A gene is regulated by a coherent type-1 feedforward loop: activator X activates gene Z both directly and indirectly through activator Y (X activates Y, Y activates Z). What happens when X is turned on as a step input?
AZ activates immediately, because the direct path from X to Z requires no intermediary
BZ activates with a delay, because activation requires BOTH the direct path (X -> Z) and the indirect path (X -> Y -> Z) to be active — the indirect path imposes a delay that filters transient signals
CZ never activates, because the two pathways cancel each other
DZ oscillates, because the two pathways create conflicting signals
In the coherent type-1 feedforward loop with AND logic at Z's promoter, Z requires both X and Y to be active. When X turns on, the direct signal from X arrives immediately, but Y takes time to accumulate (it must be transcribed and translated). Z only activates once Y reaches sufficient levels — creating a delay. Critically, if X is turned on only briefly (a transient pulse), Y never reaches the threshold and Z remains off. This 'sign-sensitive delay' filters out transient fluctuations while responding to sustained signals — a noise filter built from simple regulatory components.
Question 2 True / False
Negative autoregulation (a transcription factor represses its own promoter) is one of the most common motifs in E. coli. Its primary function is to slow down gene expression to conserve cellular resources.
TTrue
FFalse
Answer: False
Negative autoregulation actually SPEEDS UP the response time compared to unregulated expression. When the gene is first activated, the protein level is low, so there is no self-repression — the gene is transcribed at maximum rate, producing protein rapidly. As protein accumulates and begins repressing its own promoter, the production rate drops to match degradation, reaching steady state faster than a gene without autoregulation. The steady-state level is lower (which does conserve resources), but the dynamic benefit is faster response. Additionally, negative autoregulation reduces noise by dampening fluctuations around the steady state.
Question 3 Short Answer
How are network motifs identified — what makes a subgraph pattern a 'motif' rather than just a common feature of the network's degree distribution?
Think about your answer, then reveal below.
Model answer: A motif is defined statistically: a subgraph pattern qualifies as a motif if it appears significantly more often in the real network than in an ensemble of randomized networks that preserve the same degree distribution. The comparison to degree-preserving random networks is critical — it controls for the fact that some patterns are common simply because high-degree nodes participate in many subgraphs. If a pattern is enriched beyond what the degree distribution alone would predict, this suggests that natural selection has favored that wiring pattern for its functional properties, not that it is merely a statistical byproduct of the network's connectivity.
Alon's original analysis compared the E. coli transcription network to thousands of randomized networks with the same number of nodes, edges, and degree sequence. The feedforward loop, negative autoregulation, and single-input module appeared far more often than expected, while other three-node patterns (like the three-node feedback loop) were depleted. This enrichment/depletion pattern is remarkably consistent across different biological networks and organisms.