About Genetics

The Similarity of Living Things


	Plant cells (left) and Animal cells (right) have many features in common.

On a cellular level, human beings have a kinship of varying degrees with all other living beings. Human cells are cousins with cells of other vertebrates: mammals, birds, reptiles, amphibians, and fish. Internally, our similarities with other vertebrates are surprisingly strong: we have a nervous system that, through our brain, controls muscular activity and responds to external stimulus. Outside appearances may suggest great differences between humans and organisms that lack a vertebrae (such as insects, plants, or protozoa,) yet there are still similarities at the cellular level of reproduction, acquiring food, and surviving the elements.

Indeed, the basic life trait extends from the cells of the most complex living organisms to the simplest bacteria. Our most distant relative is in fact thought to have been a single molecule that provided the seed for life. The defining characteristic of this first molecule of life: its ability to reproduce a near-identical copy of itself. Similar molecules and “molecular machinery” comprise the life processes for all organisms.

Charles Darwin
1809-1892

As the milestones in genetics section indicates, the progress of science as it looks at the processes of life, has, in a sense, spanned from the “macro” (the large) to the “micro,” (the small). While Darwin hoped to understand how creatures evolved over expanses of time—millennia, eons, large chunks of time involving large populations in a kind of huge laboratory – his assertion of “survival of the fittest” caused scientists to rethink how organisms evolve, even on a molecular and scientific level. Evolution involves the selection of traits favorable to survival and reproduction, Darwin hypothesized. Those organisms in possession of favorable traits for survival will be most likely to pass these traits on to their offspring, who will in turn pass these traits on to future generations on down the line. Studies in genetics focus on how these “traits” get passed on from parent to offspring.

Inheritance and Gene Function

Children often have characteristics reminiscent of their parents. Eye traits (color, shape, overall appearance) lead us to comment that infants “have their mother’s (or father’s) eyes.” Appearance traits like hair type, mannerisms, and, later in life, height and weight, are often like those of our parents. We are said to inherit such traits from our parents. How does this process of inheritance occur?

Gregor Mendel
1822-1884

In the 1860’s, science began to study the mechanisms of inheritance in earnest. Precise observations were made about how traits are passed on from generation to generation by an Augustinian monk named GregorMendel. With experiments on pea plants, Mendel noted the manner in which characteristics of coloring, seed appearance, and stem length were developed and passed on from generation to generation. Out of these experiments came the “Mendelian Laws of Inheritance,” which were soon applied to human traits as well. These laws state that each parent contributes equally to the inheritance of the child; for each characteristic, each parent contributes one factor. One parent might contribute a factor that produces blue eyes, while the other parent might contribute a brown-eyed factor. One factor might outweigh the other, meanwhile. A person who had inherited a factor for brown eyes and one for blue eyes would be brown-eyed, because the brown-eyed factor has dominance over the blue-eyed factor. These factors have persistence, however, and the recessive blue-eyed trait might appear in future offspring.

These factors and traits came to be known, in the early 20th century, as genes, from a Greek word meaning “to give birth to.” The science of dealing with the manner in which genes are inherited and in which the characteristics are displayed is genetics. This science has come to mean more than the study of visible traits, however. The complexities of inheritance in complex organisms, which can involve multiplicities of genes working in cooperation and can even be affected by environmental conditions, require close examination of molecular mechanics.

So we need to look closely at cellular processes. During the process of cell division, for instance, material in the nucleus of cells collects into pairs of threadlike bodies called chromosomes. At the moment just before cell division (mitosis, from a Greek word for “thread”,) pairs of chromosomes pull apart. Yet chromosomes are conserved in cell division. These chromosomes consist of strings of genes which encode the traits mentioned above. All of the characteristics of an organism are the visible expression of genes, mostly in the form of proteins that produce cellular function. Understanding chromosome function requires an understanding of proteins, which are key ingredients of a cell. Proteins help determine shape, size, and activity of a cell, while genes provide the code to order proteins. A given gene typically encodes for a single protein. The relationship between genes, chromosomes (the pairs of genes,) and proteins is central to the study of inheritance.

Protein Function

Indeed, genetic function can, in part, be said to be the function of proteins. Let’s look more closely at proteins: proteins are long, unbraided molecular chains. They are built of amino acids, molecules which consist of a central carbon atom attached by one bond to an amine group and by a second bond to a carboxylic acid group (see illus.) There are twenty-two common amino acids. A string of amino acids are joined together in proteins by peptide bonds (see next illustration), and there is no limit to the number of amino acids that can be put together by peptide bonds. Polypeptides are large peptides made up of numerous amino acids. The order of the hundreds or thousands of amino acids in a protein chain determines the protein’s function. This order will also determine the protein’s shape, which is important to that protein’s function.

And what of this function? Our cells build about sixty thousand different types of proteins, each with a different function (Goodsell 19). Enzymes are protein catalysts which bring about very specific reactions, those involving only one or a few closely related compounds. Structural proteins bind to adjacent molecules, and, when stacked end to end in groupings, form the strong girders that support and move cells (Goodsell 18). Additio nally, proteins can function as carriers of other molecules, and, as in the case of hormones that are proteins, they can serve as messengers.