The Behavior of Genes on Chromosomes
After Mendel's work was rediscovered in 1900, the next great milestone came with the research of Thomas Hunt Morgan, one of the foremost American zoologists at the time. Working with fruit flies of the genus Drosophila, Morgan and his colleagues showed that factors called genes, located on recognizable cellular structures called chromosomes, were the units of inheritance that could be passed from generation to generation. For this and related discoveries, Morgan was awarded the Nobel Prize for Medicine in 1933.

DNA is the Genetic Material
While Morgan was breeding fruit flies, a series of three experiments was beginning, that would eventually show DNA to be the actual molecule that carries genetic information. The first was done by Frederick Griffith, and it showed that a cellular substance could be transmitted from one organism to another, giving the recipient organism traits of the donor, a phenomenon that came to be called transformation. The second experiment was performed in 1944 by Oswald Avery, Colin McLeod, and Maclyn McCarty. These researchers separated the different classes of cellular molecules to see which one was responsible for transformation. And they found the molecule to be deoxyribonucleic acid, or DNA. However, because the biological community was expecting for the transforming molecule to be a protein, and because the nation was preoccupied with war at the time, no one believed it. It wasn't until an experiment by Alfred Hershey and Martha Chase in 1952 that the world finally accepted that DNA was the carrier of genetic information.

Discovering the Structure of DNA
The elusive hereditary molecule had finally been found. Very little was understood about it, though; no one even knew what DNA looked like. But that was about to change. A technique called x-ray crystallography had been developed to determine the molecular structures of proteins. A chemist named Rosalind Franklin, with her colleague Maurice Wilkins, applied this technique to a DNA molecule. The resulting photograph suggested that the DNA molecule had a double-helical structure. Two scientists named James Watson and Francis Crick put the findings of Franklin and Wilkins together with the work of some earlier scientists, and reasoned out a model for the complete molecular structure of DNA and its mechanism for carrying genetic information. Franklin, Wilkins, Watson, and Crick were all important contributors to this discovery; unfortunately Franklin died of cancer before she could be recognized for her work. Watson, Crick, and Wilkins shared the Nobel Prize in 1962.

Timeline
Here is a timeline of genetics from the 1800s to the present, describing the experiments that led up the discovery of DNA, and the events, developments, and discoveries that have followed after.

The Cell Cycle
Every living creature has a life cycle, even something as small as a cell. A cell is born, carries out the metabolic processes for which it is specialized (during a phase called Gap 1), replicates its genetic information (during a phase called Synthesis), prepares to divide in two (during a phase called Gap 2 ), and then divides (during a phase called Mitosis). When one old cell divides, two newborn cells result. Because the old cell replicated its DNA, it has two copies of all of its DNA at the time it divides. One copy goes to each of the new cells. Thereby more cells are created but no genetic information is lost. And the cell cycle continues anew.

The Structure of a DNA Strand
So at a molecular level, what is actually happening to the DNA during all of this? The first thing to understand is the structure of DNA, as discovered by Watson and Crick. DNA is a polymer, meaning it is a molecule made by linking together thousands of small molecules called monomers. The monomers for DNA are called nucleotides. There are four types of nucleotides in DNA, abbreviated A, T, C, and G. The linked nucleotides pair up with each other; A always with T and C always with G. The long string of paired nucleotides twists around, very precisely, into the shape of a double helix. This is a strand of DNA.

DNA Replication
The ability of DNA to replicate itself is crucial to its role as the carrier of genetic information. The DNA strand can't replicate all on its own, though - it has some help. Proteins work together to "unzip" the DNA helix, meaning the nucleotide pairs are separated. A protein called DNA polymerase then takes new nucleotides and pairs them to the unpaired ones on each side of the "zipper." Again, A is always paired to T and C always to G. In this way the sequence of nucleotides in the DNA remains intact. When the strand has been completely replicated, there are two new double-stranded helices of DNA. The process is called "semi-conservative," because one half of the old DNA strand is incorporated into each new strand. Here, DNA Replication is described in more detail. And here is a diagram of the process to go along with the explanation.

