3S2+Bio+Essay+by+Kenneth+Tan+(09),+Marcus+Tan+(16),+Lee+Peizheng+(11),+Lim+Yao+Chong+(14)

Group members: Kenneth Tan Marcus Tan Lee Peizheng  Lim Yao Chong  The DNA, otherwise known as the deoxyribonucleic acid, is a molecule that carries genetic information which by cellular standards is considered to be a very long macromolecule that forms the vital component of chromosomes and is capable of self replicating . . The nucleotides are created out of a simple nitrogenous base which are chemically bound with phosphate groups and also sugars joined together by an ‘ester’ bond. These nucleotides are composed of three parts: a phosphate, a deoxyribose sugar, and a type of compound base. Let’s take the DNA to be a twisted ladder. Using an analogy, the phosphate and the deoxyribose sugar forms the side of the ladder, whereas the compound base forms the rung of the ladder. The deoxyribose and the phosphate are also bound together chemically with pyrimidine and purine bases, which is also known as thymine, cytosine, guanine and adenine (which are only referred to by the first letter of their name: A, G, T and C) , furthermore, the ordering of these bases is what functionally determines the nature of the genetic code specified by the DNA molecule . Next, the DNA polymer chains are bound together when base pairing occurs in which the bases that are attached to sugar groups of the nucleotides are being connected by hydrogen connections. DNA is an anti-parallel double helix. Anti-parallel refers to the fact that although the strands run parallel to each other, they run in opposite directions. One strand runs from the 3’ carbon deoxyribose (three prime carbon) to the 5’ carbon deoxyribose (five prime carbon), while the other strand runs from the 5’ carbon deoxyribose to the 3’ carbon deoxyribose. To differentiate the strands, one strand is called the “sense” strand while the other is called the “anti-sense” strand.

Also, DNA could be seen as being made up of many deoxyribose sugar subunits, each consisting of the deoxyribose sugar and the compound base of thymine, adenine, guanine or cytosine, and with the phosphate groups connecting the subunits together to form a strand of DNA. This determines the gene makeup and the structure of the proteins produced by the ribosomes.

Thymine always bonds with adenine, whereas guanine only bonds with cytosine. This is known as Chargaff's rule. This is because an adenine-thymine pair forms two hydrogen bonds, while a cytosine-guanine pair forms three hydrogen bonds, preventing wrong base pairings from occurring and also making the base pairs more structurally stable. <span style="color: rgb(0,0,255);">It is possible for thymine to bond with guanine or cytosine, but this happens only when a mutation in the DNA occurs, The same goes for the other three bases. <span style="color: rgb(0,128,0);">Because of this, the rule also states that a strand of DNA should have equal amounts of guanine and cytosine, and equal amounts of thymine and adenine. <span style="color: rgb(255,0,0);"> Every base and phosphate group is grouped individually for easy identification and isolation.

Therefore, the overall 3-dimensional structure of DNA consists of 2 polynucleotide strands that are arranged in such a way that they are wound round each other, forming a double helix. This is a short region - about 2nm in length. These two polynucleotide chains are bonded together by hydrogen bonds between the bases, thus forming base pairs. This is followed by a string of chromatin, and then a 30-nm chromatin fiber. The fiber is a stack of nucleosomes. Below that, there is a section of chromosome in extended form, as well as one that is condensed beneath. At the very last layer lies the entire mitotic chromosome, which is has the greatest length within the DNA structure.

__Pictures on the Structure of DNA

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