Biology 12

Chapter 8 Protein Synthesis • MHR 269

Figure 8.21A summary of the main steps involved in transcription and translation
in a eukaryotic cell. Which steps do not occur in prokaryotes?

A

pre-mRNA transcript

3 ′

3 ′

RNA

polymerase

DNA nucleus

5 ′ cap

5 ′

5 ′

cap

exon

exon

intron

poly-A

tail

poly-A

tail

3 ′

poly-A tail

cytoplasm

aa-tRNA

aminoacyl-tRNA

synthetase

(enzyme)

amino

acid

tRNA

A

A

AA

A

A

CC C

U U

U

GG UU UG

anticodon

codon

ribosome

ribosomal

sub-units

separating

cap

F

E

D

C

B

A

A particular stretch of DNA, identified by a

promoter sequence, serves as a template

for a pre-mRNA transcript.

A

Enzymes add a 5 ′cap and poly-A tail

to the pre-mRNA transcript, and

spliceosomes splice out introns.

B

The finished mRNA moves from the

nucleus to the cytoplasm. There, the

ribosome assembly and an initiator tRNA

come together on the mRNA strand.

C

Activating enzymes in the cytoplasm

bind amino acids to their correct tRNA

molecules to produce aa-tRNAs.

D

These aa-tRNA molecules make their way

to the ribosome assembly, where they

attach to binding sites along the mRNA

strand. The ribosome then moves along

the mRNA strand, catalyzing the formation

of peptide bonds along the growing chain

of amino acids.

E

The finished polypeptide is released and

the ribosome assembly comes apart.

F

Figure 8.22This electron microscope image of a

polyribosome complex shows a strand of mRNA being

translated by a number of ribosomes that are moving along

the strand. A lengthening polypeptide chain trails out from

behind each ribosome. Polyribosomes are found in both

prokaryotes and eukaryotes.