Page 121 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

280 Chapter 8 Gene Expression: The Flow of Information from DNA to RNA to Protein

The Genetic Code Is Almost, broad outlines of information flow between gene and
but Not Quite, Universal protein, these results did not explain exactly how the cel-
lular machinery accomplishes gene expression. This
We now know that virtually all cells alive today use the question is our focus as we present in the next sections
same basic genetic code. One early indication of this uni- the details of transcription and translation.
formity was that a translational system derived from one
organism could use the mRNA from another organism to
convert genetic information to the encoded protein. Rabbit essential concepts
hemoglobin mRNA, for example, when injected into frog
eggs or added to cell-free extracts from wheat germ, di- • The nearly universal genetic code consists of 64 codons,
rects the synthesis of rabbit hemoglobin proteins. More each one composed of three nucleotides. Sixty-one
codons specify amino acids, while three—UAA, UAG,
recently, comparisons of DNA and protein sequences have UGA—are stop codons. The code is degenerate in that
revealed a perfect correspondence according to the genetic more than one codon can specify one amino acid.
code between codons and amino acids in almost all organ- • The codon AUG specifies methionine; it also serves
isms examined. as the initiation codon establishing the reading frame
that groups nucleotides into successive, nonoverlapping
codon triplets.
Conservation of the genetic code
The universality of the code is an indication that it • Missense mutations change a codon so that it specifies
a different amino acid; frameshift mutations alter the
evolved very early in the history of life. Once it emerged, reading frame for all codons following the mutation; and
the code remained constant over billions of years, in part nonsense mutations change a codon for an amino acid
because evolving organisms would have little tolerance into a stop codon.
for change. A single change in the genetic code could
disrupt the production of hundreds or thousands of pro-
teins in a cell—from the DNA polymerase essential for
replication to the RNA polymerase required for gene ex-
pression to the tubulin proteins that compose the mitotic
spindle—and such a change would therefore be lethal. 8.2 Transcription:
From DNA to RNA

Exceptional genetic codes
Researchers were thus quite amazed to observe a few ex- learning objectives
ceptions to the universality of the code. In some species of
the single-celled eukaryotic protozoans known as ciliates, 1. Describe the three stages of transcription: initiation,
the codons UAA and UAG, which are nonsense codons in elongation, and termination.
most organisms, specify the amino acid glutamine; in other 2. Compare transcription initiation in prokaryotes and
ciliates, UGA, the third stop codon in most organisms, eukaryotes.
specifies cysteine. Ciliates use the remaining nonsense 3. List three ways by which eukaryotes process mRNA
codons as stop codons. after transcription.
Other systematic changes in the genetic code exist in
mitochondria, the semiautonomous, self-reproducing
organelles within eukaryotic cells that are the sites of Transcription is the process by which the polymerization of
ATP formation. Each mitochondrion has its own chro- ribonucleotides, guided by complementary base pairing,
mosomes and its own apparatus for gene expression produces an RNA transcript of a gene. The template for the
(which we describe in detail in Chapter 15). In the mito- RNA transcript is one strand of that portion of the DNA
chondria of yeast, for example, CUA specifies threonine double helix that constitutes the gene.
instead of leucine. Yet another exception to the code is
seen in certain prokaryotes that sometimes use the tri-
plet UAG to specify insertion of the rare amino acid RNA Polymerase Synthesizes a Single-
pyrrolysine (see Fig. 7.28c and also Problem 57 at the Stranded RNA Copy of a Gene
end of this chapter.)
The experimental evidence presented so far helped Figure 8.10 depicts the basic components of transcription
define a nearly universal genetic code. But although and illustrates key events in the process as it occurs in the
cracking the code made it possible to understand the bacterium E. coli. This figure divides transcription into