A Basic Understanding of the Gene
By: Doris Ehret
Today, the word genetics has been spoken by nearly everyone with a serious interest in the breeding of dogs. This is an easy word for us to say, but only recently in the course of history did this word even exist. Even now, researchers around the world are still attempting to better understand the complexities of heredity. Since this is a relatively new science, it is expected that few people have a clear understanding of its essential basics. The following explanations and descriptions are offered with the hope of providing some basic knowledge about genes: what they are made of and how they work. The principles of heredity will be touched on only briefly.
Each cell in the body is jam-packed with various structures, one of which is the nucleus. Inside the nucleus are structures called chromosomes. In the dog, each cell's nucleus contains 78 chromosomes (39 pairs). Together, they contain all the genetic material necessary to reproduce a dog, cell by cell. There is an exception to this rule regarding the cells of reproduction, the sperm and ova. Each of these cells contains only half the full complement of chromosomes; instead of 78, they each have 39. When these two cells combine, the resultant cell (the fertilized ovum) contains the full set of 78.
At this point, you need to be aware that in a non-reproductive cell, the two chromosomes that make up a pair differ from one another in their genetic content. These pairs split apart during the formation of the reproductive cells. Therefore, a dog's sperm will contain one or the other of these chromosomes and thus the genetic information will vary even among the sperm of one dog. This is also true of ova.
When a sperm and ovum unite under the simplest of circumstances, there are four possible combinations of chromosomes possible. This can easily be understood if we arbitrarily assign a separate design to each of the chromosomes.
As you can see, each mating relies on a certain amount of chance as to which way the chromosomes from the dam and sire match up. Therefore, even though dog breeding is becoming more technology-reliant, the element of luck still remains.
So what exactly is DNA? It is a very long molecule. Until the early 1950's, it was a nebulous concept in the minds of a few scientists. Then Francis Crick and James Watson, with the work of other scientists, produced a 3-dimensional model of DNA. They had little to go on, other than a knowledge of physics and chemistry and the extraordinary "x-ray" of DNA that a fellow scientist, Rosalind Franklin, was able to produce.
Time out for basics. A few definitions:
ATOM: The smallest unit of a chemical element. Consists of a central nucleus which has a positive charge and surrounding electrons holding negative charges. These charges produce electrostatic and electromagnetic forces which hold the components together.
MOLECULE: A group of atoms held together by electrostatic and electromagnetic forces.
DNA: A very long "super" molecule made up of smaller units called nucleotides.
NUCLEOTIDE: A molecule composed of a sugar, a nitrogen base and phosphoric acid.
Example of a nucleotide:
For those of you who have forgotten their chemistry or never had the opportunity to learn it, don't pass this diagram by! It is important because it illustrates one of the four different nucleotides that make up the vast array of genetic information known as DNA. Each of those little letters and lines has significant meaning. First of all, each letter represents an atom: O = oxygen, N = nitrogen, C = carbon, H = hydrogen, P = phosphate. Each of the lines between them represents a chemical bond, the force of attraction that "hooks" atoms together to form a molecule. Chemicals exist because of the unique ways that atoms "hook" together. In this instance, there are actually three chemicals (a sugar, a nitrogen base, and a phosphate) which have combined to form a molecule called a nucleotide. When Watson and Crick were working to reproduce DNA in a 3-dimensional form, they spent most of their time getting the atoms and molecules to link up properly. The laws of physics and chemistry required that the atoms could be combined only in one certain way in order to produce chemical bonds. The Watson-Crick team took what existed only as theory and created a tinker toy-like model to prove that DNA is a physical entity.
You've seen a diagram of one nucleotide. DNA is actually composed of 4 different nucleotides and each has its own name and structure. There are several names for each of these, but we'll just call them A, G, C or T. They are:
A: 2' deoxyadenosine 5'-triphosphate. The base is adenosine. Also called dATP.
G: 2' deoxyguanosine 5'-triphosphate. The base is guanosine. Also called dGTP.
C: 2' deoxycytidine 5'-triphosphate. The base is cytidine. Also called dCTP.
T: 2' deoxythymidine 5'-triphosphate. The base is thymidine. Also called dTTP.
