Base On A Strand Of Dna Pairs example essay topic

DNA REPLICATION 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 diffraction's 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 Maurice Wilkens was born in the year 1916 and is a British biochemist. Born in Pongaroa, New Zealand, he is currently the deputy director of biophysics research unit at the Medical Research Council. He attended the University of London from the years of 1955 to 1974 and was awarded the Noble Prize in 1962 for the discovery of the molecular structure of DNA. THE FIRST MODEL OF A DNA MOLECULE Two scientists from Cambridge University named James Watson and Francis Crick were the first ones to actually build the actual structure of the DNA molecule.

In building their DNA molecule, they discovered that there were two different types of bases, purines and pyrimidines. Purines were the larger of the two types of bases, and are the double ringed structures [example of nucleotides that fit in this category are adenine (A) and guanine (G) ]. Pyrimidines were the smaller of the two types of bases, and are the only single ringed structures [example of nucleotides that fit in this category are cytosine (C) and thymine (T) ]. The two scientists also discovered that in the DNA molecule, only two base pairings of nucleotides are possible, adenine (A) with guanine (G) and thymine (T) with cytosine (C). This is because of the improper forming of hydrogen bonds. In a G-C pairing, three hydrogen bonds are formed whereas in an A-T pairing, only two hydrogen bonds are formed.

The Watson-Crick model also suggested that the basis for copying the genetic information is complementary. For example, if the sequence for one chain of DNA is ATTGCAT, then the sequence of its copy, or complementary side would be TAACGTA. Hence, to copy a DNA molecule you would only need to unzip its structure and construct a new complementary chain along one of its unzipped sides. An example of where this occurs is in the cell division. The basis for the great accuracy of DNA replication is complementarity. A DNA molecule is a structure containing two strands that consist of complementary sequence of bases, so either strand can be used to reconstruct the other.

JAMES WATSON As a boy, James Watson was already very interested in science, particularly in birds. He was born in 1928 from the city Chicago. Watson's interest in DNA grew out of desire, first picked up as a seni our in college, to learn about the gene. By the first time he got into graduate school at Indiana University, he decided that if he was going to understand genes, he needed to understand the simplest form of life bacteria. He then headed off to Europe, as a postdoctoral fellow, to learn more about biochemistry and bacteriophages. In 1951, he ended up at the Cavendish Laboratory, where he met Francis Crick.

In 1953, Watson and Crick sparked a revolution with their discovery of the helical structure of the DNA molecule. Watson was only 25 years old when their findings were published. He was only 34 when he was awarded the Noble Prize for the discovery of the molecular structure of DNA. Since 1968, Watson has served as director of Cold Spring Harbor Laboratory, a research institute for molecular biology. FRANCIS CRICK When Francis Crick was growing up in England, he received a children's encyclopedia from his parents, which exposed him to the world of science. His fascination with this world has continued throughout his whole life.

He received his college degree in physics and was starting graduate school when the World War II began. During the war, Crick worked on weapons for the British Admiralty. He was in his late 20's by the time the war ended, but he decided to go back to school for a PhD. Around the same time, he read a book that inspired him to begin studying biology. He went to the Cavendish Laboratory of Cambridge University to pursue this interest by studying proteins. In 1951, James Watson arrived at Cavendish, and the two began the collaboration that would lead to the discovery of the structure of the DNA molecule.

Before Crick received his PhD, he completed the work that would earn him a Noble Prize. Since 1976, Crick has been at the Salk Institute in California, where he investigates topics such as the origin of life and consciousness. BIOCHEMICAL NATURE OF CELL STRUCTURE Living things and their components have distinct shapes because the architecture of their molecules is tailored for their specific tasks. For example, each of the thousand or more protein molecules has a special job. It might be involved in catalysis, in electron transfer, or in membrane construction, to name a few.

