Proteincoding RNA and the genetic code
The RNA transcripts that code for the production of proteins are called mes-sengerRNA (abbreviated mRNA). Messenger RNA's job is to transfer the information regarding what proteins need to be made from the DNA to the protein-producing machinery where proteins are assembled: the ribosomes (see the next section, "Non-protein-coding RNA").
RNA and DNA are each composed of four subunits (their four respective nucleotides). The DNA nucleotides appear in groups of three (the 64 codons, discussed in "DNA: A Molecule for Storing Genetic Information" earlier in this chapter). Proteins are composed of 20 subunits, called amino acids. The exact translation from codons to the amino acids is called the genetic code.
Back before anyone knew that DNA is the molecule where genetic information is stored, many people argued that DNA just wasn't complicated enough. They wondered how something with only four different letters could code for all the complexity of an organism. Thus, at one point, proteins were considered to be good candidates for the material used for information storage because they had 20 amino acids — or 20 different letters available in their alphabet. The breakthrough in understanding came when scientists determined that the 4 nucleotides were read in groups of 3, meaning that instead of containing 4 individual letters, DNA has an alphabet of 64 triplet letters, called codons. That discovery raised another question. Instead of wondering how to code for 20 amino acids with only 4 letters, people questioned how you would code for only 20 amino acids when you have 64 codons. The answer is that there is some redundancy in the translation of DNA into proteins.
Figure 3-2 shows all 64 codons of the genetic code. Most of the amino acids correspond to multiple codons, as you can see from the figure (the notations Phe, Leu, and so on). Some codons, though, don't code for amino acids at all; instead, they signal that protein synthesis should stop. These codons are called stop codons. They tell the protein-producing machinery to stop adding amino acids to a growing protein, and their presence indicates that the protein is finished.
Second Letter
Figure 3-2:
The 64 codons of the genetic code.
UUUlp, ucu-
UUCJ UCC
UUGJ UCG
CCU CCC CCA CCG
GCU GCC GCA GCG
UAU UAC
CAUl CAC J CAA^l CAG J
ACC I T, AAC AUAJ ACA I AAA AUG Met ACGj AAG
GAU GAC GAA"\ GAG
Ty r
Ty r
|
UGU^ UGCJ |
Cys |
U C | |
|
UGA |
Stop |
A | |
|
UGG |
Trp |
G | |
|
CGU |
U | ||
|
CGC CGA |
> Arg |
C A |
h |
|
CGG |
G | ||
|
AGU^ AGCJ |
Ser |
U C |
e tt r |
|
AGA^ AGGJ |
Arg |
G | |
|
GGU |
U | ||
|
GGC GGA |
> Gly |
C A | |
|
GGG |
G |
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