Sunday, February 28, 2021

Secret Science Club Zoom Lecture: What is Life?

Today, my great and good friends of the Secret Science Club for a Zoom lecture by Dr Paul Nurse, 2001 Nobel Prize winner in Physiology and Medicine, and director of London's Francis Crick Institute.  The topic of the lecture was the subject of his book What Is Life? Five Great Ideas in Biology.  This Zoom lecture represents Dr Nurse's triumphant return to the Secret Science Club, the good doctor lectured back in 2008, while heading Rockefeller University... my one big blogging regret is that I didn't start the blog earlier, so I could have summarized those lectures.

Having a crazy schedule these days, I entered the Zoom about fifteen minutes late, entering while Dr Nurse was discussing Gregor Mendel's study of heredity in pea plants, showing slides of the monk's gardens to illustrate his painstaking study.

Chromosomes, the molecules which regulate heredity were originally found in onion root cells.  Chromosomes are made of deoxyribonucleic acid (DNA).  DNA splits and the strands are templates for reproduction.  The sequence of bases (adenine, cytosine, guanine, thymine) in a gene is coded in RNA which acts as a messenger to the cytoplasm to regulate proteins- one gene per protein.  Heredity is written in the four letter nucleobase code (ACGT) in linear form.  

Life is chemistry.  Louis Pasteur, while studying the fermentation of sugar beets to ensure the production of alcohol rather than acid- he determined that yeast, a single cell organism, was responsible for producing alcohol, while bacteria would produce acid.  He stated that fermentation is a physiological process.  Proteins, polymers of amino acids, in the yeast acted as enzymes to alter carbohydrates, polymers of carbon, oxygen, and hydrogen.  There are twenty amino acids which can combine to form complex molecular structures (DNA is rather simple).  The various combinations of these twenty amino acids allow a wide range of proteins to be harnessed to power life functions.

Enzymes act as molecular machines which can reconfigure carbohydrates and lipids, they can transport other chemicals, they can turn sunlight to carbohydrates and break down other chemicals.  Because they interact, enzymes need to be encapsulated to prevent harmful reactions.  This separation allows the mechanism of life to occur.

Life is information, it is a complex system involving the management of information.  The structure of DNA, iconic and beautiful, only makes sense when seen as a digital information system combined of four amino acids.  Regulation is also necessary for life.  There are governors to the processes of life- negative feedback loops to reduce synthesis of chemicals, and positive feedback loops to initiate growth.  Homeostasis must be maintained, routines must be managed.

Life evolves, and life evolves by means of natural selection.  Charles Darwin's father, Erasmus Darwin, believed in the evolution of life from simple forms to more complex forms, but did not name a process by which such evolution occurred.  If life only evolved once, using four amino acids, all life is related.  Charles Darwin proposed the mechanism of natural selection as the means for evolution- it is a consequence of a population having variations dependent on heritable traits.  Traits which provide for better adaptation, and increased reproduction, will be passed on until there are sufficient changes to cause variable populations to cease to be able to interbreed.

Dr Nurse gave a brief rundown of Charles Darwin's gradual discovery of evolution by means of natural selection while serving on HMS Beagle.  He also displayed an illustration of the famous Galápagos finches, comparing their beaks to various types of pliers:

The different beak shapes were selected for by the necessity to obtain different types of food.  Natural selection requires reproduction, heritability, and mutability in heritability.  Chromosomes determine cell properties, chromosomes replicate, and mutations arrive, which can be selected.  

Dr Nurse ended his discussion of evolution by means of natural selection with this beautiful quote by Charles Darwin, who asserted that biology has laws, just like physics:

"Whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved."

Dr then shifted the topic to intermediate-life forms, viruses.  Viruses can reproduce and evolve, but can only do so in the cells of other organisms- they hijack the mechanism of cells to reproduce.  One could say that viruses only live while inside other organisms, but are lifeless chemicals while outside cells.  All life forms depend on other organisms- humans cannot synthesize certain amino acids, plants need bacterial symbiotes to produce nitrogen.  There is a spectrum of interdependence among the organisms of earth.  All life is interdependent.

Towards the end of the lecture, Dr Nurse summed up the various topics he had discussed in a series of bullet points.

Living things are independent bounded physical entities.  The basic unit of life on Earth is the cell

Life forms are entities that can undergo evolution, they reproduce, they have a hereditary system, their progeny is subject to natural selection.

Life forms are chemical, physical, and informational machines which build their own metabolism to grow and reproduce.

Life is based on carbon polymer chemistry, polymers produce nucleic acid information storage  Life is based on information storage in linear form encoded in polymers.

All known life on Earth descends from a common ancestor.  We are all related to our fellow denizens of the planet.

If there is life beyond earth, it will be based on polymers, but might be based on different chemistry, utilizing different forms of energy.

All living organisms are related, and interact.  We depend on our relationships to other inhabitants of Earth.  Together, we are the great survivors, the currently living descendants of a single organism back in Deep Time.  Because we are perhaps the only organisms to understand this, we have an obligation to protect and preserve life.

The lecture was followed by a Q&A session.  The first question involved horizontal gene transfer, which can move genes from one branch of the Tree of Life to another- this doesn't alter natural selection, but does alter heritability.  A question regarding consciousness evoked a call for increased study of the brain, and its chemical and electrical processes- Dr Nurse stated that the study of the brain will be the great scientific study of the next one-hundred years.  Another question involved mitochondrial migration in neurons- neurons often being of great length, mitochondria might have to migrate to ensure proper energy supplies throughout the cells.  How likely or unlikely was the eukaryotic revolution?  Having studied the mechanism of cell division, the way in which human cells control cell division is identical to the way in which yeast cells control cell division- this is an extraordinary level of conservation, unchanged over a billion years.  One could even take a human gene and implant it in a yeast cell, and cell division would not change.  Questioned about the Drake equation, Dr Nurse noted that the universe is so vast, with enormous numbers of potential homes for life, the universe is most likely home to other lifeforms than earthly ones.

The Q&A session was short, as Dr Nurse had a prior claim on his time, which is just as well, because I had to get ready for work.  

For a taste of the Secret Science Club experience, here is a video by Dr Nurse covering the topic he lectured on today:

Kudos to Dr Nurse, Dorian, and Margaret for another great Secret Science Club lecture.  Now, folks, soak in that SCIENCE!

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