Bio+Essay+by+Kai+Cong,+Glendon,+Li+Ke+and+Kon+Kit

Hong Kai Cong-black Glendon Goh-blue Cheng Li Ke-green Wong Kon Kit-red DNA, or deoxyribonucleic acid is a molecule that carries genetic information necessary for all cellular functions which include cell division and cell differentiation. Genetic information found in DNA is transferred from one generation to another in the form of genes which are the basic units of heredity. DNA is a nucleic acid as it is a weak acid, first found in the nuclei of cells. It is a polymer, composed of monomers called nucleotides.

Each nucleotide is made up of a sugar called deoxyribose, a phosphate group and a nitrogen-containing base. There are 4 types of nitrogen-containing bases in DNA - adenine (A), thymine (T), cytosine (C) and guanine (G). These nucleotides can then join up together through a condensation reaction between the phosphate group of on nucleotide and the hydroxyl groups on carbon 3 of the sugar of the other nucleotide to form a polynucleotide chain, i.e. phosphodiester bonds link the nucleotides together. Two anti-parallel polynucleotide chains will make up a DNA molecule. Therefore, DNA is a double stranded molecule, with the two strands of polynucleotide chains winding around each other to form a double helix. The sugar phosphate bonds form the backbone of the DNA molecule. The two strands are joined together by hydrogen bonds between the bases. 2 hydrogen bases exist between a A-T base pair while 3 hydrogen bonds exist between a G-C base pair. The base pairs therefore form the “rungs” of the DNA molecule. The base pairs are specific. A only binds to T (and T with A), and C only binds to G (and G with C). These are called complementary base pairs. Hence, knowing the sequence of bases along one strand will enable the deduction of the sequence of bases on the complementary strand. Every three nucleotides bases, is called a base triplet. A base triplet can code for one specific amino acid. As there are 4 nitrogenous bases and they are arranged in triplets, there 64 possible combinations of base triplets. However, there are only 20 amino acids that can be coded. Therefore, there are cases when more that one base triplet can code for the same amino acid. Therefore, the whole polynucleotide chain of the anti-sense strand of a DNA molecule can code for a protein. A gene is a segment of DNA coding for a polypeptide. Therefore, genetic information coded in genes can be expressed as the features of an individual.