3B1+Bio+Essay+by+Ernest+Aw+(2)+Cheong+Rong+Hao+(4)+Edwin+Tay+(29)+Zhou+Shicheng+(38)

Ernest Rong Hao Edwin Zhou Shicheng

DNA stands for Deoxyribonucleic Acid. DNA is a polymer; polymers are composed of many individual units called monomers, linked together in a chain.

One strand of DNA has a backbone consisting of a polymer of the simple sugar deoxyribose bonded to something called a phosphate unit. The Backbone of a strand of DNA looks like this :


 * sugar-phosphate-sugar-phosphate-sugar-phosphate-sugar-phosphate-... ||

The monomer of DNA is the nucleotide. The nucleotide itself is a complex molecule, consisting of three separate parts, namely sugar, nitrogen bases and phosphoric acid component. The sugar component in DNA is a pentose (contains five carbon atoms) sugar called 2’-deoxyribose. The nitrogenous bases are attached to the sugar in the nucleotide. In DNA, any one of four nitrogenous bases can be attached to the sugar. These are adenine (A) and guanine (G), which are purines and thymine (T) and cytosine (C), which are pyrimidines. These bases are arranged in a specific order according to our genetic ancestry. The order of these base units makes up the code for specific characteristics in the body, such as eye color or nose-hair length. Just as we use 26 letters in various sequences to code for the words you are now reading, our body's DNA uses 4 letters (the 4 nucleotide bases) to code for millions of different characteristics. Each molecule of DNA is actually made up of 2 strands of DNA cross-linked together. Each nucleotide base in the DNA strand will cross-link (via hydrogen bonds) with a nucleotide base in a second strand of DNA forming a structure that resembles a ladder. These bases cross-link in a very specific order: A will only link with T (and vice-versa), and C will only link with G (and vice-versa). Thus our picture of DNA now looks like this: 


 * sugar- || phosphate- || sugar- || phosphate- || sugar- || phosphate- || sugar- || phosphate- || ... ||
 * G ||  || A ||   || C ||   || T ||   || ... ||
 * C ||  || T ||   || G ||   || A ||   || ... ||
 * sugar- || phosphate- || sugar- || phosphate- || sugar- || phosphate- || sugar- || phosphate- || ... ||
 * sugar- || phosphate- || sugar- || phosphate- || sugar- || phosphate- || sugar- || phosphate- || ... ||

 The nucleotides are linked together by their phosphate groups to form a polymer, the polynucleotide. The phosphate group of one nucleotide joins to the carbon ring of the sugar in the next nucleotide. The linkage between the nucleotides is a phosphodiester bond. DNA is a helical molecule with a double helix. There are seven important features of the double helix. The double helix consists of two polynucleotides. The nitrogenous bases are on the inside of the helix with the sugar phosphate backbone on the outside. The bases of the two complementary polynucleotides are bonded by hydrogen bonding. The double helix turns every ten base pairs. The two polynucleotide strands are antiparallel, that is one strand runs one direction and the other strand runs the opposite direction. The double helix contains two different grooves, a major and a minor groove. The double helix is right handed, this means that if the double helix was a spiral staircase, if you climbed up it, the banister (the sugar phosphate backbone) would be on your right hand side.  DNA is the hereditary material in humans and almost all other organisms. Nearly every cell in a person’s body has the same DNA. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA). The information in DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Human DNA consists of about 3 billion bases, and more than 99 percent of those bases are the same in all people. The order, or sequence, of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences. DNA bases pair up with each other, A with T and C with G, to form units called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule. Together, a base, sugar, and phosphate are called a nucleotide. Nucleotides are arranged in two long strands that form a spiral called a double helix. The structure of the double helix is somewhat like a ladder, with the base pairs forming the ladder’s rungs and the sugar and phosphate molecules forming the vertical sidepieces of the ladder. An important property of DNA is that it can replicate, or make copies of itself. Each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases. This is critical when cells divide because each new cell needs to have an exact copy of the DNA present in the old cell.

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