Regulation of Gene Expression (Genetic Control)
1. Define:
Operon - An operon is a segment of DNA that includes a series of structural genes and the control elements regulating the transcription of those genes. The structural genes within a single operon are regulated together (by a single promoter), and transcribed as a single unit (transcription is polycistronic).
Promoter - The promoter site is a portion of DNA and serves as one of several control elements within an operon. The promoter is the nucleotide sequence recognized and bound by the sigma factor of RNA polymerase to initiate transcription.
Operator - The operator site is a portion of DNA serving as one of several control elements within an operon. The operator is the nucleotide sequence to which a functional repressor protein binds to stop (repress) transcription. The location of operators sites within operons is variable.
Catabolite repression - Catabolite repression is a mechanism allowing bacteria to utilize metabolic pathways for which the enzymes are constitutive in favor of those for which the enzymes must be induced. It increases cellular effeciency and energy conservation.
Constitutive enzyme - Constitutive enzymes are those enzymes essential to cell function and always being made, regardless of changes in the environment (intracellular or extracellular). Constitutive enzymes are coded for by genes that are constantly expressed (constitutive genes).
2. feedback inhibition or end product inhibition
3. allosteric inhibition/ This mechanism is less efficient than end product repression in terms of energy conservation. Cells expend considerable amounts of energy when they synthesize m-RNA molecules and protein molecules (form phosphodiester and peptide bonds). If the proteins synthesized are not used because they are being inhibited, then this energy is wasted.
4. feedback or end-product inhibition
5. structural gene/ When transcription is polycistronic, several genes are transcribed together, forming one, long m-RNA molecule. When this m-RNA is translated, a series of different polypeptides will be formed.
6. operon/ polycistronic
7. operon/ promoter
8. The promoter is the nucleotide sequence (region of DNA) where the sigma factor of RNA polymerase binds to begin transcription. Without a promoter site, transcription will not occur.
9. repressor protein/ operator
10. repressor protein/ tryptophan
11. repressible operon/ corepressor/ operator
12. inducible operon/ permease/ allolactose
13. allolactose/ catabolite activating protein
14. b-galactosidase/ Allolactose binds to the repressor and inhibits the activity of this protein. The repressor then releases the operator site, and transcription is allowed to proceed. Transcription is thus said to be induced (however, it will not proceed unless the promoter site is attractive to sigma factor).
15. It is active alone. It does not require a corepressor.
16. The beta-galactoside permease is an enzyme that catalyzes the reactions necessary to bring lactose through the cell membrane and into the cell. In E. coli, b-galactoside permease is coded for by the lac Y gene of the lactose operon.
17. lactose utilization operon (lac operon)/ allolactose
18. Catabolite repression/ cyclic AMP
19. glucose/ constitutive/ cyclic AMP
20. constitutive/ catabolite repression.
Copyright 2002 Sierra College Biological Sciences Department
Last updated: 01/18/02.
