Biology 12

Chapter 8 Protein Synthesis • MHR 259

degree of accuracy does not pose a significant
problem for the cell. At the same time, the absence
of proofreading speeds up the process of protein
synthesis. In eukaryotes, transcription occurs at
the rate of 40 to 60 nucleotides per second.

Figure 8.9Many molecules of RNA polymerase are working
along this DNA strand to transcribe mRNA molecules from a
gene. The increasing length of the mRNA strands indicates
the direction of transcription.

Termination of Transcription

Once transcription has been successfully initiated,

the RNA polymerase continues along the DNA

molecule until it encounters terminator sequences

on the non-transcribed DNA strand. Like the

promoter sequences, the terminator nucleotide

sequences are highly specific. Figure 8.10 shows

the terminator sequences found in E. coli.

Once transcription is terminated, the mRNA

molecule separates from the RNA polymerase. In

turn, the RNA polymerase molecule dissociates

from the DNA molecule. The RNA polymerase can

then bind to another promoter sequence and begin

a new transcription process, either on the same

gene or on a different gene. In prokaryotes, the

mRNA molecule is now ready to begin directing

the process of protein synthesis. In fact, since both

transcription and translation take place in the

cytoplasm of the cell in prokaryotes, the mRNA

molecule may begin translation even before

transcription is finished.

RNA

RNA polymerase

DNA

A A

A

A

A

C

C

CC

C

C

C

C

C

C

C

C

G

G

GG

G G G G G G

G

G

T T

T

T

T

TT

T

T

T

T

T

A

A A

A A

A

A

A

A A

U

U

U

Figure 8.8During elongation, RNA polymerase tracks along
the sense strand of DNA, synthesizing a strand of mRNA by
adding nucleotides in the 5 ′to 3 ′direction. In contrast to
the synthesis of a new strand of DNA, no primer is needed.

Only a short strand of the DNA helix is opened at any one

time. As the helix re-forms, the elongating mRNA strand

separates and trails out behind the RNA polymerase.

3 ′ 5 ′

5 ′ 3 ′

GGGTGTCGGCGGTCAAGGCGACCGCCGTAAAATTGAAAGAAATTACT

5 ′ CCCACAGCCGCCAGTTCCGCTGGCGGCATTTTAACTTTCTTTAATGA 3 ′

DNA

terminator sequences

template strand of DNA

transcription

stop site

direction of transcription

section of mRNA produced

CCCACAGCCGCCAGUUCCGCUGGCGGCAUUUU

Figure 8.10The terminator sequences, illustrated here
on E. coliDNA, are transcribed onto the mRNA molecule.
Transcription ends immediately after the final terminator
has been transcribed. As with the promoter sequences,

the terminator sequences are, by convention, written

according to their nucleotide sequence along the non-

transcribed DNA strand.