Cycle 8 - Eukaryotic Gene Regulation

Question 1

Compare eukaryotic vs prokaryotic cells with respect to gene expression

Answer 1

Prokaryotic transcription and translation occur simultaneously (coincident) in the cytoplasm, and regulation occurs at the transcriptional level.

Eukaryotic gene expression is regulated during transcription and RNA processing, which take place in the nucleus, and during protein translation, which takes place in the cytoplasm.

• This affords a whole new area of regulation and complexity for eukaryotic cells

Question 2

Draw the structure of a eukaryote gene and describe each component’s function

Answer 2

• Enhancer: site for activator binding, functions to increase transcription
• Promoter proximal region​ contains promoter proximal elements ​that increase/decrease rate of transcription when regulatory proteins bind to them
• Promoter contains a TATA box​ where transcription factors bind to the TATA box (using motifs) and then recruit RNA polymerase
• Targetting sequence: brings proteins to specific organelle
• Splice signal: site of snRNA pairing for splicing
• PolyA signal: attracts an RNA-binding enzyme that cuts the message which signals polymerase to stop

Question 3

Describe the mechanism of transcription termination in eukaryotes

Answer 3

At the 3’ UTR, there is a cleavage site and a polyadenylation signal (found in the DNA) that attracts an RNA-binding enzyme that cuts the message which signals polymerase to stop

PolyA polymerase then extends the end by adding As to lessen degradation

polyA signal is transcribed

Question 4

Describe protein motifs common in DNA binding proteins

Answer 4

Protein motifs: Highly specialized region in a protein produced by the three-dimensional arrangement of amino acid chains within and between domains

Found in DNA-binding domains of regulatory proteins, such as activators

Question 5

Describe the mechanism of targeting proteins to cellular organelles

Answer 5

• If a newly made protein in the cytoplasm must go to an organelle, that gene will have an organelle-targeting sequence (ex., chloroplast –> chloroplast-targeting sequence)
• This target peptide is eventually lost UNLESS it is brought back to the nucleus (nucleus-targeting sequence)
  
  • This is necessary for when the nuclear membrane dissolves; the proteins must get back inside

Question 6

Where do endomembrane proteins get translated?

Answer 6

• Endomembrane proteins are targeted for co-translational import into the ER
• Translation begins in the cytoplasm but the ER tag is recognised by the signal recognition particle and facilitates translation on the rough ER

Question 7

Which gene expression components cross the nuclear membrane to get from where they are made to where they function?

Answer 7

1. rRNA, tRNA are made in the nucleus but function in the cystol
2. Proteins/enzymes are synthesized in the cytoplasm but may function in the nucleus so they must cross the membrane
3. snRNA never crosses the nuclear membrane because it functions in the nucleus for snipping
4. mRNA must cross the membrane to be translated

Question 8

What is epigenetic regulation?

Imprinting?

Answer 8

• NOT mutations, just chemical modifications of the DNA
• Imprinting: epigenetic marks on genome (some turned up, some turned down)
  
  • Ex., Father’s know it’s in their evolutionary best interest for their newborns to be big and get lots of resources from their mother (best chance of survival)
    
    • Father cranks up growth factor genes in gametes
    • Mother’s survival doesn’t matter
  • Mother’s know large babies put their lives at risk, so they want to have a small child

**mutation could occur in father’s gene that makes it have low expression, combined with mother’s low expression –> dwarfism**

Question 9

What changes are needed in genes if they are moved between prokaryotic and eukaryotic systems.

Answer 9

Add enhancer, promoter proximal elements, targeting sequence, polyadenylation signal

Remove operator, attenuator, SD box, terminator sequence