DNA

Authors Note: I chose to write about the topic of DNA. The story behind this is that a few weeks ago I decided to write a report on the double helix. That's when I found out how hard it is to write a report about a shape and thought that a report on DNA would be simpler. I was right. My biggest focus when writing this was transitions.

DNA is, without a doubt, the most amazing of natures creations. Each DNA molecule contains the information and instruction on how to make an organism, including you. DNA is responsible for the characteristics of an organism from appearance to the way someone behaves. Even aging has been programmed into our DNA. Without our DNA, we probably wouldn’t even be here. There’s a lot to know about DNA. What better place to begin than the beginning?

DNA was first discovered in 1869 by Friedrich Miescher in Switzerland. When he discovered it in White blood cells he gave it the name “nuclein”. However, the real beginning of this study began in 1865 when Gregor Mendel conducted his pea plant experiments in heredity leading to Mendel laws of heredity. These laws are the law of segregation, the law of independent assortment and the law of dominance. While not the actual discovery of DNA, Mendel’s research was the beginning of the study of genetics. Unfortunately, Mendel’s research was not accepted until the early 1900s when it was rediscovered by Carl Correns, Hugo de Vries and Erich von Tschermak-Seysenegg. Many new discoveries followed, but it wasn’t until 1951, almost 80 years after Miescher’s discovery, that anyone actually wondered what DNA looked like.

These people were Rosalind Franklin, Francis Crick and James Watson. Franklin used a technique called x-ray diffraction to take a photo of a DNA molecule. The photo suggested that DNA had a helical shape. After seeing Franklins photo, Watson and Crick created a three dimentional model of a DNA double helix. A double helix is, as its name suggests, a shape made up of two helices curving around the same axis. The DNA double helix has been described by many to have an appearance similar to that of a ladder, two strands connected by many “rungs”. It has been observed that many cells and molecules in our body tend to take on a helical form. However, DNA is the only double helix. Why does DNA take on its twisted form?

The answer might lie in its makeup. DNA is made of things called nucleotides. Each nucleotide is made of phosphate, sugar and a base. There are four bases in DNA: adenine, thymine, guanine and cytosine. The sugar and phosphate make up the two strands of a DNA molecule and the rungs are made of two bases each. A base will only connect to a complementary base. The complement of adenine is thymine and guanines complement is cytosine. The double helical shape of a DNA molecule might have something to do with how these components react to water. Sugar and phosphate are hydrophilic and attract water. The bases, however, are hydrophobic and are repelled by water. This is a problem as 80% of the human body is water. The shape of DNA might be designed to keep water away from the hydrophobic bases by only exposing the hydrophilic strands. What is the point of the bases though? Why are they needed?

They are needed because they make up the genetic code. A gene is a long string of nucleotides with information about creating a specific trait or characteristic. The human body contains a lot of DNA, which allows for many genes and many characteristics. The bases are arranged in groups of three. These groups are called codons. Inside the core of a cell, called the nucleus, one side, and it doesn’t matter which, of the DNA double helix is copied by mRNA or messenger RNA. Once the copy is made, the mRNA leaves the nucleus and goes to a part of the cell called the cytoplasm. In the cytoplasm is a cell subunit called the ribosome. The mRNA is fed through the ribosome one codon at a time. Inside the ribosome, tRNA, or transport RNA, carrying an amino acid from the cytoplasm, matches the base code on the mRNA with the complementary codon. The tRNA then releases the corresponding amino acid into a growing “chain” of amino acids. Once this chain grows big enough, it becomes a new protein molecule. RNA is also used in a different way.

It is used in the process of DNA replication. DNA replicates its self when a cell divides. To begin the process, an enzyme called a helicase. The helicase splits the DNA molecule in half by breaking the hydrogen bonds between the bases and creates a structure called a replication fork. Both strands are coated with Single Strand DNA Binding protein or SSB. This keeps the strands from rejoining. In a DNA molecule, each of the two strands has a 3’ and 5’, or three prime and five prime side and a DNA strand always moves in what’s called the 5’ to 3’ direction. The 5’ side of one strand of a DNA molecule is in the opposite direction than the 5’ of the other strand. In other words the strands are opposite of each other. DNA replication can only happen in the 5’ to 3’ direction and can’t start from nothing. It needs a starting point with an open 3’ end. This is where RNA comes in. RNA makes what’s called a primer. This primer creates a starting point for something called a DNA polymerase. A DNA polymerase is responsible for the creation of new DNA. In the process of replication each strand has a name. The strand moving away from the replication for is called the leading strand. The one moving into the replication fork is called the lagging strand. The leading strand needs only one RNA primer, but the lagging strand needs multiple primers. On the lagging strand the DNA polymerase will only create make a strand from one primer to the next, but won’t connect them, resulting in many fragments of a strand instead on one while one. These are called Okazaki fragments. After a RNA primer has been used, it must be removed. This task is done by an RNase H. Then the polymerase fills in the gaps where the primers used to be. Then a DNA ligase joins the Okazaki fragments. The end result is two new DNA molecules. But what would happen if a mistake was made in the replication process?

The most probable outcome would be a genetic mutation. This is caused by a change in the genetic code. That means something changed in the arrangement of the bases. The most common forms of mutation are the addition, removal or substitution of a base. the effects of mutation could be harmful or they can improve a trait of a person. They can even do nothing at all. Usually only one base is affected, but this one change can cause conditions like a disease called sickle cell disease. This condition results from a mutation that changes the codon for Glutamic amino acid to the codon for valine. This causes the shape of red blood cells to change. These new cells don’t carry oxygen as well as normal cells and are more prone to clotting. Mistakes don’t just occur in the replication process. They happen when mRNA copies a DNA strand or when DNA repairs its self when damaged and it can even happen as a result of external sources.

An external physical or chemical agent that causes mutation is called a mutagen. Examples of mutagens are UV radiation and x-rays.

The study of genetics has come a long way since Mendel’s pea plants. Things like genetic engineering and using DNA to track criminals are some of the possibilities. These kinds of things would’ve been thought impossible in Mendel’s time. Imagine what the research and study of DNA will accomplish in the future.