additional genes, currently unrecognized, also lie within this portion
of the human genome.
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.
From DNA to RNA
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,
gene A
gene B
DNA
TRANSCRIPTION
RNA
TRANSCRIPTION
RNA
TRANSLATION
TRANSLATION
A A A A A
A A A A A
A A A A A
A A A A A
A A A A A
B
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.