It is the sequence of these 4 nucleotides as they link up with one another that is responsible for all the genetic information contained in one organism. To visualize this, picture a string of beads. Imagine that each of the beads represents one of the nitrogen bases in a nucleotide. The sugar and phosphate portion of the nucleotide make up the "string" that holds the beads (nitrogen bases) together. In reality, the sugar and phosphate are on the "outside" and the nucleotides are on the "inside". This simple schematic drawing represents DNA as two such strands of beads:
Now picture the two strands coming together so that the two sets of nitrogen bases meet in the middle. They "hook up" to one another by way of a hydrogen bond (H) between them:
Finally, imagine the two strands now linked in the middle as one big strand. Then twist it. You'll get what is called a double helix structure. This is what DNA looks like. The length of the strands can be several million nucleotides long! Between each twist in the strand will be ten nucleotides. The way the nucleotides face one another in this big strand follows a very specific order. "A" can pair only with "T" and vice versa. "G" can pair only with "C" and vice versa.
Now, let's untwist the strand and lay it horizontal to see how this pairing process between the nucleotides works:
Remember, DNA is a very long molecule, a string of nucleotides located on a chromosome. There will be one specific DNA molecule per chromosome.
Now that we know what DNA looks like, we can then ask, what is a gene? A gene is simply a segment of the DNA molecule. There may be hundreds to thousands of genes on one DNA molecule, each with its own sequence and number of nucleotides. Not all DNA is made of genes, however. There are also non-genetic sequences of nucleotides which exist between the genes. Also, remember that we're actually dealing with two strands of linked nucleotides. Different genes can be located on either strand:
Each gene codes for its own specific bit of information. Some genes work independently to create a genetic factor, and others work together to code for a common trait. These polygenic genes may or may not be near each other on the DNA molecule and they may even exist on different chromosomes. This may explain why the study of polygenic traits is so difficult.
How does a gene work or express itself? Several processes are involved, the first of which is transcription. During transcription, the twisted strands unravel and separate and a matching pattern of nucleotides is made from one of the strands which acts as a template. This becomes a single strand molecule called RNA (ribonucleic acid). There are several types of RNA, one of which is mRNA (messenger RNA). The mRNA leaves the cell nucleus and travels to ribosomes which exist in the cell's cytoplasm. When the RNA leaves the nucleus DNA strands resume their original double helix structure.
Now, a process called translation occurs at the site of the ribosomes. This involves the translation of the RNA nucleotides into amino acids. The amino acids will eventually become a protein. Proteins are also very large molecules. Just as the basic sub-unit of DNA is a nucleotide, a protein also consists of a basic sub-unit called an amino acid. The specific sequence of nucleotides on an RNA molecule translates into a specific protein composed of amino acids. A genetic code has been determined to decipher which nucleotides translate into which amino acids. Take the 4 nucleotides, A, T, C and G, and arrange them in as many combinations of 3 that you can. You'll get 64 possible combinations. Each of these 64 triplets (or codons) can be translated into one of the 20 amino acids that exist in the dog. This doesn't make sense until you know that several triplets can code for the same amino acid. For example, the amino acid alanine may result from the combination of GAU, GAC, GAA or GAG. Now you ask, where did the "U" come from? The nucleotide "T"/Thymidine becomes "U"/ Uracil during the earlier process of transcription.
As amino acids are formed, they link up in various ways to produce proteins. This creation of proteins is the end result of genetic expression. GENES EXIST TO PRODUCE PROTEINS. Proteins serve a myriad of functions. They form tendons, bones and cartilage; they allow for muscle contraction; they act as enzymes responsible for all biochemical reactions; they act to move other molecules about the body; they regulate cellular metabolism; they store molecules for future use and they protect us from infection. They also control the processes of transcription and translation. In short, a living organism develops and functions only because this vast array of proteins exists courtesy of our genes. It takes only one mistake in the process of transcribing one nucleotide from a gene on the DNA molecule to the RNA molecule to create a problem with a protein. The result might be inconsequential or it might be significant, or even devastating.
This simple explanation of the gene should not mislead you into thinking that genes are simple entities. The complexities of genes and their expression have not been touched on in this short discourse. The complexities of nature are what keep researchers in pursuit of further knowledge. The mapping of the canine genome to discern exactly where certain specific genes reside on the chromosomes is an ongoing project. The hope for dog breeders is that tests will be developed to determine which genes are faulty in their dogs. Treatment in the future may be directed at correcting these genes so that during the process of transcription and translation, genetic faults will not be passed on from generation to generation.
Brown, T. A. Genetics: a molecular approach, 2nd ed. London: Chapman & Hall; 1992; 12-13, 26-39, 44-45, 52-73,76.
Watson, J. D. The double helix. New York: Penguin Books; 1968.
Willis, M. B. Practical genetics for dog breeders. New York: Howell House, 1992; 22-24.