DNA to RNA Transcription
If DNA were simply to sit around in the nucleus of the cell and replicate, it wouldn't do any good. The information encoded in the DNA must be used by the cell to make proteins. This requires a couple of steps. First, the information coding for the desired protein must be transferred from the DNA to a new molecule, one capable of traveling out of the nucleus to the protein-making machines (called ribosomes ) of the cell. This process is called Transcription, and the new molecule that is made is called mRNA. Transcription is very similar to replication. It begins the same way, except that instead of unzipping the helix completely, only the region of the desired gene unzips. An enzyme called RNA polymerase adds slightly different nucleotides to the open area of the DNA strand, but the sequence is still kept intact. Once the gene has been completely transcribed, the mRNA strand detaches and the DNA helix recloses. The mRNA strand then leaves the nucleus and heads for the ribosomes.

RNA to Protein Translation
Finally, the mRNA must be used to make a protein. Proteins, like DNA strands, are polymers. The monomers for proteins are called amino acids . Amino acids must be linked together in the proper sequence for the protein to perform its specific function. The information contained in the DNA and carried by the mRNA codes for that amino acid sequence. Every protein in the body has a specific gene in the DNA that contains the code for its exact sequence.
So how does the protein get made? The mRNA strand travels to a ribosome, where the mRNA is "read" and interpreted into the proper sequence of amino acids. Each series of three nucleotides in the mRNA is called a codon , and each codon represents a particular amino acid. Another type of RNA, called tRNA recognizes each codon and responds to it by adding the correct amino acid to the chain. This process is called Translation. There are 20 different amino acids that make up all of the proteins in nearly all living organisms. And the genetic code is practically universal among all these organisms, meaning that each amino acid is represented by the same codon, in nearly every creature on the planet.

DNA and Reproduction
Every cell of your body goes through the same cell cycle, except for the sex cells, or gametes - eggs and sperm. Instead of dividing by mitosis, these cells divide by a special process called meiosis, which yields new cells with only half of the genetic information of the old cell. This is because the purpose of eggs and sperm is to combine with a gamete from the opposite gender to create a new individual with the correct total amount of DNA. Errors during meiosis can cause several genetic disorders in the new child; most meiotic errors can be detected by a test called a karyotype. Here, Meiosis and its effects are explained in greater detail.

Genetic Engineering
Believe it or not, humans have been manipulating genes since prehistoric times. As early as 10,000 years ago, people selectively bred wild animals to produce livestock with the traits most beneficial to mankind. But recently, we've discovered a new way to manipulate genes. Genetic Engineering is a fundamental technique in much of biotechnology. Genetic Engineering simply means taking DNA from one organism's genome and inserting it into that of another, making the second organism transgenic. Genetic engineering has provided many benefits for mankind since it's invention, and also sparked many concerns. In a society changing so rapidly, only time can tell the role it will play in the future of human science.

Genetics in Agriculture
Agriculture has been one of the fields most influenced by biotechnololgy. Genetic engineering has been used to give fruits and vegetables longer shelf lives; to enable crops to resist disease, drought, and pesticides; to increase the vitamin content of foods such as rice, that are staples in much of the world. Agricultural Biotechnology has the potential to solve many problems for mankind. But will it present more problems in their place? Here is an extensive list of the pros and cons of genetically engineering crop plants.

Gene Therapy
One of the most exciting possibilities that DNA technology holds is the potential for gene therapy. Diseases that for thouands of years have been largely untreatable - such as cystic fibrosis, Tay-Sachs disease, sickle-cell anemia, and some cancers - may all someday soon have cures. Gene Therapy also has risks and is still largely experimental. Hopefully in the coming years it will become better understood, safter, and applicable to an even broader spectrum of diseases.

Cloning
Cloning. For years it's been a thing of science fiction. Then in 1996, Drs. Ian Wilmut and Keith Campbell of the Roslin Institute in Scotland produced the first successfully cloned animal, a sheep named Dolly. And cloning has been surrounded by controversy from then on. But what does cloning really mean? What does it entail? What are its advantages and risks? What philosophical questions does it pose for us to answer? What standpoints do the world's major religious groups take on cloning? It's still to early to tell whether cloning's ultimate role in our world will be one of science fiction or science fact.

