Biology 12

Chapter 8 Protein Synthesis • MHR 277

Figure 8.32The main control points in the protein synthesis
pathway in eukaryotic cells. The packing of DNA in the
eukaryotic genome, the processing of mRNA, and the
separation of transcription (in the nucleus) from translation
(in the cytoplasm) offer opportunities for regulation not
found in prokaryotic cells.

Pre-transcriptional Control

In Chapter 7, you saw that eukaryotic DNA
undergoes several levels of packaging, and that
different stretches of the genome may be more
condensed than others during most of the cell’s
life cycle (see Figure 8.33). In general, the more

condensed regions of DNA are not transcribed.

Their more complex structure presents a physical

barrier that keeps regulatory proteins and RNA

polymerase from reaching their gene sequences to

transcribe pre-mRNA. On the other hand, genes

that are found on the more loosely packaged

regions of DNA in various cells are more available

for transcription, meaning that their sequences are

the most likely to be expressed.

Figure 8.33Sections of this chromosome from an

amphibian egg cell have de-condensed to allow transcription

to take place. The visible loops are stretches of DNA that

have been released from the proteins that normally hold

them in their tightly packed array. The mRNA transcripts will

be stored in the egg cell until it is fertilized, at which point

translation may take place.

Transcriptional Control

Researchers have not discovered any examples of

operons in eukaryotic cells. Genes that code for

different enzymes involved in the same metabolic

pathway are much more likely to be found some

distance from one another — even on different

chromosomes — than to be grouped together in

operons. As a result, transcription in eukaryotes

relies on both the presence of the promoter DNA

sequence and the action of regulator proteins called

transcription factors. Before transcription begins,

one transcription factor binds to the promoter.

This action helps to bring together additional

transcription factors. The resulting protein-protein

interactions establish an initiation complex that

allows RNA polymerase to bind to the promoter

sequence of each gene.

As in prokaryotes, the rate of transcription in

eukaryotes may rely on the action of operator

functional

protein

plasma

membrane

polypeptide

chain

post-translational

control

post-transcriptional

control

transcriptional

control

pre-transcriptional

control

translational

control

nuclear

envelope

nuclear

pore

mRNA

pre-

mRNA

intron

DNA

chromatin

histone