3B1+Bio+Essay+by+Chen+Shenghui+(03),+Lim+Mingxun+(15),+Yeo+Kai+Yuan+(35)

 Colour Code :D 1. Shenghui 2. Mingxun  3. Kai Yuan

DNA stands for Deoxyribonucleic acid and is a molecule that carries genetic information. Each DNA molecule consists of two parallel strands twisted around each other to form a double helix. It is wrapped around proteins to form a single chromatin thread. In every DNA molecule, a gene is a small segment that contains information used to make a single protein. The basic unit of DNA is called a nucleotide and each of them is made up of a deoxyribose sugar, a phosphate group and a nitrogen-containing base, all connected to one another. Under the nitrogen-containing bases in DNA, there are four different types of them: adenine (A), cytosine (C), guanine (G) and thymine (T).

< Diagram of the arrangements of components in a single DNA molecule 

Nucleotides can be joined together to form long chains called polynucleotides. A molecule of DNA is made up of two parallel polynucleotide chains running in opposite directions, said to be anti-parallel. The bases of one chain are bonded to those of the opposite chain according to the **rule of base pairing**. Under the rule of base pairing, **adenine (A) will always bond with thymine (T)** and vice versa. Thus, they are known as complementary bases. Likewise, **cytosine (C) always bonds with guanine (G)**.

 The nucleotide itself is a complex molecule, comprising of three parts: i. **The sugar component **: in DNA, the sugar component of the nucleotide is a pentose (contains five carbon atoms) sugar called 2’-deoxyribose. ii. **The nitrogenous bases **: these 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. iii. **The phosphoric acid component **: also attached to the sugar.

- References: [|http://www.massey.ac.nz/~wwbioch/DNA/DNAmake/framset.htm] [] [] [|http://www.colorado.edu/Outreach/BSI/pdfs/DNA_components.pdf[[http://dna.microbiologyguide.com/s/10002/pics/dnabases.gif] [| http://www.accessexcellence.org/RC/VL/GG/dna_molecule.php]