The Human Genome Project
In 1990 a massive project began between the U.S. Department of Energy and the National Institutes of Health with the goal of sequencing the entire human genome. It was an ambitious undertaking, especially since at the time not even the simplest bacterial genome had been completely sequenced. But the Human Genome Project has been a huge success, achieving most of its goals ahead of schedule. In 2001 a rough draft of the human genome was published, and the complete sequence is expected to be known by the spring of 2003. The above link is to the Department of Energy's Human Genome Project site - it has excellent information not only about the HGP but also about genetic processes, technologies, ethics, and news.

Bioinformatics
Rapidly improving computer technology was one of the main reasons why the human genome was sequenced so quickly. Advances in computer science combined with advances in biology have led the the formation of a whole new field of science: Bioinformatics. Bioinformatics uses computational tools and algorithms to predict the sequence of complex biological molecules like DNA and proteins. It also creates databases to store the massive amounts of information which research uncovers, and to search these databases for genes or patterns. Bioinformatics is an invalubale science for this, and, like much of biotechnology, it is still growing.

Forensics
Another field that's been heavily impacted by biotechology is forensic science. DNA Fingerprinting has been key in a number of famous criminal trials, and is becoming ever more valuable in solving cases of violent crime and sexual assault. DNA testing has also led to exonerations of people convicted for crimes, and in almost every state Innocence Projects have been organized for this purpose. And forensic DNA analysis has many applications beyond criminal trials as well; paternity questions, immigration disputes, and identification of bodies that time or violence have left beyond visual recognition have all been solved with DNA testing. The Forensics page of the government Human Genome Project website describes forensic testing in more detail and lists some interesting cases involving forensic DNA analysis.

Phylogeny
One more area of science influenced by DNA is the study of evolution. Comparing the DNA sequences of different species can help to determine how closely they are related. It can tell us how organisms should be classified (Taxonomy), and the pattern in which organisms are descended from each other (Phylogeny). Evolution is yet another field in which the use of biotechnology is rapidly growing. Here is an amazing Phylogeny website - it is still under construction and will be for some time, but already describes the evolutionary relationships of a huge number of species of life on earth, in a format that is easy to follow and fun to read.

Mendel's Peas
What did Mendel really see, over a hundred years ago, that led him to formulate his laws of inheritance? This web activity on Mendelian Inheritance lets you experiment with his plants yourself...instananeously, of course. The activity requires the Shockwave plugin but you can download it (free) from the site.

Extract DNA at Home
This experiment is molecular biology made simple! It describes a procedure for Extracting DNA from common household items using common household chemicals and equipment. The explanations along the way as to why each step works are helpful and easy to understand, too.

Translate Your Own Proteins
You've probably heard of Babel Fish, an internet site that translates text from one language to another for you. This Translation Tool is the Babel Fish of the protein world. If you are a molecular biologist who has sequenced a gene and you'd like to see what the protein it encodes looks like, you can enter the gene sequence and get the translation. Or if you are simply an interested student of DNA, you can make up a DNA or RNA sequence (Remember, the DNA code is made up of the letters A, T, C, and G. RNA uses the letters A, U, C, and G.), and type it in, and the site translates your sequence into the amino acids of a protein. It displays the sequence using the one-letter abbreviations for the amino acids; here is a list of amino acid abbreviations so that you can see the full names of the amino acids in your protein.

Clones 'R' Us
I first encountered this site via the "Cloning Myths" page and was pretty shocked by it...until I got to the homepage and realized that this is a spoof site. The company Dream Technologies International does not really exist and everything on the website is complete fiction (if you don't believe me read the fine print of their disclaimer from the section titled "Important Legal Information"). But nonetheless, it's very thought-provoking about the roles that reproductive cloning could possibly play in our future. And some sections of the site should give you a good laugh, too.

DNA Jokes
Several pages worth of Jokes about DNA. Warning: you may have to be a hard core biology geek to appreciate some of the humor here.

Dolly's Cloning Emporium
And finally, Dolly's Cloning Emporium is full of clever and funny DNA links. This site is just plain fun.

This website was created by Melanie Ktorides at the University of Connecticut, Spring 2003. Feel free to email me with any questions or comments!