Molecular Genetics - Controlling Gene Expression

Question 1

Why do organisms regulate the expression of their genes?

Answer 1

• Continuous expression of all genes in genome is very inefficient
• The cell would be overloaded with non-essential proteins, while depleting resources needed to make them

Question 2

What are the 4 categories of gene regulation?

Answer 2

1. Transcriptional: during mRNA synthesis
2. Post-transcriptional: during mRNA processing
3. Translational: during protein synthesis
4. Post-translational: after protein synthesis

Question 3

What is an operon?

Answer 3

• A cluster of genes controlled by a single promoter
• Used by prokaryotes to regulate transcription of genes
• The genes are transcribed as single piece of mRNA and translated simultaneously, entire metabolic pathway becomes active at the same time

Question 4

What is an operon made up of?

Answer 4

• Promoter: region to which RNA polymerase will bind
• Operator: specialized recognition sequence for a repressor protein (on-off switch)
• Structural genes: genes that code for various proteins/enzymes involved in particular metabolic pathway

Question 5

What is a regulatory gene?

Answer 5

• Codes for a repressor protein
• Located upstream from operon
• This gene is always active and produces repressor protein responsible for binding to operator of operon
• By binding to operator, repressor protein prevents RNA polymerase from transcribing the genes

Question 6

What are the 2 types of operons?

Answer 6

1. Inducible operons: normally turned off but can be activated when needed
2. Repressible operons: normally turned on but can be repressed when not needed

Question 7

What is the Lac operon?

Answer 7

• Inducible operon
• Usually turned off since lactose is not usual food for bacteria (repressor protein synthesized in its active conformation)
• When bacteria exposed to lactose, operon will be turned on
• Lactose binds to repressor protein, changing its shape so that it “falls off” the operator
• RNA polymerase can now bind to promoter and transcribe operon so that enzymes for lactose metabolism can be produced
• When all lactose used up (including inducer molecules), repressor changes back to active shape, binds to operator + shuts down operon again

Question 8

What is the Trp Operon

Answer 8

• Tryptophan is an amino acid needed for protein synthesis, so pathway is usually active (repressor protein synthesized in inactive conformation)
• If tryptophan builds up in cell or available from environment, cell will shut down production (turn of operon)
• Trp molecule binds to repressor protein, changing shape so it can bind to operator (gene can no longer be expressed)
• Trp is corepressor (signal molecule that binds to repressor to reduce expression of operon’s genes)
• When all trp used up (including corepressor molecule), repressor protein returns to inactive shape + falls off operator
• Operator turned back on, trp synthesis can begin again

Question 9

What are the four categories of gene regulation in eukaryotes?

Answer 9

1. Transcriptional
2. Post-transcriptional
3. Translational
4. Post-translational

Question 10

What is transcriptional regulation + examples?

Answer 10

• Allows cells to regulate which genes are transcribed + rate of transcription
• Controlling access to promoter: to turn transcription of gene on or off
• Activating or repressing promotor: to increase/decrease rate of transcription of gene
• Methylation of promoter: methyl groups bond to promoter, prevent binding RNA polymerase (preventing transcription of gene)

Question 11

What is post-transcriptional regulation + examples?

Answer 11

• Control availability of mRNA to ribosomes
• Alternative splicing: allows pre-mRNA to be modified into variety of related proteins by varying exons kept in final mRNA
• Masking proteins: bind to mRNA, preventing translation (can be removed when translation is required)
• Regulatory molecules: used to control rate of mRNA degradation (controlling how long it is actively being translated)

Question 12

What is translational regulation + examples?

Answer 12

• Controls how often + how rapidly mRNA is translated into proteins
• Altering length of poly-A-tail: increasing or decreasing time required to translate mRNA into protein

Question 13

What is post-translational regulation + examples?

Answer 13

• Controls when proteins become fully functional, how long they are functional, + when they degrade (controls availability of functional proteins)
• Processing mechanisms: used by cell to finalize protein structures, can be regulated to control availability of activated/functional proteins
• Chemical modification: addition or removal of chemical groups can activate or inhibit proteins
• Ubiquitination: binding of small protein called ubiquitin, signals protein for degradation (allows cell to control how long protein is active)