Organic Chemistry

052 CHAPTER 27 Nucleosides, Nucleotides, and Nucleic Acids

3 ′

a. 5 ′

OH

anticodon

U

G

C

C

C

G

G

C

U

C

AGGC C

C

G

C

A

A

C

C

C

A

G

C

C

U

G

C

U

G U

G

U

G

G

U

G

AU

A

A

U

C

G

C

G

UA

G

C C

C

G

G

G

C

G

G

G

C G

C

G C

all tRNAs have

CCA at the 3′-end

Figure 27.13N
(a) a transfer RNA that
carries alanine. Compared with
other RNAs, tRNA contains a high
percentage of unusual bases
(shown as empty circles). These
bases result from enzymatic
modification of the four normal
bases. (b) The anticodon is
shown in red; the serine binding
site is shown in yellow.

tRNASer:

tRNAAla,

b.

3-D Molecule:

tRNA

(^3) Sedimentation constants are not additive, which is why a 50S and a 30S can combine to form a 70S.
The S stands for the sedimentation constant, which designates where a given compo-
nent sediments during centrifugation.^3
27.12 Transfer RNA
Transfer RNA (tRNA) is much smaller than mRNA or rRNA. It contains only 70 to 90
nucleotides. The single strand of tRNA is folded into a characteristic cloverleaf struc-
ture strung out with three loops and a little bulge next to the right-hand loop
(Figure 27.13a). There are at least four regions with complementary base pairing. All
tRNAs have a CCA sequence at the The three bases at the bottom of the loop
directly opposite the and are called an anticodon(Figures 27.13a and b).
Each tRNA can carry an amino acid bound as an ester to its terminal group.
The amino acid will be inserted into a protein during protein biosynthesis. Each tRNA
can carry only one particular amino acid. A tRNA that carries alanine is designated as
tRNAAla.
3 ¿-OH
5 ¿- 3 ¿-ends
3 ¿-end.
30S
50S
40S
80S
70S
prokaryotic ribosome eukaryotic ribosome
MW 2,500,000 MW 4,200,000
60S