Essential Cell Biology 5th edition

232 CHAPTER 7 From DNA to Protein: How Cells Read the Genome
Although RNA polymerase catalyzes essentially the same chemical reaction
as DNA polymerase, there are some important differences between
the two enzymes. First, and most obviously, RNA polymerase uses ribonucleoside
triphosphates as substrates, so it catalyzes the linkage of
ribonucleotides, not deoxyribonucleotides. Second, unlike the DNA polymerase
involved in DNA replication, RNA polymerases can start an RNA
chain without a primer and do not accurately proofread their work. This
sloppiness is tolerated because RNA, unlike DNA, is not used as the permanent
storage form of genetic information in cells, so mistakes in RNA
transcripts have relatively minor consequences for a cell. RNA polymerases
make about one mistake for every 10 4 nucleotides copied into RNA,
whereas DNA polymerase makes only one mistake for every 10 7 nucleotides
copied.
Cells Produce Various Types of RNA
The majority of genes carried in a cell’s DNA specify the amino acid
sequences of proteins. The RNA molecules encoded by these genes—
which ultimately direct the synthesis of proteins—are called messenger
RNAs (mRNAs). In eukaryotes, each mRNA typically carries information
transcribed from just one gene, which codes for a single protein; in bacteria,
a set of adjacent genes is often transcribed as a single mRNA, which
therefore carries the information for several different proteins.
The final product of other genes, however, is the RNA itself. As we see
later, these noncoding RNAs, like proteins, have various roles, serving
as regulatory, structural, and catalytic components of cells. They play
key parts, for example, in translating the genetic message into protein:
ribosomal RNAs (rRNAs) form the structural and catalytic core of the ribosomes,
which translate mRNAs into protein, and transfer RNAs (tRNAs) act
as adaptors that select specific amino acids and hold them in place on a
ribosome for their incorporation into protein. Other small RNAs, called
microRNAs (miRNAs), serve as key regulators of eukaryotic gene expression,
as we discuss in Chapter 8. The most common types of RNA are
summarized in Table 7–1.
In the broadest sense, the term gene expression refers to the process
by which the information encoded in a DNA sequence is converted into
a product, whether RNA or protein, that has some effect on a cell or
organism. In cases where the final product of the gene is a protein, gene
expression includes both transcription and translation. When an RNA
molecule is the gene’s final product, however, gene expression does not
require translation.
TABLE 7–1 TYPES OF RNA PRODUCED IN CELLS
Type of RNA
messenger RNAs (mRNAs)
ribosomal RNAs (rRNAs)
microRNAs (miRNAs)
transfer RNAs (tRNAs)
Other noncoding RNAs
Function
code for proteins
form the core of the ribosome’s structure and
catalyze protein synthesis
regulate gene expression
serve as adaptors between mRNA and amino acids
during protein synthesis
used in RNA splicing, gene regulation, telomere
maintenance, and many other processes