Whereas DNA is contained entirely within the cell’s nucleus, however, protein
synthesis takes place on ribosomes contained in the cytoplasm, which requires the
DNA diagram to be copied precisely within the nucleus and transported to the
ribosomes. This role is performed by RNA.
In its principles, the mechanism is the same as for DNA autoreplication. Genetic
information is not transcribed directly from DNA to proteins; rather, it is first
transcribed to mRNA. One strand of DNA serves as the information chain here,
with mRNA created to complement its base sequence. The enzyme that catalyzes
this reaction is DNA-dependent RNA polymerase. When mRNA synthesis occurs,
the base matched to adenine (A) from the DNA is not thymine (T), but uracil (U).
Once the necessary information has been transcribed, the mRNA exits the nucleus
and enters the cytoplasm, while the DNA returns to its original double helix shape.
The name “mRNA,” referring to the RNA that transports the protein diagram, is
short for “messenger RNA.” Other types of RNA include transfer RNA (tRNA),
which carries the amino acids that form the raw material for proteins to the ribosome within the cytoplasm, and ribosomal RNA (rRMA), which is a component of
the ribosomes. All of them are polynucleotide strands formed with DNA as a
template (Figs. 2.14 and 2.15).
The question of how amino acid combination sequences form through DNA
molecules is a very intriguing one. It is very likely that names of the amino acids are
somehow encoded rather than written individually in sequence. In 1954, George
Gamow found that in the four kinds of bases that form DNA, one sequence of three
bases corresponded to a single amino acid (Gamow 1954). If a single base coded for
a single amino acid, it would be impossible to designate more than four; if two bases
coded for one amino acid, the result would be 16 types (4 Â 4), which would not be
enough to account for the 20 existing types. If three bases coded for a single amino
acid, however, there would be 64 possible types (4 Â 4 Â 4), which would be more
than adequate even if several different codes existed for the same amino acid. Indeed,
genetic experiments found codons to be composed of three bases, with each codon
designating an amino acid. These three-base codons are also known as “triplets.”
The first amino acid to be “decoded” was phenylalanine. Its code was a series of
three adenine bases (AAA) in DNA, and three consecutive uracil bases (UUU) in
the messenger RNA transcribing it. Decoding of all 64 types was complete by
around 1966, or 100 years after Mendel. Typically, the term “genetic code” refers
to the base triplets transcribed to mRNA. Table 2.2 shows gene codes based on
comparison of the DNA base sequence and the amino acid sequence in the resulting
proteins.
In this table, the sequences UAG, UAA, and UGA do not code for any amino
acid; when these units appear, they indicate the end of protein synthesis. The
sequence AUG is a start codon that initiates reading of the code, but codes for
methionine if it appears once reading has begun. As can be seen from the genetic
code table, nearly all amino acids correspond to multiple codons, a phenomenon
referred to as codon degeneracy. The number of codons conforms to the frequency
with which their amino acids appear in proteins.
2.9 What Is Genetic Information?
41
synthesis takes place on ribosomes contained in the cytoplasm, which requires the
DNA diagram to be copied precisely within the nucleus and transported to the
ribosomes. This role is performed by RNA.
In its principles, the mechanism is the same as for DNA autoreplication. Genetic
information is not transcribed directly from DNA to proteins; rather, it is first
transcribed to mRNA. One strand of DNA serves as the information chain here,
with mRNA created to complement its base sequence. The enzyme that catalyzes
this reaction is DNA-dependent RNA polymerase. When mRNA synthesis occurs,
the base matched to adenine (A) from the DNA is not thymine (T), but uracil (U).
Once the necessary information has been transcribed, the mRNA exits the nucleus
and enters the cytoplasm, while the DNA returns to its original double helix shape.
The name “mRNA,” referring to the RNA that transports the protein diagram, is
short for “messenger RNA.” Other types of RNA include transfer RNA (tRNA),
which carries the amino acids that form the raw material for proteins to the ribosome within the cytoplasm, and ribosomal RNA (rRMA), which is a component of
the ribosomes. All of them are polynucleotide strands formed with DNA as a
template (Figs. 2.14 and 2.15).
The question of how amino acid combination sequences form through DNA
molecules is a very intriguing one. It is very likely that names of the amino acids are
somehow encoded rather than written individually in sequence. In 1954, George
Gamow found that in the four kinds of bases that form DNA, one sequence of three
bases corresponded to a single amino acid (Gamow 1954). If a single base coded for
a single amino acid, it would be impossible to designate more than four; if two bases
coded for one amino acid, the result would be 16 types (4 Â 4), which would not be
enough to account for the 20 existing types. If three bases coded for a single amino
acid, however, there would be 64 possible types (4 Â 4 Â 4), which would be more
than adequate even if several different codes existed for the same amino acid. Indeed,
genetic experiments found codons to be composed of three bases, with each codon
designating an amino acid. These three-base codons are also known as “triplets.”
The first amino acid to be “decoded” was phenylalanine. Its code was a series of
three adenine bases (AAA) in DNA, and three consecutive uracil bases (UUU) in
the messenger RNA transcribing it. Decoding of all 64 types was complete by
around 1966, or 100 years after Mendel. Typically, the term “genetic code” refers
to the base triplets transcribed to mRNA. Table 2.2 shows gene codes based on
comparison of the DNA base sequence and the amino acid sequence in the resulting
proteins.
In this table, the sequences UAG, UAA, and UGA do not code for any amino
acid; when these units appear, they indicate the end of protein synthesis. The
sequence AUG is a start codon that initiates reading of the code, but codes for
methionine if it appears once reading has begun. As can be seen from the genetic
code table, nearly all amino acids correspond to multiple codons, a phenomenon
referred to as codon degeneracy. The number of codons conforms to the frequency
with which their amino acids appear in proteins.
2.9 What Is Genetic Information?
41