Evolution__3rd_Edition

..
Information encoded in the DNA
...
. . . is first transcribed to mRNA ...
. . . and then translated into protein
CHAPTER 2 / Molecular and Mendelian Genetics 25
adenine (A), cytosine (C), guanine (G), and thymine (T). Adenine and guanine belong
to the chemical group called purines; cytosine and thymine are pyrimidines. In the
double helix, an A nucleotide in one strand always pairs with a T nucleotide in the
other; and a C always pairs with a G (as in Figure 2.2b). If the nucleotide sequence
in one strand was ...AGGCTCCTA..., then the complementary strand would be
...TCCGAGGAT.... Because the sugar and phosphate are constant, it is often more convenient
to imagine a DNA strand as a sequence of bases, like the ...AGGCTCCTA...
sequence above.
2.3 Information in DNA is decoded by transcription
and translation
There are four types of nucleotide, but 20 different amino acids. A one-to-one code of
nucleotide encoding an amino acid would therefore be impossible. In fact a triplet of
bases encodes one amino acid; the nucleotide triplet for an amino acid is called a codon.
The four nucleotides can be arranged in 64 (4 × 4 × 4) different triplets, and each one
codes for a single amino acid. The relation between triplet and amino acid has been
deciphered and is called the genetic code.
The mechanism by which the amino acid sequence is read off from the nucleotide
sequence of the DNA is understood in molecular detail. The full detail is unnecessary
for our purposes, but we should distinguish two main stages. RNA (ribonucleic acid) is
a class of molecules that has a similar composition to DNA. Messenger RNA (mRNA) is
one of the main forms of RNA. Messenger RNA is transcribed from the DNA and the
process is called transcription. Messenger RNA is single-stranded and, unlike DNA,
uses a base called uracil (U) instead of thymine (T). The DNA sequence AGGCTCCTA
would therefore have an mRNA with the following sequence transcribed from it: UCC-
GAGGAU. The genetic code is usually expressed in terms of the codons in the mRNA
(Table 2.1). The mRNA sequence UCCGAGGAU, for example, codes for three amino
acids: serine, glutamic acid, and aspartic acid. The beginning and end of a gene are signaled
by distinct base sequences, which (in a sense) punctuate the DNA message. As
Table 2.1 shows, three of the 64 triplets in the genetic code are for “stop.” Only 61 of the
64 code for amino acids.
Transcription takes place in the nucleus. After the mRNA molecule has been
assembled on the gene, it then leaves the nucleus and travels to one of the structures
in the cytoplasm called ribosomes (see Figure 2.1); ribosomes are made of another
kind of RNA called ribosomal RNA (rRNA). The ribosome is the site of the second
main stage in protein production. It is where the amino acid sequence is read off from
the mRNA sequence and the protein is assembled. The process is called translation.
The actual translation is achieved by yet another kind of RNA, called transfer RNA
(tRNA). 1
1 That completes the three main kinds of RNA: mRNA, rRNA, and tRNA. By the way, both rRNA and tRNA
molecules originate by transcription from genes in the DNA. It is therefore not always true that genes code for
proteins, as stated above a some genes code for RNA.