Essential Cell Biology 5th edition

262 CHAPTER 7 From DNA to Protein: How Cells Read the Genome
RNA-based systems
RNA
EVOLUTION OF RNAs THAT
CAN DIRECT PROTEIN SYNTHESIS
RNA- and protein-based systems
RNA
DNA TAKES OVER AS GENETIC
MATERIAL; RNA BECOMES AN
INTERMEDIATE BETWEEN DNA
AND PROTEIN
present-day cells
DNA
RNA
protein
Figure 7–51 RNA may have preceded
DNA and proteins in evolution. According
to this hypothesis, RNA molecules provided
genetic, structural, and catalytic functions in
the earliest ECB5 cells. e7.48/7.51
DNA is now the repository
of genetic information, and proteins carry
out almost all catalysis in cells. RNA now
functions mainly as a go-between in protein
synthesis, while remaining a catalyst for
a few crucial reactions (including protein
synthesis).
QUESTION 7–6
protein
Discuss the following: “During the
evolution of life on Earth, RNA lost
its glorious position as the first selfreplicating
catalyst. Its role now is as
a mere messenger in the information
flow from DNA to protein.”
in polynucleotides, is easier to detect and repair in DNA than in RNA (see
Figure 6−24). This is because the product of the deamination of cytosine
is, by chance, uracil, which already exists in RNA, so that such damage
would be impossible for repair enzymes to detect in an RNA molecule.
However, in DNA, which has thymine rather than uracil, any uracil produced
by the accidental deamination of cytosine is easily detected and
repaired.
Taken together, the evidence we have discussed supports the idea that
RNA—with its ability to provide genetic, structural, and catalytic functions—preceded
DNA in evolution. As cells more closely resembling
present-day cells appeared, it is believed that RNAs were relieved of many
of the duties they had originally performed: DNA took over the primary
storage of genetic information, and proteins became the major catalysts,
while RNA remained primarily as the intermediary connecting the two
(Figure 7–51). With the rise of DNA, cells were able to become more complex,
for they could then carry and transmit more genetic information
than could be stably maintained by RNA alone. Because of the greater
chemical complexity of proteins and the variety of chemical reactions
they can catalyze, the shift from RNA to proteins (albeit incomplete) also
provided a much richer source of structural components and enzymes,
enabling cells to evolve the great diversity of appearance and function
that we see today.
ESSENTIAL CONCEPTS
• The flow of genetic information in all living cells is DNA → RNA →
protein. The conversion of the genetic instructions in DNA into RNAs
and proteins is termed gene expression.
• To express the genetic information carried in DNA, the nucleotide
sequence of a gene is first transcribed into RNA. Transcription is
catalyzed by the enzyme RNA polymerase, which uses nucleotide
sequences in the DNA molecule to determine which strand to use as
a template, and where to start and stop transcribing.
• RNA differs in several respects from DNA. It contains the sugar ribose
instead of deoxyribose and the base uracil (U) instead of thymine (T).
RNAs in cells are synthesized as single-stranded molecules, which
often fold up into complex three-dimensional shapes.
• Cells make several functional types of RNAs, including messenger
RNAs (mRNAs), which carry the instructions for making proteins;
ribosomal RNAs (rRNAs), which are the crucial components of ribosomes;
and transfer RNAs (tRNAs), which act as adaptor molecules in
protein synthesis.
• To begin transcription, RNA polymerase binds to specific DNA sites
called promoters that lie immediately upstream of genes. To initiate
transcription, eukaryotic RNA polymerases require the assembly of
a complex of general transcription factors at the promoter, whereas
bacterial RNA polymerase requires only an additional subunit, called
sigma factor.
• Most protein-coding genes in eukaryotic cells are composed of a
number of coding regions, called exons, interspersed with larger,
noncoding regions, called introns. When a eukaryotic gene is transcribed
from DNA into RNA, both the exons and introns are copied.
• Introns are removed from the RNA transcripts in the nucleus by RNA
splicing, a reaction catalyzed by small ribonucleoprotein complexes
known as snRNPs. Splicing removes the introns from the RNA and
joins together the exons—often in a variety of combinations, allowing
multiple proteins to be produced from the same gene.