3G2+Tan+Zhi+Rong,+Ting+Chang+Hui,+Oon+Ming+Shen,+Brandon+Ng

Tan Zhi Rong - green  Brandon Ng (19) - orange Oon Ming Shen - red Ting Chang Hui 3G2 (25) - blue

DNA stands for deoxyribonucleic acid. It is a molecule that carries genetic information necessary for all cellular functions which include cell division and cell differentiation. Therefore, DNA is present in most of the nuclei in cells as vital functions cannot be carried out without it. The main role of DNA molecules is the long-term storage of information.

The DNA chain is 22 to 26 Ångströms wide (2.2 to 2.6 nanometres), and one nucleotide unit is 3.3 Å (0.33 nm) long. In living organisms, DNA does not exist as a single molecule, but instead as a pair of molecules that are held tightly together in the shape of a double helix. This anti-parallel polynucleotide chains run in opposite directions, the bases of one chain bonding to the opposite chain according to the rule of base pairing. The rule of base pairing states that Adenine (A) always bonds with Thymine (T), and Cytosine (C) with Guanine (G). These bonds are known as hydrogen bonds, and as a result can be broken or rejoined relatively easy, either by mechanical force or high temperature. These respective pairs of the the four types of nitrogen-containing bases are known as complementary bases. This allows all the information in a DNA helix is replicated in the other strand, which is required for DNA replication. This reversible and specific interaction between the complementary base pairs is responsible and vital for all the functions of DNA in living oraganisms. Also, a molecule of DNA is wrapped around proteins to form a single chromatin thread, which coil tightly into chromosomes during cell division.

The basic unit of DNA is called a nucleotide. The nucleotide repeats contain the segment of the backbone of the molecule, which holds the chain together, and a base, which interacts with the other DNA strand in the helix.  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 different bases then form different nucleotides with the phosphate group and deoxyribose sugar - adenine nucleotide, thymine nucleotide, cytosine nucleotide and guanine nucleotide. These nucleotides can then join up together by a condensation reaction through a condensation reaction between the phosphate group of on nucleotide and the hydroxyl group on carbon 3 of the sugar of the other nucleotide to form polynucleotides. Each gene is made up of a variety of sequences of nucleotides, giving rise to the presence of many different genes. As there are 4 different nucleotides (adenine nucleotide, thymine nucleotide, guanine nucleotide, cytosine nucleotide), for a gene made up of n nucleotides, there are 4 to the power of n different combinations of nucleotides. I  t is the sequence of these four bases along the backbone that encodes information. This information is read using the genetic code, which specifies the sequence of the amino acids within proteins. References: G.C.E. 'O' Level Biology Matters, Marshall Cavendish Education [] [] [] []