144
Z. Zheng
happen at the level of each unit. Recent studies of the so-called complex networks
stimulated by milestone works on small-world [22] and scale-free networks [23]
provide a powerful platform in studying complex systems. For example, it is a significant topic to explore network properties of biological gene, DNA, metabolic, and
neural networks, social and ecological networks, WWW and internet networks, and
so on. Another important topic is the study of dynamical processes on networks, such
as synchronization, propagation processes, and network growth. Interested readers
may refer related monographs and reviews for more knowledge of network science
[24–30].
4.3 Emergence of Rhythms
4.3.1 Biological Rhythms: An Introduction
Biological rhythm is an old question and can be found ubiquitously in various living
systems [31]. The 2017 Nobel Prize in Physiology or Medicine was awarded to J. C.
Hall, M. Rosbash and M.W. Young for their discoveries of molecular mechanisms
that control circadian rhythms [32, 33]. Circadian rhythms are driven by an internal
biological clock that anticipates day-night cycles to optimize the physiology and
behavior of organisms. The exploration of the emergence of circadian rhythms from
the microscopic level (e.g. molecular or genetic levels) aroused a new era of studies
on biological oscillations [34] (Fig. 4.2).
Observations that organisms adapt their physiology and behavior to the time of
the day in a circadian fashion have been recorded for a long time. The very early
observations of leaf and flower movements in plants, for example, the leaves of
mimosa plants close at night and open during the day presented interesting biological
clocks. In 1729, the French scientist de Mairan observed that the leaves of a mimosa
plant in the dark could still open and close rhythmically at the appropriate time of the
day, implying an endogenous origin of the daily rhythm rather than external stimulus
[35].
The genetic mechanism responsible for the emergence of circadian rhythms was
first explored by S. Benzer and R. Konopka from the 1960s. In 1971, their pioneering
work identified mutants of the fruit fly Drosophila that displayed alterations in the
normal 24-h cycle of pupal eclosion and locomotor activity, which was named as
period (PER) [36]. Later on, Hall and Rosbash at Brandeis University [37] and Young
at Rockefeller University [38] isolated and molecularly characterized the period gene
(Fig. 4.3).
Further studies by Young, Hardin, Hall, Rosbash and Takahashi revealed that
the molecular mechanism for the circadian clock relies not on a single gene but on
the so-called transcription-translation feedback loop (TTFL), i.e. the transcription
of period and its partner gene timeless (TIM) are repressed by the PER and TIM
proteins, generating a self-sustained oscillation [39–42]. These explorations led to
Z. Zheng
happen at the level of each unit. Recent studies of the so-called complex networks
stimulated by milestone works on small-world [22] and scale-free networks [23]
provide a powerful platform in studying complex systems. For example, it is a significant topic to explore network properties of biological gene, DNA, metabolic, and
neural networks, social and ecological networks, WWW and internet networks, and
so on. Another important topic is the study of dynamical processes on networks, such
as synchronization, propagation processes, and network growth. Interested readers
may refer related monographs and reviews for more knowledge of network science
[24–30].
4.3 Emergence of Rhythms
4.3.1 Biological Rhythms: An Introduction
Biological rhythm is an old question and can be found ubiquitously in various living
systems [31]. The 2017 Nobel Prize in Physiology or Medicine was awarded to J. C.
Hall, M. Rosbash and M.W. Young for their discoveries of molecular mechanisms
that control circadian rhythms [32, 33]. Circadian rhythms are driven by an internal
biological clock that anticipates day-night cycles to optimize the physiology and
behavior of organisms. The exploration of the emergence of circadian rhythms from
the microscopic level (e.g. molecular or genetic levels) aroused a new era of studies
on biological oscillations [34] (Fig. 4.2).
Observations that organisms adapt their physiology and behavior to the time of
the day in a circadian fashion have been recorded for a long time. The very early
observations of leaf and flower movements in plants, for example, the leaves of
mimosa plants close at night and open during the day presented interesting biological
clocks. In 1729, the French scientist de Mairan observed that the leaves of a mimosa
plant in the dark could still open and close rhythmically at the appropriate time of the
day, implying an endogenous origin of the daily rhythm rather than external stimulus
[35].
The genetic mechanism responsible for the emergence of circadian rhythms was
first explored by S. Benzer and R. Konopka from the 1960s. In 1971, their pioneering
work identified mutants of the fruit fly Drosophila that displayed alterations in the
normal 24-h cycle of pupal eclosion and locomotor activity, which was named as
period (PER) [36]. Later on, Hall and Rosbash at Brandeis University [37] and Young
at Rockefeller University [38] isolated and molecularly characterized the period gene
(Fig. 4.3).
Further studies by Young, Hardin, Hall, Rosbash and Takahashi revealed that
the molecular mechanism for the circadian clock relies not on a single gene but on
the so-called transcription-translation feedback loop (TTFL), i.e. the transcription
of period and its partner gene timeless (TIM) are repressed by the PER and TIM
proteins, generating a self-sustained oscillation [39–42]. These explorations led to