Recall that in Chapter 2 you read that a geneis the
genetic code for one protein. This is a simplification,
and the functioning of genes is often much more com-
plex. We have genes with segments that may be shuf-
fled or associated in many combinations, with the
potential for coding for many more proteins. A full
explanation is beyond the scope of our book, so for the
sake of simplicity, and in the following discussion, we
will say that a gene is the code for one protein. Recall
too that a protein is a specific sequence of amino acids.
Therefore, a gene, or segment of DNA, is the code for
the sequence of amino acids in a particular protein.
The code for a single amino acid consists of three
bases in the DNA molecule; this tripletof bases may
be called a codon(see Fig. 3–4). There is a triplet of
bases in the DNA for each amino acid in the protein.
If a protein consists of 100 amino acids, the gene for
that protein would consist of 100 triplets, or 300 bases.
Some of the triplets will be the same, since the same
amino acid may be present in several places within the
protein. Also part of the gene are other triplets that
start and stop the process of making the protein,
rather like capital letters or punctuation marks start
and stop sentences.
RNA AND PROTEIN SYNTHESIS
RNA, the other nucleic acid, has become a surprising
molecule, in that it has been found to have quite a few
functions. It may be involved in the repair of DNA,
and it is certainly involved in gene expression. The
expression of a gene means that the product of the
gene is somehow apparent to us, in a way we can see
or measure, or is not apparent when it should be.
Examples would be having brown eyes or blue eyes, or
having or not having the intestinal enzyme lactase to
digest milk sugar. Although these functions of RNA
are essential for us, they too are beyond the scope of
our book, so the roles of RNA in the process of pro-
tein synthesis will be our focus.
The transcription and translation of the genetic
code in DNA into proteins require RNA. DNA is
found in the chromosomes in the nucleus of the cell,
but protein synthesis takes place on the ribosomes in
the cytoplasm. Messenger RNA (mRNA) is the
intermediary molecule between these two sites.
When a protein is to be made, the segment of DNA
that is its gene uncoils, and the hydrogen bonds
between the base pairs break (see Fig. 3–4). Within
the nucleus are RNA nucleotides (A, C, G, U) and
enzymes to construct a single strand of nucleotides
that is a complementary copy of half the DNA gene
(with uracil in place of thymine). This process is tran-
scription, or copying, and the copy of the gene is
mRNA, which now has the codons for the amino acids
of the protein, and then separates from the DNA. The
gene coils back into the double helix, and the mRNA
leaves the nucleus, enters the cytoplasm, and becomes
attached to ribosomes.
As the copy of the gene, mRNA is a series of triplets
of bases; each triplet is a codon, the code for one
amino acid. Another type of RNA, called transfer
RNA(tRNA), is also found in the cytoplasm. Each
tRNA molecule has an anticodon, a triplet comple-
mentary to a triplet on the mRNA. The tRNA
molecules pick up specific amino acids (which have
come from protein in our food) and bring them to
their proper triplets on the mRNA. This process is
translation; that is, it is as if we are translating from
one language to another—the language of nucleotide
bases to that of amino acids. The ribosomes contain
enzymes to catalyze the formation of peptide bonds
between the amino acids. When an amino acid has
been brought to each triplet on the mRNA, and
all peptide bonds have been formed, the protein is
finished.
The protein then leaves the ribosomes and may be
transported by the endoplasmic reticulum to wherever
it is needed in the cell, or it may be packaged by the
Golgi apparatus for secretion from the cell. A sum-
mary of the process of protein synthesis is found in
Table 3–3.
Thus, the expression of the genetic code may be
described by the following sequence:Each of us is the sum total of our genetic charac-
teristics. Blood type, hair color, muscle proteins, nerve
cells, and thousands of other aspects of our structure
and functioning have their basis in the genetic code of
DNA.
If there is a “mistake” in the DNA, that is, incorrect
bases or triplets of bases, this mistake will be copied by
the mRNA. The result is the formation of a malfunc-
tioning or non-functioning protein. This is called a
geneticor hereditary disease, and a specific example
is described in Box 3–2: Genetic Disease—Sickle-Cell
Anemia.DNA RNA Proteins:Structural EnzymesCatalyze ReactionsHereditary CharacteristicsProteins58 Cells