Genomics - L8

What is an operon?

• A eukaryotic gene cluster with multiple start sites
• A single bacterial gene controlled by its own promoter
• A set of adjacent bacterial genes transcribed together under a single promoter
• A protein complex that blocks transcription

Answer explanation

An operon is a cluster of bacterial genes regulated by a shared promoter and transcribed as one mRNA.

What happens in positive regulation of the lac operon?

• An activator protein responds to low glucose by enhancing transcription
• A repressor protein binds directly to the RNA polymerase
• The operon is activated only in high glucose conditions
• A terminator sequence is formed in response to lactose

Answer explanation

In low glucose, cAMP levels rise, allowing the CRP protein to activate lac operon transcription.

How is the lac operon negatively regulated?

• The repressor protein binds to the operator to block transcription
• RNA polymerase is inactivated by glucose
• The CRP protein represses transcription
• The promoter sequence is removed

Answer explanation

The Lac repressor binds the operator to prevent RNA polymerase from initiating transcription.

What is the regulatory mechanism of the ara operon?

• AraC represses the operon in the presence of glucose
• AraC acts only as an activator
• AraC functions as both a repressor and an activator depending on arabinose
• The ara operon is regulated by the Lac repressor

Answer explanation

AraC represses by DNA looping, but activates transcription when arabinose binds and changes its conformation.

How is the trp operon regulated?

• Only through CRP-cAMP activation
• By a repressor and transcriptional attenuation based on tryptophan levels
• Through the presence of glucose and lactose
• By silencing mRNA using small RNAs

Answer explanation

The trp operon is controlled by a repressor that binds in presence of tryptophan, and by attenuation at the leader sequence.

A set of adjacent bacterial genes that are transcribed together are known as a…

• cluster
• polyprotein
• operon
• regulator

Answer explanation

An operon is a functional unit of DNA containing a cluster of genes under control of a single promoter.

What is the idea behind operons?

• Genes are separated for individual expression
• Each gene uses its own promoter and repressor
• Functionally related genes are grouped for coordinated regulation
• Operons only work in eukaryotes

Answer explanation

Operons allow bacteria to regulate genes involved in similar functions all at once.

Which of these is not a classic example of an operon?

• lac
• ara
• trp
• actin

Answer explanation

Lac, ara, and trp are all well-known bacterial operons; actin is a eukaryotic structural protein gene.

Operons are sets of genes that…

• share the same repressor binding site
• have the same ribosome
• have the same promoter
• code for identical proteins

Answer explanation

Operon genes are co-transcribed under a shared promoter, allowing simultaneous regulation.

Lac operon contains 3 genes coding for E. coli proteins to use lactose. Which of these is NOT one of those proteins?

• Lac permease
• Beta-galactosidase
• Galactoside transacetylase
• Hexokinase

Answer explanation

Hexokinase is not part of the lac operon; it’s involved in glucose metabolism, not lactose.