WHAT IS DNA?
DNA is a molecule that has a repeating chain of identical five-carbon sugars (polymers) linked together from head to tail. It is composed of four ring shaped organic bases (nucleotides) which are Adenine (A), Guanine (G), Cytosine (C) and Thymine (T). It has a double helix shape and contains the sugar component deoxyribose.
THE PROCESS OF DNA REPLICATION
How DNA replicates is quite a simple process. First, a DNA molecule is "unzipped". In other words, it splits into two strands of DNA at one end of the DNA molecule. This separation will cause a formation of a replication fork. After the replication fork has been established the strands of DNA are ready for the next stage. On each strand is a sequence of nucleotides. These nucleotides act as a template for complementary nucleotides to bind. Hence, it is the site where the synthesis of a new complementary strand will be formed. Because of the DNA "unzipping", there will be two single strands of DNA. Hence, because there is two single strands of DNA, there will be two new daughter strands synthesized. However, each of these daughter cells is synthesized in different ways. The first strand of DNA is built by simply adding nucleotides to its end. This strand grows inward towards the replication fork as the DNA molecule unzips. This strand ends with a hydroxide (OH) group and is called the 3` prime or 3`end. The enzyme that catalyzes this process is called DNA polymerase. The second strand is built by having a polymerase jump ahead on the strand and fill in the complementary nucleotides backwards. This strand moves in the outward direction, hence away from the replication fork. The DNA polymerase for this strand starts a burst of synthesis at the point of the replication fork. The addition of nucleotides to the 3` end of a short new chain until this new segment fills in a gap of 1000 to 2000 nucleotides between the replication fork and the end of the growing chain to which the previous segment was added. Hence, this new short chain is then added to the growing chain, and the polymerase jumps ahead again to fill in another gap. Thus in short, the polymerase copies the template strand in segments about 1000 nucleotides long and stitches each new fragment to the end of the growing chain. This process of replication is referred to as discontinuous synthesis.
RELATIONSHIPS OF THE NITROGENOUS BASES
In the late 1940's, a scientist from Columbia University by the name of Erwin Chagraff discovered a few relationships between the nitrogenous bases.
The first discovery Chagraff made was that the amount of adenine present in all DNA molecules is equal to the amount of thymine. The second discovery Chagraff made was that the amount of guanine was equal to the amount of cytosine. The third discovery Chagraff made was that the amount of adenine plus thymine often differs greatly from the amount of guanine plus cytosine.
HOW THE IMAGE OF DNA WAS FIRST DISCOVERED
After Chagraff's discoveries, two British scientists by the name Rosalind Franklin and Maurice Wilkens were able to apply his observations. Thus, these two scientists were the first ones to discover the image of what a DNA molecule actually looked like in three-dimension.
How these scientists manage to obtain the image of a DNA molecule was by the use of "x-ray crystallographic analysis". In this process, DNA molecules are bombarded with a x-ray beam. These x-rays encounter atoms, which in turn causes their paths to bent or diffract. The pattern created by the sum of total of these diffractions are then captured on a photographic film. The pattern is then interpreted into the image of the molecule through careful analysis. Thus, because of this research it led to the first theory and model structure of DNA.
Maurice Wilkens was born in the year 1916 and is a British biochemist. Born in...
References: Raven, P.H. and G.B. Johnson, (1988) Understanding Biology. Times Mirror/Mosby: United States
Biotech – www.accessexcellence.org/AB/WYW/wkbooks/SFTS/biography.htm
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