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Circadian Clocks A circadian clock also known as a circadian oscillator is a biochemical system in living organisms that oscillates within a period of 24 hours. It is normally coordinated with the day- night cycle. The circadian clocks are main the central mechanisms which drive circadian rhythms…
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Download file to see previous pages In cyanobacteria, strains with a clock period length that matched the environmental photoperiod outgrow strains with no clock or with an out of phase period length in competitive culture conditions (Woefle et al., 2004). This does gesture to the probability of the clock being an important evolutionary step that allowed survival of early organisms. Circadian rhythms can influence sleep-wake cycles, hormone release, body temperature and other important intrinsic functions. Organization and function Function: The main principle of circadian clocks is oscillatory gene activation. The initial gene activation is regulated by the last one in the sequence, making up an auto-regulatory feedback loop that lasts about 24 hours. That is, feedback loops of transcription and translation, whereby the protein product of a clock gene will indirectly shut off its own expression. Such feedback loops are controlled and probably lengthened by post-translational modifications of most proteins involved (Harms et al., 2004) Organization: They have three major components: 1) A central oscillating mechanism with a period of about 24 hours. 2) Several output pathways associated with distinct phases of the oscillator. These control the activities of the organism. 3) Several input pathways to this central oscillator to allow programming of the circadian clock. Metazoans possess circadian clocks in most cells of the body. Each of these clocks is autonomous (Welsh et al., 1995). This means that each of the cells has to be entrained autonomously as well. But, since the oscillations of the clocks are roughly 24 hours in length, there has to be a form of synchronization. In the case of direct light entrained organisms, such as the drosophila, this entrainment occurs in cells independently (Plautz et al.,1997) (Whitmore et al., 2000). In drosophila, the Drosophila ring gland and Malpighian tubules show some sort of rhythms. This autonomous cell entrainment may also be seen in higher organisms where the parasympathetic system is responsible for entrainment (Ishida et al., 2005). It is itself entrained by the central master clock. In higher organisms, the process is more complex and requires what is known as hierarchical entrainment. This is discussed below. In higher organisms such as man, the entrainment of the clock is hierarchical. This is similar to most other vertebrates. In mammals, the suprachiasmic nucleus acts as the main/master clock. It is a paired neuronal structure located at the base of the hypothalamus. It is just above the optic chiasm/bifurcation (Klein et al.1991). It facilitates entrainment and synchrony of all other tissues by direct and indirect methods. These include use of hormones, temperature regulation, feeding regulation and metabolism. The suprachiasmic nucleus undergoes entrainment by relation with the optic apparatus via optic tract. This is the retionohypothalamic tract (Ben-Schlomo and Kyriacou, 2002). The SCN consists of a mixed population of neuronal and glial cells, but which types of cells might be capable of acting as circadian oscillators is presently unknown . Since they are autonomous in function, several theories have been proposed as to their synchronization. One is the synchrony of circadian rhythm due to production of melatonin by the pineal gland. It ...Download file to see next pagesRead More
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