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Note that with four kinds of bases, each group of three bases of DNA can represent as many as 4 x 4 x 4 = 64 possible amino acids. Since there are only 20 amino acids plus a
stop signal, the genetic code has evolved some redundancy, so that some amino acids are represented by more than one codon. Every possible three bases of DNA—each codon—
represents some amino acid apart from the three codons that represent a stop signal.
The chart in Figure 8-1
shows how the various bases combine to form amino acids. There are many interesting things to note about the genetic code. For our purposes, the
most important is redundancy—the way more than one codon translates to the same amino acid. Well program this using character classes and regular expressions, as youll
soon see.
[2] [2]
Also note that the genetic code in Figure 8-1
is properly based on RNA, where uracil appears instead of thymine. In our programs, were going to go directly from DNA to amino
acids, so our codons will use thymine instead of uracil.
Figure 8-1. The genetic code
The machinery of the cell actually starts at some point along the RNA and reads the sequences codon after codon, attaching the encoded amino acid to the end of the growing
protein sequence. Example 8-1
simulates this, reading the string of DNA three bases at a time and concatenating the symbol for the encoded amino acid to the end of the
growing protein string. In the cell, the process stops when a codon is encountered.
8.3.2 Translating Codons to Amino Acids
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The first task is to enable the following programs to do the translation from the three- nucleotide codons to the amino acids. This is the most important step in implementing the
genetic code, which is the encoding of amino acids by three-nucleotide codons.
Heres a subroutine that returns an amino acid represented by a one-letter abbreviation given a three-letter DNA codon:
codon2aa A subroutine to translate a DNA 3-character codon to an
amino acid sub codon2aa {
mycodon = _; if codon =~ TCAi { return S }
Serine elsif codon =~ TCCi { return S }
Serine elsif codon =~ TCGi { return S }
Serine elsif codon =~ TCTi { return S }
Serine elsif codon =~ TTCi { return F }
Phenylalanine elsif codon =~ TTTi { return F }
Phenylalanine elsif codon =~ TTAi { return L }
Leucine elsif codon =~ TTGi { return L }
Leucine elsif codon =~ TACi { return Y }
Tyrosine elsif codon =~ TATi { return Y }
Tyrosine elsif codon =~ TAAi { return _ } Stop
elsif codon =~ TAGi { return _ } Stop elsif codon =~ TGCi { return C }
Cysteine elsif codon =~ TGTi { return C }
Cysteine elsif codon =~ TGAi { return _ } Stop
elsif codon =~ TGGi { return W } Tryptophan
elsif codon =~ CTAi { return L } Leucine
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elsif codon =~ CTCi { return L } Leucine
elsif codon =~ CTGi { return L } Leucine
elsif codon =~ CTTi { return L } Leucine
elsif codon =~ CCAi { return P } Proline
elsif codon =~ CCCi { return P } Proline
elsif codon =~ CCGi { return P } Proline
elsif codon =~ CCTi { return P } Proline
elsif codon =~ CACi { return H } Histidine
elsif codon =~ CATi { return H } Histidine
elsif codon =~ CAAi { return Q } Glutamine
elsif codon =~ CAGi { return Q } Glutamine
elsif codon =~ CGAi { return R } Arginine
elsif codon =~ CGCi { return R } Arginine
elsif codon =~ CGGi { return R } Arginine
elsif codon =~ CGTi { return R } Arginine
elsif codon =~ ATAi { return I } Isoleucine
elsif codon =~ ATCi { return I } Isoleucine
elsif codon =~ ATTi { return I } Isoleucine
elsif codon =~ ATGi { return M } Methionine
elsif codon =~ ACAi { return T } Threonine
elsif codon =~ ACCi { return T } Threonine
elsif codon =~ ACGi { return T } Threonine
elsif codon =~ ACTi { return T } Threonine
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elsif codon =~ AACi { return N } Asparagine
elsif codon =~ AATi { return N } Asparagine
elsif codon =~ AAAi { return K } Lysine
elsif codon =~ AAGi { return K } Lysine
elsif codon =~ AGCi { return S } Serine
elsif codon =~ AGTi { return S } Serine
elsif codon =~ AGAi { return R } Arginine
elsif codon =~ AGGi { return R } Arginine
elsif codon =~ GTAi { return V } Valine
elsif codon =~ GTCi { return V } Valine
elsif codon =~ GTGi { return V } Valine
elsif codon =~ GTTi { return V } Valine
elsif codon =~ GCAi { return A } Alanine
elsif codon =~ GCCi { return A } Alanine
elsif codon =~ GCGi { return A } Alanine
elsif codon =~ GCTi { return A } Alanine
elsif codon =~ GACi { return D } Aspartic Acid
elsif codon =~ GATi { return D } Aspartic Acid
elsif codon =~ GAAi { return E } Glutamic Acid
elsif codon =~ GAGi { return E } Glutamic Acid
elsif codon =~ GGAi { return G } Glycine
elsif codon =~ GGCi { return G } Glycine
elsif codon =~ GGGi { return G } Glycine
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elsif codon =~ GGTi { return G } Glycine
else { print STDERR Bad codon \codon\\n;
exit; }
}
This code is clear and simple, and the layout makes it obvious whats happening. However, it can take a while to run. For instance, given the codon GGT for glycine, it has
to check each test until it finally succeeds on the last one, and thats a lot of string comparisons. Still, the code achieves its purpose.
Theres something new happening in the codes error message. Recall filehandles from Chapter 4
and how they access data in files. From Chapter 5
, remember the special filehandle STDIN that reads user input from the keyboard. STDOUT and STDERR are
also special filehandles that are always available to Perl programs. STDOUT directs output to the screen usually or another standard place. When a filehandle is missing
from a
print statement, STDOUT is assumed. The print statement accepts a filehandle
as an optional argument, but so far, weve been printing to the default STDOUT. Here, error messages are directed to STDERR, which usually prints to the screen, but on many
computer systems they can be directed to a special error file or other location. Alternatively, you sometimes want to direct STDOUT to a file or elsewhere but want
STDERR error messages to appear on your screen. I mention these options because you are likely to come across them in Perl code; we dont use them much in this book see
Appendix B for more information.
8.3.3 The Redundancy of the Genetic Code
