Circadian Rhythms Quiz for Neuroscience Learners

Luciferase reports gene expression because it produces light when its substrate reacts with the enzyme. In Drosophila, luciferase is commonly fused to promoters such as the per promoter. This allows researchers to quantify transcriptional rhythms by measuring oscillations in emitted light. Since locomotor activity, CRY degradation, and PER phosphorylation are not directly measured through luciferase, mRNA production is the most accurate and relevant measurement.

Entrainment requires periodic environmental cues. LL, or constant light, removes the external timing signal needed for synchronization to a 26-hour cycle. Light–dark cycles such as LD14:12, LD13:13, and DL1:25 (dark–light cycles) provide transitions essential for resetting the circadian clock each day. Without shifts between light and dark, organisms cannot align endogenous rhythms to an imposed schedule, making LL ineffective for entrainment.

Double plotting presents each day's activity twice on an actogram to help visualize rhythmic patterns more clearly. By overlapping the plot, subtle advances or delays become easier to detect compared to single plotting. Researchers use this method to analyze free-running or entrained rhythms, identify phase shifts, and interpret behavioral trends across days. Other options do not relate to actogram visualization, making double plotting the correct interpretation.

The SCN determines circadian period based on the donor tissue’s genotype. In tau/tau mutant hamsters, the endogenous period is approximately 20 hours. When a wildtype hamster receives a tau/tau SCN transplant, its rhythmicity is restored but now reflects the donor's shortened period. This demonstrates that the SCN, not peripheral systems, sets the overall circadian timing, making the 20-hour rhythm the expected outcome.

The SCN acts as the central circadian pacemaker located in the hypothalamus. It coordinates daily rhythms by integrating environmental light signals and synchronizing peripheral clocks throughout the body. While temperature regulation, digestion, and immune responses may be influenced indirectly by circadian rhythms, the SCN's primary role is to maintain internal temporal order. Thus, it serves as the master clock rather than a direct regulator of physiological systems.

In Drosophila, TIM forms a complex with PER to inhibit transcription driven by CLK and CYC, completing the transcription–translation feedback loop. This negative regulation sets the oscillatory cycle that underlies circadian timing. Other molecules like CLK and CYC act as positive regulators, while DBT phosphorylates PER to adjust degradation timing. Therefore, TIM best represents the core negative regulatory component within the Drosophila circadian system.

A free-running rhythm occurs under constant conditions where no external cues exist to reset the internal clock. The rhythm persists with a period slightly longer or shorter than 24 hours depending on species. This indicates endogenous rhythmicity independent of external inputs. Entrained rhythms require environmental cues, ultradian rhythms are shorter than 24 hours, and masking effects modify behavior without altering internal timing. Thus, free-running is the correct concept.

Light is the strongest zeitgeber for most organisms because photoreceptors directly reset the circadian pacemaker in response to changes in illumination. Neural pathways convey light information to the SCN, adjusting phase and period as needed for alignment with the environment. Humidity, magnetic fields, and barometric pressure show much weaker or inconsistent entraining effects. Therefore, light remains the dominant and most reliable cue for circadian synchronization.

Activity appearing earlier each day indicates a shortening of the circadian period under constant conditions. When the endogenous period is less than 24 hours, the rhythm advances slightly each cycle, causing activity onset to shift earlier across days. This progressive advance is characteristic of species with shorter free-running periods. Stable entrainment would show no drift, while arrhythmic behavior lacks a discernible pattern.

Constant darkness removes external resetting cues but preserves internal rhythmicity, allowing the free-running period to manifest. Depending on species, DD often leads to slight lengthening or shortening of the endogenous cycle. Rhythms do not lock to exactly 24 hours, nor do they disappear unless the circadian system is damaged. Therefore, constant darkness primarily reveals natural timing variations rather than abolishing rhythmic behavior.