Organic Chemistry

PROBLEM 18

If methionine is the first amino acid incorporated into a heptapeptide, what is the sequence

of the amino acids encoded for by the following stretch of mRNA?

PROBLEM 19

Four C’s occur in a row in the segment of mRNA in Problem 18. What polypeptide would

be formed from the mRNA if one of the four C’s were cut out of the strand?

PROBLEM 20

UAA is a stop codon. Why does the UAA sequence in mRNA in Problem 18 not cause pro-

tein synthesis to stop?

PROBLEM 21

Write the sequences of bases in the sense strand of DNA that resulted in the mRNA in

Problem 18.

PROBLEM 22

List the possible codons on mRNA that specify each amino acid in Problem 18 and the an-

ticodon on the tRNA that carries that amino acid.

U¬A¬A¬A¬C¬A¬C¬ 3 ¿

5 ¿¬G¬C¬A¬U¬G¬G¬A¬C¬C¬C¬C¬G¬U¬U¬A¬U¬

Section 27.13 Biosynthesis of Proteins: Translation 057

3-D Molecules:

Chloramphenicol complexed

to acetyl-transferase;

Tetracycline

A sculpture done by Robert Holley.

Robert W. Holley (1922–1993)was

born in Illinois and received a

bachelor’s degree from the University

of Illinois and a Ph.D. from Cornell

University. During World War II he

worked on the synthesis of penicillin

at Cornell Medical School. He was a

professor at Cornell and later at the

University of California, San Diego.

He was also a noted sculptor.

SICKLE CELL ANEMIA

Sickle cell anemia is an example of the damage that

can be caused by a change in a single base of DNA

(Problem 55 in Chapter 23). It is a hereditary disease caused

when a GAG triplet becomes a GTG triplet in the sense strand of

a section of DNA that codes for the -subunit of hemoglobin. As

a consequence, the mRNA codon becomes GUG—which signals

b

for incorporation of valine—rather than GAG, which would

have signaled for incorporation of glutamic acid. The change

from a polar glutamic acid to a nonpolar valine is sufficient to

change the shape of the deoxyhemoglobin molecule and induce

aggregation, causing it to precipitate in red blood cells. This

stiffens the cells, making it difficult for them to squeeze

through a capillary. Blocked capillaries cause severe pain and

can be fatal.

Normal red blood cells Sickle red blood cells