3S2+Bio+Essay+Low+Rui+Hao,+Tai+Jia+Ching,+Seow+Wen+Jun,+Dylan+Fones

Done By: Dylan Fones(04), Low Rui Hao(15), Seow Wen Jun(23), Tai Jia Ching(25)

Task: Write a 300-word essay describing the **components of DNA** and **how these associate** with each other to generate the **overall, 3-dimensional structure** of DNA.

Essay:  DNA stands for DeoxyriboNucleic Acid. DNA is a polymer; polymers are composed of many individual units called monomers, linked together in a chain. The monomer of DNA is the nucleotide. The nucleotide itself is a complex molecule, consisting of three seperate parts: The sugar component, the nitrogenous bases and the phosphoric acid component. These phosphate groups, deoxyribose sugars and nitrogenous bases combine to form four different nucleotide molecules:adenine nucleotide, thymine nucleotide,guanine nucleotide and cytosine nucleotide. They can then be merged together to form polynucleotide chains. There are two complementary polynucleotide chains in DNA .The nitrogenous bases on one strand will form hydrogen bonds with the bases on the other strand, forming base pairs. These strands are also antiparallel which means that they run in opposite directions. The two strands coil tightly around each other and the bases are stacked together(which improves stability) thus DNA exist as a double helix.

Continuing from the previous point. Either an adenine-thymine pair forms a two-hydrogen bond together, or a cytosine-guanine pair forms a three-hydrogen bond. Therefore, the base pairing is restricted due to the above point. However, this restriction is essential when the DNA is being copied: the DNA-helix is first "unzipped" in two long stretches of sugar-phosphate backbone with a line of free bases sticking up from it, like the teeth of a comb. Each half will then be the template for a new, complementary strand. Biological machines inside the cell put the corresponding free bases onto the split molecule and also "proof-read" the result to find and correct any mistakes. After the doubling, this gives rise to two exact copies of the original DNA molecule. Thus, we can see that the restriction of base pairing is important to the DNA not just in giving it its 3D shape, but also in copying of DNA.

The key features of the DNA structure includes the sugar-phosphate backbone that runs on the outside of the helix structure and is created when one phosphate gropu of one nucleotide is joined with a phosphodiester bond to a pentose sugar of another by a condensation reaction . This joining up forms a polynucleotide chain. This was concluded from Rosalind Franklin's report. This arrangement was appealing because it put the relatively hydrophobic nitrogenous bases in the molecule's interior and thus away from the aqueous solution. The helix makes one full turn every 3.4nm along its length with the bases stacked just 0.34nm apart, each base being 36 degrees from the previous base. Thus, there are 10 layers of base pairs in each turn of the helix. The nitrogenous base are paired in a specific combination: Adenine and Thymine, Cytosine and Guanine. The pairing are done by trail and error at first by Watson and Crick but later found out that only a Purine and a Pyrimidine results in a uniform diameter. And with that, Watson and Crick found out that Adenine forms 2 hydrogen bonds with Thymine and Guanine forms 3 hydrogen bonds with Cytosine. Also, the Watson and Crick model explained the Chargraff's rule where whenever one strand of a DNA molecule have an A, the partner strand must have a T; whenever one strand of a DNA molecule have a G, the partner strand must have a C. Thus, the amount of adenine equals to the amount of thymine and the amount of guanine equals to that of cytosine. This bonding is called specific or complemetart base pairing.

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References: 1. <span style="color: rgb(0, 53, 255);">Campbell, Neil A. (2005). //International Edition, Biology seventh edition//. San Francisco: Pearson Education, Benjamin Cummings. 2. Image from: <span style="color: rgb(0, 53, 255);">http://upload.wikimedia.org/wikipedia/commons/b/b8/DNA-structure-and-bases.png 3: <span style="color: rgb(241, 121, 9);">http://genetics.suite101.com/article.cfm/the_structure_of_dna <span style="color: rgb(0, 128, 0);">4. <span style="color: rgb(0, 128, 0); font-family: verdana,arial,sans-serif; font-size: 11px; line-height: 22px;">. (n.d.). <span style="color: rgb(241, 121, 9); font-family: verdana,arial,sans-serif; font-size: 11px; line-height: 22px;">//<span style="color: rgb(0, 128, 0);">DNA structure. // <span style="color: rgb(0, 128, 0); font-family: verdana,arial,sans-serif; font-size: 11px; line-height: 22px;"> Retrieved July 3, 2009, from http://www.biochem.uwo.ca/meds/MEDNA/doublehelix.html 5.eslie, P. A. (n.d.). <span style="font-family: verdana,arial,sans-serif; font-size: 11px; line-height: 22px;">//<span style="color: rgb(0, 128, 0);">Http: // <span style="font-family: verdana,arial,sans-serif; font-size: 11px; line-height: 22px; color: rgb(0, 128, 0);">www.nature.com/scitable/topicpage/Discovery-of-DNA-Structure-and-Function-Watson-397. <span style="color: rgb(0, 128, 0);">// <span style="color: rgb(0, 128, 0); font-family: verdana,arial,sans-serif; font-size: 11px; line-height: 22px;"> Retrieved July 3, 2009, from http://www.noaanews.noaa.gov/stories/s1119.htm <span style="color: rgb(0, 128, 0);"> <span style="color: rgb(219, 15, 15);">5. http://nobelprize.org/educational_games/medicine/dna_double_helix/readmore.html