دانلود کتاب Lehninger Principles of biochemistry

In this twenty-first century, a typical science education often leaves the philosophical underpinnings of science unstated, or relies on oversimplified definitions. As you contemplate a career in science, it may
be useful to consider once again the terms science, scientist, and scientific method. Science is both a way of thinking about the natural world and the sum of the information and theory that result from such thinking. The power and
success of science flow directly from its reliance on ideas that can be tested: information on natural phenomena that can be observed, measured, and reproduced and theories that have predictive value. The progress of science rests on a foundational assumption that is often unstated but crucial to the enterprise: that the laws governing forces and phenomena existing in the universe are not subject to change. The Nobel laureate Jacques Monod referred to this underlying assumption as the “postulate of objectivity.” The natural world can therefore be understood by applying a process of inquiry— the scientific method. Science could not succeed in a universe that played tricks on us. Other than the postulate of objectivity, science makes no inviolate assumptions about the natural world. A useful scientific idea is one
that (1) has been or can be reproducibly substantiated, (2) can be used to accurately predict new phenomena, and (3) focuses on the natural world or universe. Scientific ideas take many forms. The terms that scientists use to describe
these forms have meanings quite different from those applied by nonscientists. A hypothesis is an idea or assumption that provides a reasonable and testable explanation for one or more observations, but it may lack extensive experimental substantiation. A scientific theory is much more than a hunch. It is an idea that has been substantiated to some extent and
provides an explanation for a body of experimental observations. A theory can be tested and built upon and is thus a basis for further advance and innovation. When a scientific theory has been repeatedly tested and validated on many fronts, it can be accepted as a fact. In one important sense, what constitutes science or a scientific idea is defined by whether or not it is published in the scientific literature after peer review by other working scientists. As of late 2014, about 34,500 peerreviewed scientific journals worldwide were publishing some 2.5 million articles each year, a continuing rich harvest of information that is the birthright of every human being.
Scientists are individuals who rigorously apply the scientific method to
understand the natural world. Merely having an advanced degree in a
scientific discipline does not make one a scientist, nor does the lack of such a
degree prevent one from making important scientific contributions. A
scientist must be willing to challenge any idea when new findings demand it.
The ideas that a scientist accepts must be based on measurable, reproducible
observations, and the scientist must report these observations with complete
honesty.
The scientific method is a collection of paths, all of which may lead to
scientific discovery. In the hypothesis and experiment path, a scientist poses a
hypothesis, then subjects it to experimental test. Many of the processes that
biochemists work with every day were discovered in this manner. The DNA
structure elucidated by James Watson and Francis Crick led to the hypothesis
that base pairing is the basis for information transfer in polynucleotide
synthesis. This hypothesis helped inspire the discovery of DNA and RNA
polymerases.
Watson and Crick produced their DNA structure through a process of
model building and calculation. No actual experiments were involved,
although the model building and calculations used data collected by other
scientists. Many adventurous scientists have applied the process of
exploration and observation as a path to discovery. Historical voyages of
discovery (Charles Darwin’s 1831 voyage on H.M.S. Beagle among them)
helped to map the planet, catalog its living occupants, and change the way we
view the world. Modern scientists follow a similar path when they explore
the ocean depths or launch probes to other planets. An analog of hypothesis
and experiment is hypothesis and deduction. Crick reasoned that there must
be an adaptor molecule that facilitated translation of the information in
messenger RNA into protein. This adaptor hypothesis led to the discovery of
transfer RNA by Mahlon Hoagland and Paul Zamecnik.
Not all paths to discovery involve planning. Serendipity often plays a
role. The discovery of penicillin by Alexander Fleming in 1928 and of RNA
catalysts by Thomas Cech in the early 1980s were both chance discoveries,
albeit by scientists well prepared to exploit them. Inspiration can also lead to
important advances. The polymerase chain reaction (PCR), now a central part
of biotechnology, was developed by Kary Mullis after a flash of inspiration
during a road trip in northern California in 1983.
These many paths to scientific discovery can seem quite different, but
they have some important things in common. They are focused on the natural
world. They rely on reproducible observation and/or experiment. All of the
ideas, insights, and experimental facts that arise from these endeavors can be
tested and reproduced by scientists anywhere in the world. All can be used by
other scientists to build new hypotheses and make new discoveries. All lead
to information that is properly included in the realm of science.
Understanding our universe requires hard work. At the same time, no human
endeavor is more exciting and potentially rewarding than trying, with
occasional success, to understand some part of the natural world.