6.10.2 Eukaryotic Control Mechanisms: Posttranscriptional and Posttranslational Controls

Question 1

note

Answer 1

• Gene expression can be controlled at the level of pretranscription or transcription.
  • Review: Examples of pretranscription control includes DNA methylation and histone acetylation. Transcription-level controls include transcription factors and enhancers.
• After transcription yields an mRNA molecule, the cell still has many other opportunities to regulate protein processing.

Question 2

Gene expression can also be controlled at the level of:

Answer 2

· post-transcription
· pre-translation
· translation
· post-translation

Question 3

Post-transcription control:

Answer 3

• Review: RNA processing involves the addition of a cap and a tail to a new mRNA transcript. This addition prevents nuclease enzymes present in the cytoplasm from hydrolyzing the transcript.
• One hypothesis is that a build-up of the protein product can wear down the protective additions, enabling the break down of the mRNA transcript and preventing translation.

Question 4

Pre-translation control:

Answer 4

• Review: During the initiation of translation, certain proteins (not pictured in the illustration) must be physically present for translation to occur.
• Deactivating the initiation factors will prevent translation.

Question 5

Translation control

Answer 5

• Translation of mRNA strands may be blocked by the
  attachment of regulatory proteins that bind to sequences near the 5’ end of the mRNA strand and prevent the attachment of ribosomes.

Question 6

Post-translation control:

Answer 6

• Proteasomes are giant complexes of proteins. They function to break down the unneeded proteins of a cell. The illustration to the left depicts the break down of proteins by a proteosome enzyme. The steps are as follows:
  · Enzymes in the cytosol attach ubiquitin molecules to the protein to be degraded, thereby marking it.
  · A proteosome recognizes the ubiquitin and
  degrades the protein

Question 7

Control of mRNA translation can be accomplished by the degradation of the mRNA. The first step in this process is

Answer 7

• enzymatic shortening of the 3′ poly A tail

Question 8

Place the following steps of protein degradation in the correct order.

i. Enzymatic components of the proteasome chop protein into small parts.
ii. Cytosolic enzymes attach ubiquitin to protein.
iii. A proteasome recognizes protein for degradation, denatures and encapsulates it.
iv. Proteasome and ubiquitin are recycled.

Answer 8

• ii, iii, i, iv

Question 9

Normal control mechanisms for the regulation of gene expression in eukaryotes include:

i. tRNA-mediated amino acid substitutions
ii. regulation of DNA transcription
iii. regulation of RNA translation
iv. control of protein degradation

Answer 9

• ii, iii, iv

Question 10

What is the role of ubiquitin in protein degradation?

Answer 10

• It flags proteins for degradation

Question 11

Which of the following does not play a role in the regulation of mRNA translation?

Answer 11

• regulation of DNA replication