Organic Chemistry

054 CHAPTER 27 Nucleosides, Nucleotides, and Nucleic Acids

Table 27.2 The Genetic Code

Middle position

UCA G

U Phe Ser Tyr Cys U

Phe Ser Tyr Cys C

Leu Ser Stop Stop A

Leu Ser Stop Trp G

C Leu Pro His Arg U

Leu Pro His Arg C

Leu Pro Gln Arg A

Leu Pro Gln Arg G

A Ile Thr Asn Ser U

Ile Thr Asn Ser C

Ile Thr Lys Arg A

Met Thr Lys Arg G

G Val Ala Asp Gly U

Val Ala Asp Gly C

Val Ala Glu Gly A

Val Ala Glu Gly G

5 ¿-Position 5 ¿-Position

The genetic code was worked out
independently by Marshall Nirenberg
and Har Gobind Khorana, for which
they shared the 1968 Nobel Prize in
physiology or medicine. Robert
Holley, who worked on the structure
of tRNA molecules, also shared that
year’s prize.

Marshall Nirenbergwas born in
New York in 1927. He received a
bachelor’s degree from the University
of Florida and a Ph.D. from the Uni-
versity of Michigan. He is a scientist
at the National Institutes of Health.

Har Gobind Khoranawas born
in India in 1922. He received a
bachelor’s and a master’s degree
from Punjab University and a Ph.D.
from the University of Liverpool.
In 1960 he joined the faculty at the
University of Wisconsin and later
became a professor at MIT.

attaching the acyl adenylate to tRNA and one for hydrolyzing the acyl adenylate. The

acylation site is hydrophobic, so valine is preferred over threonine for the tRNA acyla-

tion reaction. The hydrolytic site is polar, so threonine is preferred over valine for the

hydrolysis reaction. Thus, if threonine is activated by the aminoacyl-tRNA synthetase

for valine, it will be hydrolyzed rather than transferred to the tRNA.

27.13 Biosynthesis of Proteins: Translation

A protein is synthesized from its N-terminal end to its C-terminal end by reading the

bases along the mRNA strand in the direction. A sequence of three bases,

called a codon, specifies a particular amino acid that is to be incorporated into a pro-

tein. The bases are read consecutively and are never skipped. A codon is written with

the -nucleotide on the left. Each amino acid is specified by a three-base sequence

known as the genetic code(Table 27.2). For example, UCA on mRNA codes for the

amino acid serine, whereas CAG codes for glutamine.

Because there are four bases and the codons are triplets, different codons

are possible. This is many more than are needed to specify the 20 different amino

acids, so all the amino acids—except methionine and tryptophan—have more than one

codon. It is not surprising, therefore, that methionine and tryptophan are the least

abundant amino acids in proteins. Actually, 61 of the bases specify amino acids, and

three bases are stop codons. Stop codonstell the cell to “stop protein synthesis here.”

Translationis the process by which the genetic message in DNA that has been

passed to mRNA is decoded and used to build proteins. Each of the approximately

100,000 proteins in the human body is synthesized from a different mRNA. Don’t

43 = 64

5 ¿

5 ¿¡ 3 ¿

CH 3

O

CH 3 CH

+NH 3

CHCO−

valine

OH

O

CH 3 CH

+NH 3

CHCO−

threonine