A protein molecule has a shape uniquely suited for its assignment. The hemoglobin molecule, for example, has a pocket for carrying oxygen or carbon dioxide during respiration. The rod-shaped collagen molecule stiffens tissues and organs. The same notion of fit-to-function applies to most other cell molecules. DNA is designed for the storage of genetic information, phospholipids for use in membranes, and ATP for the storage of usable energy. Every living system has a blueprint for replication, or making copies of itself.

This blueprint is commonly called heredity. The key structure of the hereditary process is the long, spiral DNA molecule. DNA consists of two complementary strands coiled around each other to form a twisting ladder called a double helix (see Genetics). The strands are made up of varying sequences of chemical groups which are called nucleotides.

A nucleotide consists of a sugar and a phosphate group plus either of two purine bases -- adenine (A) and guanine (G) -- or either of two pyrimidine bases -- thymine (T) and cytosine (C). DNA contains the genetic code for making proteins from smaller molecules called amino acids. Each base on a strand of DNA pairs only with its complement on the other strand; that is, A pairs only with T, and G pairs only with C. Moreover, each set of three bases on a strand, such as, AGC, , or CGT, codes for a specific amino acid (or in the case of a few triplets, for an end to the protein-making process). Thus, a base triplet corresponds to a particular amino acid in the same way that a unit of the Morse telegraph code corresponds to an alphabet letter. In this manner, DNA directs the sequencing of the amino acids that grow into proteins. In many organisms, DNA is restricted to the cell nucleus, while protein synthesis goes on at the endoplasmic reticulum, a system of membrane-lined tubes in the cytoplasm.

Ordinarily attached to the endoplasmic reticulum are the ribosomes, 'workbenches' for protein construction. Since the ribosomes are away from the nucleus, the building code must somehow be communicated from DNA to the ribosomes. This is done through ribonucleic acid (RNA). RNA is closely related to DNA and can carry genetic messages. First, DNA unwinds and separates its strands so that complementary strands of RNA can be assembled on them.

A strand of so-called messenger RNA (m RNA) then travels out of the nucleus to the ribosomes, where protein synthesis begins. The m RNA strand, like its DNA 'parent,' contains the total genetic information needed for sequencing amino acids into a particular protein. Imagine a protein containing only the two amino acids A and B strung out in this unvarying sequence: A -- B -- A -- B -- A -- B (the sequence is deliberately shortened because proteins usually contain several hundred amino acids). A strand of m RNA has the series of complementary base triplets that codes for this sequence. However, another type of RNA called transfer RNA (t RNA) must carry the amino acids to the ribosome for assembly. When the m RNA code calls for amino acid A, the appropriate t RNA carries it in a form ready for peptide bonding with the next amino acid in line.

In a peptide bond, the tail-end carbon atom of one amino acid is linked to the nitrogen atom of the next. When the code calls for it, another t RNA carries amino acid B. Bit by bit, the polypeptide chain grows to the desired length, guided by the m RNA directions. At the end of the operation, the newly formed protein is kicked off the ribosome. The protein instantly folds up in the most stable way. Synthesis proceeds at a fast pace. A protein containing 400 amino acids can be synthesized in about 20 seconds.

(For more information about the role of DNA in protein synthesis, see Genetics.) Of all the molecules that DNA could direct to be built, one might wonder why the information encoded in DNA is limited solely to the manufacture of protein. The reason is that so long as DNA can direct the making of protein enzymes, no other direction is necessary because enzymes aid in the building of all other cell molecules. Most of the details of protein synthesis have been omitted from this discussion so that key events could be stressed. However, one procedure merits mention. Before an amino acid can be assembled into a polypeptide chain, it must first be modified to a so-called acyl amino acid, which is more reactive than an unmodified one. This important acyl conversion is powered by the energy stored in a molecule called adenosine tri phosphate (ATP).

Bibliography

Raven, P.H. and G.B. Johnson, (1988) Understanding Biology. Times Mirror / Mosby: United States Biotech - web.