Circadian Rhythms Exhibit Characteristic Features

Circadian rhythms arise from cyclic phenomena that are defined by three parameters:

1. Period, the time between comparable points in the repeating cycle. Typically the period is measured as the time between consecutive maxima (peaks) or minima (troughs) (Figure 24.15A).

2. Phase2, any point in the cycle that is recognizable by its relationship to the rest of the cycle. The most obvious phase points are the peak and trough positions.

3. Amplitude, usually considered to be the distance between peak and trough. The amplitude of a biological rhythm can often vary while the period remains unchanged (as, for example, in Figure 24.15C).

In constant light or darkness, rhythms depart from an exact 24-hour period. The rhythms then drift in relation to solar time, either gaining or losing time depending on whether the period is shorter or longer than 24 hours. Under natural conditions, the endogenous oscillator is

2 The term phase should not be confused with the term phase change in meristem development, discussed earlier.

A typical circadian rhythm. The period is the time between comparable points in the repeating cycle; the phase is any point in the repeating cycle recognizable by its relationship with the rest of the cycle; the amplitude is the distance between peak and trough.

A circadian rhythm entrained to a 24 h light - dark (L-D) cycle and its reversion to the free-running period (26 h in this example) following transfer to continuous darkness.

12D 12L i 12D 12L i 12D 12L I

A circadian rhythm entrained to a 24 h light - dark (L-D) cycle and its reversion to the free-running period (26 h in this example) following transfer to continuous darkness.

(C)

Light

(h)

1

Suspension of a circadian rhythm in continuous bright light and the release or restarting of the rhythm following transfer to darkness.

Suspension of a circadian rhythm in continuous bright light and the release or restarting of the rhythm following transfer to darkness.

Typical phase-shifting response to a light pulse given shortly after transfer to darkness. The rhythm is rephased (delayed) without its period being changed.

FIGURE 24.15 Some characteristics of circadian rhythms.

Typical phase-shifting response to a light pulse given shortly after transfer to darkness. The rhythm is rephased (delayed) without its period being changed.

FIGURE 24.15 Some characteristics of circadian rhythms.

entrained (synchronized) to a true 24-hour period by environmental signals, the most important of which are the light-to-dark transition at dusk and the dark-to-light transition at dawn (see Figure 24.15B).

Such environmental signals are termed zeitgebers (German for "time givers"). When such signals are removed— for example, by transfer to continuous darkness—the rhythm is said to be free-running, and it reverts to the cir-cadian period that is characteristic of the particular organism (see Figure 24.15B).

Although the rhythms are generated internally, they normally require an environmental signal, such as exposure to light or a change in temperature, to initiate their expression. In addition, many rhythms damp out (i.e., the amplitude decreases) when the organism is in a constant environment for some time and then require an environmental zeitgeber, such as a transfer from light to dark or a change in temperature, to be restarted (see Figure 24.15C). Note that the clock itself does not damp out; only the coupling between the molecular clock (endogenous oscillator) and the physiological function is affected.

The circadian clock would be of no value to the organism if it could not keep accurate time under the fluctuating temperatures experienced in natural conditions. Indeed, temperature has little or no effect on the period of the free-running rhythm. The feature that enables the clock to keep time at different temperatures is called temperature compensation. Although all of the biochemical steps in the pathway are temperature-sensitive, their temperature responses probably cancel each other. For example, changes in the rates of synthesis of intermediates could be compensated for by parallel changes in their rates of degradation. In this way, the steady-state levels of clock regulators would remain constant at different temperatures.

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