What is Life and Aging?
A Basic Introduction to Biology

This is a fundamental question, but many people are very confused about what life is from a biological perspective. Therefore, this guide will try to clearly explain the biology of life. (Unlike the other articles on this website, this is more of a reference article rather than a "how-to" article.) 

Let us assume you have never taken a single science course in your life, and we will begin from there. Even if you have, this will refresh your memory.

The Molecules That Make Up Life

First, a brief description of the chemistry and physics. Matter, or the stuff around us, as well as ourselves, is made of atoms. Each atom is composed of one or more positive charges in the nucleus, and one or more orbiting electrons forming a cloud of negative charge around it. There are many naturally-occurring elements which each have a unique number of positive charges in the nucleus, beginning with 1, all the way up to 92, and can be organized into a periodic table where elements with similar properties are grouped together. Carbon is one of those elements (atomic number 12, where it has 12 positive charges in its nucleus) and life is mainly based on carbons that link to one another. (You may have heard Bill Gates say that computers are silicon based and life forms are carbon based.) 

The carbons link to each other through a type of bonding called covalent bonding, where the electrons from neighbouring carbons are so close together that they stick together since they are attracted to the opposite carbon's positive charge. Other common elements within your body include oxygen (atomic number 16) and hydrogen (atomic number 1). Some less common elements include trace elements, and there are varying amounts of those other atoms in your body. 

The basic unit of a live being is the cell. Single-celled organisms, such as amoebae and bacteria, can exist in nature. As well, there are multicellular organisms, such as humans. Of course, as a multicellular organism, we each have different types of cells on different areas of the body that each perform specialized functions. For example, we have skin cells, liver cells, kidney cells, among other cells in the body. However, each of these cells display remarkable unity in that they share common features.

Some of the most important of these common features include the possession of DNA (abbreviation for deoxyribonucleic acid), and protein, both of which you may have heard of but not really sure what they mean. We will briefly describe these components just so that you can have a working knowledge of them. They are such common terms these days but many people do not really understand what they mean.

In order to understand what the term DNA is, we must delve into its structure. Deoxyribonucleic acid, or DNA, is a polymer. A polymer is a chemical molecule that is made up of repeating units linked together to form a macromolecule, or large molecule. In DNA, each of these subunits contain 

1) a deoxyribose sugar in the centre of each subunit (a name given to a type of sugar with a particular arrangement of carbons, oxygens, and hydrogens) 
2) a  phosphate group that is linked to the sugar (a particular arrangement of phorphorus and oxygen atoms), and 
3) a "base," another arrangement of carbons, hydrogens, and oxygens that is also linked to the sugar

The base is really what varies between each subunit. There are four main types of bases (the abbreviations of the bases are A, C, G, and T), which means that DNA is comprised of four different types of subunits, each linked to its own deoxyribose sugar and phosphate. Each subunit, in turn, are linked to each other through a sugar-phosphate-sugar-phosphate-sugar phosphate (and so on) type of arrangement, where the base is thus dangling out from the sugar. The sequence of bases is quite variable in different regions of the DNA. For example, there may be four consecutive A's, followed by some Gs, then a C, then another G, and then a T, and so on. In other regions, the sequence may be totally unlike the one above. The sequence of bases comprises the genetic sequence of organisms, and the total genetic sequence in a particular organism is its genome.

DNA is a double stranded molecule, where the two strands of polymers are linked together through a hydrogen bond (a weaker type of bond than a covalent bond) via their bases. There are certain rules as to which base likes to link with which, but we will not go into too much more detail. If you wish to learn more about this and other related topics that will be discussed, you may also wish to check out some other references.

Now that we've explained the "deoxyribose" part in "deoxyribonucleic acid," where does the term "nucleic acid" come in? Well, if you take away the deoxyribose sugar, what you have left is a nucleic acid. It is an "acid" simply because it contains the phosphate group, which is acidic (it likes to donate hydrogens when in water). It is "nucleic" simply because DNA is found in the nucleus, or near the centre, of the cell. There is actually another type of nucleic acid in the cell as well, termed RNA (or ribonucleic acid). The difference between RNA and DNA is the type of sugar and that RNA is single, not double, stranded.

So, how are DNA, RNA, and protein associated with one another? Proteins generally comprise the functional component of the cell, although certain types of RNA have functions within the cell too. By functional component, it means that proteins are the molecules that actually do things for the cell. For example, proteins may play a structural role, such as in the protein keratin that forms your hair, maintaining the hair's texture and feel. Proteins may also form a catalytic role, speeding up and helping out chemical reactions in the cell like the enzyme in your saliva that breaks down bread. Proteins can also be transporters, such as hemoglobin, which carries oxygen from your lungs to the rest of the cells in your body. 

What is the role of DNA and RNA then? DNA contains the sequence (through the sequence of the bases as mentioned earlier) that acts as a template for the sequence of RNA. (Special proteins build up RNA based on the DNA sequence). The sequence of RNA, in turn, acts as a template for the sequence of protein, which is also a polymeric structure, as we will discuss below.

The three-dimensional appearance and structure of a protein, which determines its ultimate function in an organism, is determined by its one-dimensional structure, or its sequence (the primary sequence). Proteins are comprised of mainly 20 distinct types of subunits called amino acids, which once again, can be arranged into any number of ways depending on how nature sees fit.

