DNA

The complete set of instructions for making an organism is called its genome. It contains the master blueprint for all cellular structures and activities for the lifetime of the cell or organism. The genome consists of tightly coiled threads of Deoxyribo Nucleic Acid [DNA] and associated protein molecules, organized into structures called chromosomes. If unwound and tied together, the strands of DNA will stretch more than 5feet and will be only 50 trillionths of an inch wide. For each living organism, the components of these slender threads encode all the information necessary for building and maintaining life. DNA molecule consists of two strands that wrap around each other giving a double-helical structure made of sugar and phosphate molecules and is connected by rings of nitrogen containing chemicals called bases. Each strand is a linear arrangement of repeating similar units called nucleotides. A nucleotide is composed of one sugar [five carbon], one phosphate and a nitrogenous base. The two strands of a DNA molecule are anti-parallel and they are held together by weak hydrogen bonds between the bases.

Four different bases are present in DNA. They are Adenine [A], Cytosine[C], Guanine [G] and Thymine [T]. The particular order of the bases arranged along the sugar-phosphate backbone is called DNA sequence. Each DNA strand has two different ends that determine its polarity: the 3’end and 5’end. The chemical structure of DNA consists of a particular bond of two linear sequences of bases. Strict base pairing rules are adhered to. Adenine will only combine with thymine and cytosine always pairs with guanine. This is called complementarity of the DNA. It is also known as Watson-Crick complementarity. Thus if two complementary single stranded DNA molecules come together they bind to form double helical structure.

For example, if one strand of DNA is of the sequence

AGTACT

The other strand must be of the sequence

TCATGA

Polymerase and ligase are the two enzymes that are vital for DNA replication and sticking together of DNA molecules when they come into close proximity in a linear fashion.