Back To CourseHigh School Biology: Help and Review
36 chapters | 570 lessons
Jeff is a Biology teacher and has a Doctorate in Educational Leadership
Did you know that you share many similarities with monkeys, fruit flies, ostriches, great white sharks, and even worms and bacteria? At first glance it may not appear as if these creatures have anything in common. However, if you examine their molecular components, you will see that all of these organisms are actually distant cousins of each other.
In 1953, there was a race within the scientific community to discover the structure of the DNA molecule. At this point, it was known that DNA carried the genetic code, or all the instructions a cell needs to make proteins and enzymes, but it was unclear what it looked like. Knowing what this genetic molecule looked like would provide much insight into how it worked because in biology, structure and function are very closely related. While many researchers around the world were investigating, the discovery was made by a team from King's College in London. Englishman Francis Crick and American James Watson, along with the assistance of Rosalind Franklin and Maurice Wilkins, figured out what the DNA molecule looked like by using, surprisingly enough, a model building kit!
The story goes that Rosalind Franklin, who was an x-ray crystallographer, had taken a picture of the DNA molecule, but didn't know what it was. One day, Watson happened to be in her lab and stumbled across the picture. He knew right away exactly what he was looking at. He returned to his lab and he and Crick started to put together a 3-D model that showed what DNA looked like. This structure came to be known as a double helix.
The double helix looks like a ladder that has been twisted at both ends. It is made of three component parts; a sugar, phosphate groups, and nitrogen bases. The sugar that makes up the backbone of the DNA molecule is called deoxyribose. It is a 5-carbon sugar that is interspersed with the phosphate groups, forming a repeating pattern. A phosphate group is a functional group (a compound that gives a carbon chain or ring a particular function) made up of phosphorous attached to four oxygen molecules.
The nitrogen bases are nitrogen bearing compounds, and those found in the DNA molecule are called adenine, guanine, thymine, and cytosine (A, G, T, and C for short). Adenine and guanine are called purines because they have a double carbon ring. Cytosine and thymine are called pyrimidines and have a single carbon ring (see Figure 1). When combined, each of these three parts makes a structure called a nucleotide, which is the monomer, or component part, of all nucleic acids. The sugar and phosphate groups make up the 'backbone' of the ladder and the 'rungs' are where the nitrogen bases are located.
Prior to Watson and Crick's discovery of the structure of the DNA molecule, another scientist named Erwin Chargraff determined that the amount of adenine was equal to the amount of thymine and that the amount of cytosine was equal to the amount of guanine. Called Chargraff's rule, this relationship allows for what is called complementary base pairing. Due to the shape of each of these nitrogen bases, adenine can only pair with thymine and cytosine can only pair with guanine.
To show Chargraff's rules, consider the following random sequence of nitrogen bases from a DNA strand:
Since adenine has to bond with thymine and cytosine has to bond with guanine, the complementary strand, or the other half of this piece of DNA would be:
Figure 2 shows the twisted ladder structure of the double helix and how the nitrogen bases align with each other. The phosphate groups link the deoxyribose molecules together and the strands are held together with hydrogen bonds. Notice that the strands run in opposite directions. One end of each strand is called the 3' (Three Prime) end and the other end is called the 5' (Five Prime). This is important to consider when the DNA copies itself during cell division.
DNA is referred to as the genetic molecule. A gene is a segment of DNA that is responsible for the physical characteristics of an organism. This means that DNA carries all the instructions needed to make up all the genes, or genome, and in turn, produce all the traits of an organism. Genes make up about two percent of the total DNA in a person's genome. It is the sequence of the nitrogen bases that ultimately produces the traits you express, or your phenotype. For example, brown hair, blue eyes, and Alzheimer's disease are all traits caused by genes. While there are millions of base pairs in your DNA, not all of them will show traits. It turns out that those genes that show traits, called exons, are interspersed with non-showing sequences called introns. During the creation of proteins (the process that is needed to show your traits), the introns are removed so that the exons can be activated to show the traits.
As technology has advanced, more and more studies into the structure of the DNA molecule have been completed. Because of these studies, it is now known that humans and chimpanzees share 99% of their DNA. This means that it is only a 1% difference in the genetic code that accounts for all the differences between you and a chimp! We humans have 99.9% the same DNA. Only 0.01% makes you different from me, from your brother, from your best friend, or from anyone else on the planet. That 0.01% also is what determines if you are going to develop Alzheimer's disease when you get older, or possibly develop cancer, or any number of other genetic diseases.
In 2003, a massive, 13-year investigation to map every gene in the human organism was completed by the U.S. Department of Energy and the National Institutes of Health. Called the Human Genome Project, this study found the location of all 20-25,000 genes found in human DNA and discovered that the average gene contains 3,000 base pairs. However, this number varies greatly from gene to gene. There also seems to be more C and G found in the genome than A or T.
Scientists started this project by mapping the DNA of many non-human organisms in order to perfect their techniques and get a sense of how the human genome could be organized. Fruit flies, worms, and many bacteria have all had their genomes mapped because of this effort. In comparison to the human genome, many of these organisms have more organized patterns of genes on their DNA than humans do, with their genes spaced evenly throughout. In addition, it was found that while there are many similarities between human and other organisms' genomes, humans have three times as many kinds of proteins, but more than 40% of them are shared with fruit flies and worms!
In the 10 years since the Human Genome Project was completed, there have been many advancements in science and medicine. Prior to this, many diseases had to be studied one gene at a time. This was a very labor-intensive process. Now that the entire genetic code is known, it is possible to study multiple genes at once, which provides more accurate findings about how the diseases can infect people, how they reproduce, and how they use proteins. This new knowledge has allowed for gene sequences associated with breast cancer, muscle disease, deafness, and blindness to be identified.
DNA is the genetic molecule of living things. It provides the blueprint for all of the traits an organism expresses over its lifetime. Since its structure was discovered in 1953, there has been a tremendous effort to investigate what it does and how it behaves. The knowledge obtained from the Human Genome Project has now been used around the world in providing applications in the fields of medicine, evolution, agriculture, and ecology.
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Back To CourseHigh School Biology: Help and Review
36 chapters | 570 lessons