The Nature of Life

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We know that that life exists in at least one "corner" of the universe. Does life exist elsewhere in our solar system, galaxy or beyond? At the present time we do not know the answer to this question. We do know, however, that the solar system and galaxy is rich with many of the physical "necessities of life".

The Physical Basis of Life

Carbon! One element - one word sums up the key to the physical basis of life. The element carbon is the backbone on which all of the organic compounds essential to life as we know it are built.

Figure 20.1 Artist's depiction of the role of carbon in creating the molecular conditions for life in the universe. (image NASA / Jenny Mottar)

What makes carbon so special?

• Carbon is a relatively abundant and low mass element that is produced via nucleosynthesis in supernovae
• Carbon grains (like graphite) are produced in the outer atmospheres of highly evolved, red giants. This material enriches future solar systems.
• The atomic structure of carbon enables it to form molecules readily with a wide variety of other elements
• Carbon atoms can join together to form long, robust chains or tight rings that can bind with other elements to from very large, stable molecules

Figure 20.1 is an artist's "homage" to carbon and also shows the formation of polycyclic aromatic hydrocarbons (PAHs) which are an important precursor to more complex molecules often associated with life.

Example 20.1 Science fiction writers often describe humans as "carbon based life-forms". They sometimes also refer to "silicon based life-forms". Why?

Solution: All of the essential biochemistry of our bodies is based on carbon molecules. In that sense we are "carbon based life-forms". The only other element that we know of that could form as diverse a repertoire of molecules is silicon so it is possible to have silicon based molecules that would share some similarities with carbon based molecules so perhaps there is a form of life based on silicon.

Storing (and reading) the Information of Life

Because carbon can produce long, stable chain molecules it forms the substrate on which the underlying information of life can be stored. One of the greatest discoveries of the 20th century was the discovery that the genetic blueprint for creating you and me is written in the carbon based molecule Deoxyribonucleic Acid or DNA. The video clip shown in Figure 20.2 shows a strand of DNA which appears as two, intertwined helical shaped molecules.

Figure 20.2 Stick representation of DNA showing the helical structure of the molecule and the cross linking between the helices. (video produced with VMD 1.8.7)

This "double helix" acts as a "cookbook" that directs the creation of proteins and other molecules essential to life. It does this by linking sets of proteins called amino acids in very specific ways. Figure 20.3a shows a space-filling version of a DNA molecule while Figure 20.3b shows a version of the same molecule that highlights the linking between pairs of amino acid molecules. These pairs are called base pairs and it is the patterns that they form which encode the genetic information that will direct the creation of proteins in a cell.

Figure 20.3a Spacefilling representation of the double helix DNA molecule	Figure 20.3b Double stranded DNA showing cross linking of base pairs (image courtesy U.S. National Library of Medicine/NIH)

The information coded within DNA is "read" through processes of transcription and replication. In transcription, enzymes read segments of the DNA that contain instructions or "recipes" to direct the building of proteins and other molecules necessary for a cell to function. Replication is the process of copying the entire DNA sequence. As an analogy consider the following:

Transcription is analogous to you copying down a recipe from a page of a cook book and then taking it to the kitchen to prepare a dish. There are lots of recipes and the particular page that you choose in the book will determine which recipe you use. Replication is the act of copying the entire cookbook to preserve (and perhaps pass on) the information contained in the book. The information in your DNA is your genetic information - ie genetic traits.   How DNA works in concert with the cell and its proteins is extremely complex and only partially understood at this time.

Figure 20.4 A "cookbook analogy" for DNA replication and transcription.

Changing the Information of Life

Replication is a fundamental property of DNA and it is what makes possible the transfer of genetic information from one generation to the next. A child, for example receives genetic information from each parent and this information, coded as DNA contains the blueprint from which the child will be built.

The replication process needs to be accurate but, paradoxically, not too accurate. The information stored in DNA can be subtlety altered by external influences. Changing a base pair somewhere along the DNA strand could produce changes in the resulting organism. The change is called mutation. Small changes often go unnoticed and are harmless. Occasionally the mutations are harmful and can lead to the death of the organism. Even rarer still, however, are mutations that are beneficial and confer an advance to the mutant that the parent organisms did not have.

The ability of genetic information to change is essential for biological evolution since it makes it possible for a species to adapt to changing environmental conditions. The process of natural selection operates by favoring those members of a species whose genetic information makes them best adapted to the environment at that time.

Example 20.2 How could proximity to a supernovae have harmful effects on an organism?

Solution: A supernova will emit intense x-ray and gamma-ray photons. If you are located close to a supernova then you will experience an increased level of exposure to these high energy photons. When a high energy photon strikes a DNA molecule it can disrupt one of more base-pairs and hence change the generic information coded at that location. If you suffer too many such DNA disruptions then the chances of harmful mutation rise rapidly. A common analog to this is exposure to radiation from nuclear sources on earth. The harmful mutations that result often are in the form of cancers.

Example 20.2 begs the question "How close is too close to a supernova?" Calculations show that a supernova event within 1000 light years would produce enough of an increase in gamma and x-ray levels to pose a significant health risk. At one time supernovae where considered as a possible cause for the extinction of the dinosaurs.

Practice

1. How is information stored in a DNA molecule?
2. Why is it important that gentic information can change? How does this change occur?
3. What is natrual selection?