56
Chapter 3
the site of protein synthesis is accomplished by RNA mole-
cules, whose synthesis is governed by the information coded
in DNA. Genetic information fl ows from DNA to RNA
and then to protein (
Figure 3–16
). The process of transfer-
ring genetic information from DNA to RNA in the nucleus
is known as
transcription.
The process that uses the coded
information in RNA to assemble a protein in the cytoplasm is
known as
translation.
transcription
translation
DNA
⎯⎯⎯⎯⎯→
RNA
⎯⎯⎯⎯⎯→
Protein
As described in Chapter 2, a molecule of DNA consists
of two chains of nucleotides coiled around each other to form
a double helix. Each DNA nucleotide contains one of four
bases—adenine (A), guanine (G), cytosine (C), or thymine
(T)—and each of these bases is specifi cally paired by hydro-
gen bonds with a base on the opposite chain of the double
helix. In this base pairing, A and T bond together and G and
C bond together. Thus, both nucleotide chains contain a spe-
cifi cally ordered sequence of bases, with one chain comple-
mentary to the other. This specifi city of base pairing forms
the basis of the transfer of information from DNA to RNA
and of the duplication of DNA during cell division.
The genetic language is similar in principle to a written
language, which consists of a set of symbols, such as A, B, C,
D, that form an alphabet. The letters are arranged in specifi c
sequences to form words, and the words are arranged in linear
sequences to form sentences. The genetic language contains
only four letters, corresponding to the bases A, G, C, and T.
The genetic words are three-base sequences that specify par-
ticular amino acids—that is, each word in the genetic language
is only three letters long. This is termed a triplet code. The
sequence of three-letter code words (triplets) along a gene in
a single strand of DNA specifi es the sequence of amino acids
in a polypeptide chain (
Figure 3–17
). Thus, a gene is equiva-
lent to a sentence, and the genetic information in the human
genome is equivalent to a book containing 30,000 to 40,000
sentences. Using a single letter (A, T, C, G) to specify each of
the four bases in the DNA nucleotides, it would require about
550,000 pages, each equivalent to this text page, to print the
nucleotide sequence of the human genome.
The four bases in the DNA alphabet can be arranged
in 64 different three-letter combinations to form 64 triplets
(4
×
4
×
4 = 64). Thus, this code actually provides more than
enough words to code for the 20 different amino acids that
are found in proteins. This means that a given amino acid
is usually specifi
ed by more than one triplet. For example,
the four DNA triplets C—C—A, C—C—G, C—C—T, and
C—C—C all specify the amino acid glycine. Only 61 of the
64 possible triplets are used to specify amino acids. The trip-
lets that do not specify amino acids are known as
“stop” sig-
nals.
They perform the same function as a period at the end
Transcription
Translation
RNA
Nucleus
Cytoplasm
Proteins having
other functions
Proteins
Amino acids
Enzymes
Substrates
Products
RNA
DNA
Figure 3–16
The expression of genetic information in a cell occurs through
the
transcription
of coded information from DNA to RNA in the
nucleus, followed by the
translation
of the RNA information into
protein synthesis in the cytoplasm. The proteins then perform the
functions that determine the characteristics of the cell.
T
A
C
A
A
A C
C
A A
G
G C
C
A A
C
C
G
T
A
A
A
G
Met
Phe
Gly
Ser
Gly
Trp
His
Phe
Portion of
a gene in one
strand of DNA
Amino acid
sequence coded
by gene
Figure 3–17
The sequence of three-letter code words in a gene determines the sequence of amino acids in a polypeptide chain. The names of the amino
acids are abbreviated. Note that more than one three-letter code sequence can specify the same amino acid; for example, the amino acid
phenylalanine (Phe) is coded by two triplet codes, A—A—A and A—A—G.
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