In the centre of each amino acid is a central carbon. All 20 amino acids share a common structure, and differ at one end of its central carbon (the side chain). There are four links that emanate from the central carbon: one link from this centre carbon branches out to a carboxyl group (carbon and two oxygens), one link of this centre carbon branches to a hydrogen, a third link branches to an amino group (a nitrogen and two hydrogens), and finally the fourth link branches to the side chain, which is the varying group is composed of different arrangements of different elements. Each of the 20 amino acids, which each has a specific side chain, has specific properties that give the protein its final shape. For example, some amino acids may have a negative charge, some amino acids may have a positive charge, which is, of course, due to that fact that its side chain has that particular property, since everything else in the amino acid is constant. A positive charge from one amino acid and a negative charge from a different amino acid can cause two different parts of a protein to stay in contact with one another since opposite charges attract. This is thus a type of force that maintains the protein's overall structure. 

Besides DNA, RNA, and protein, the other major class of molecules in cells are lipids. They are sort of like fat. They comprise components of the cell such as the cell membrane, which acts as a barrier between cells. Membranes within the cell also help segregate some components of the cell from each other. Lipids also have many other roles, but we won't go into detail. An example of a famous lipid is cholesterol, which is just a special arrangement of carbon and hydrogen atoms that has a property of either thickening or thinning down cell membranes, making the membranes, such as membranes of the endothelial cells that line your blood vessels, more fluid or more stiff. This, in turn, as you probably are aware, affects your health.

Cells are the unifying entities that encapsulate DNA, RNA, and protein. The main functions of a cell are either to (1) grow and divide into two cells, or (2) just to stay there and maintain its present activity. An example of the latter is the case of dormant cells such as your nerve cells, which, once you reach a certain age, normally do not continue to grow and divide. The cell normally has a cell cycle that it progresses through. Once nerve cells hit the dormant stage, though, they just stay there. However, dividing cells progress through four phases: two growth phases that intercalate between a dividing phase and a synthesis phase. The synthesis phase is when DNA is duplicated so that the new daughter cells each have their own copies of DNA. Proteins and other components aid in these processes. 

Other commonly-heard biological terminology

How about the terms "genes" and "chromosomes" and "genetic engineering," terms which may have been heard commonly? Each cell in a human being contains 23 pairs of chromosomes, or 46 in total. Every cell in a particular species of organisms contains the same set of chromosomes. For example, all the cells in the body of a dog contain 39 pairs of chromosomes. Each of these chromosomes is simply very well-packaged, coiled-up DNA. Within each chromosome, are many genes. Genes are the distinct characteristic traits that are possessed by organisms. For example, there is a gene that controls the colour of your hair. Each gene is comprised of a specific DNA sequence. 

It gets a bit complicated because since we have pairs of chromosomes, there is a gene on each chromosome specifying the same trait and depending on whether that gene is the dominant gene, it gets expressed (i.e. becomes what is displayed) or repressed (not displayed). Even though each cell contains the same set of chromosomes, each type of cell has its unique set of proteins that are produced. This is essentially developmental biology so we will not go into it in too much detail, but here it is in a nutshell: certain proteins will initially be expressed depending on what type of cell the embryo becomes. These initial proteins will, in turn, control the expression of other proteins so that, for example, liver cells become liver cells, heart cells become heart cells, and nerve cells become nerve cells. 

Genetic engineering is modifying the particular nucleic acids so to change a particular gene characteristic of the organism. The resulting protein that is produced will be different from the original. Genetic engineering aims to produce these changes for a useful purpose. 

A Philosophical Viewpoint

Therefore, the above paragraphs covered the molecular details of what life is comprised of. Just as a philosophical question to the meaning of life, though, what is the purpose of all this - you become alive, you live, and then you die. What is the purpose of all this? Not going into religious details or anything of that nature, here is a quote from Walter T. Stace:The cosmic eye...sees the human episode 
Poised for an instant of light between two eternities of darkness...
For one infinitesimal instant there flickers this light of Man, 
Flickers and dies out...,
Yet shall even the cosmic eye say: 
There was that light!...
And [t]hat light was light and not darkness.

Aging

As for why we die and why we age, there are many current theories, as well as much research is currently being conducted. First, how did humans get into this predicament of aging? Well, one reason is that mother nature wanted humans to be able to reproduce offspring so that future generations would continue. However, once humans had produced their offspring, there would be little purpose for them to continue to live. Therefore, this results in the decline in bodily functions as one gets older. 

One theory for this decline in bodily functions is that this aging is due to a degeneration of homeostasis in the body. Homeostasis is defined as the intricate balance in life that is required to maintain normal functions. For example, to maintain homeostasis even when we are doing nothing and sitting there requires many proteins and other molecules. As we get older, this decline in the ability to maintain homeostasis, such as a weakening immune system, may account for aging. 

Another theory maintains that aging may be due to free radical production which damages proteins. Yet another theory believes that aging is due a gradual shortening of the DNA with each division of the cells into daughter cells throughout one's lifetime. No one really knows the reason for aging. In fact there may be multiple reasons and interactions that cause aging. 

If you are afraid of dying, I must recommend this book Life 101: Everything We Wish We Had Learned About Life in School--But Didn't, for in it, it states that since we were nothingness before we were born, and we did not worry about it, why should we worry about becoming nothingness again after we die? (There are also other well-reasoned explanations in that book about death, as well as many other topics about life.) But, then, you may ask, so why live?  For the reason mentioned above by Stacy. How, though, do you determine what will be the meaning and course of your life? Visit The Meaning and Course of Your Life article. 

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