8.1 The Central Dogma and the Genetic Code
252 MHR • Unit 3 Molecular Genetics
When you look at a set of building plans (such as
those in Figure 8.1), it may seem obvious that they
provide a set of instructions for building a house.
Consider, however, the processes by which the two-
dimensional information contained on those plans
ultimately achieves expression in the form of a
three-dimensional structure made of wood, bricks,
and concrete. In the case of houses, we know that
these processes include concrete work, carpentry,
plumbing, electrical wiring, and roofing. Genetics
researchers in the 1950s knew that genes contained
the instructions for building proteins, and that these
genes were made up of stretches of double-helical
DNA. The challenge was to determine the processes
by which the information contained in DNA (an
essentially linear arrangement of nucleotides)
could be transformed into three-dimensional
proteins (structures made out of polypeptides).Figure 8.1In much the same way that building plans
contain the instructions to build a house, genes contain
the instructions needed to build proteins.By a fortunate coincidence, British scientist
Frederick Sanger established that proteins are made
up of an identifiable sequence of amino acids in
1953 — the same year that Watson and Crick
published their model for the structure of DNA.
Sanger, who was working at the same university as
Watson and Crick, showed that each molecule of
the protein insulin contains precisely the same
linear arrangement of amino acids. He went on to
demonstrate that the linear arrangement of amino
acids can lead automatically to the characteristic
three-dimensional, folded structure of the
functioning protein. This structure results from
chemical interactions among the amino acids that
cause the polypeptide chain to fold in specific
ways. Once scientists understood that a given set
of amino acids arranged in a specific order could
produce a particular protein, they began to
consider a new possibility. Perhaps there was a
connection between the sequence of nucleotides
along a DNA molecule and the sequence of amino
acids in a protein.The Triplet Hypothesis
Crick took up the challenge of cracking the genetic
code. He knew that proteins are made of 20 different
amino acids but that DNA contains only four
different nucleotides. Crick thus hypothesized that
the code must be made up of “words,” or codons,
consisting of a minimum of three nucleotides each.
He reasoned that if one nucleotide corresponded to
one amino acid, the code could account for only
four amino acids. If a combination of any two
nucleotides was needed to code for a single amino
acid, the code could generate a maximum of
42 = 16 combinations. However, if the code relied
on a combination of three nucleotides to specify
one amino acid, it could generate a maximum of
43 = 64 different combinations. These combinations
would be more than enough to account for the
20 amino acids. This theory, which suggests thatEXPECTATIONSExplain the general roles of DNA and RNA in protein synthesis.
Describe the process of discovery that helped researchers decipher
the genetic code, and outline their contributions.
Discuss the significance of the main features and universality of
the genetic code.