DNA

What is DNA?

DNA is basically a molecule that contains coded instructions for the cells. Everything the cells do is coded in the DNA - which cells should grow/die and when, which cells should make hair, where it shuld grow, and it even determines the colour of your eyes. Our DNA is inherited from our parents, and the only reason we resemble our parents is because our bodies were formed using the DNA we inherited from them to guide the process.
DNA is a long molecule, like a chain, where the links of the chain are small pieces called nucleotides:


Figure 1: DNA Helix

What are Nucleotides?

Nucleotides are the single units that make up nucleic acids – These are literally the building blocks of life. From a chemical point of view, Nucleic Acids are long linear condensation (link to glossary) polymers of Nucleotide sub-units (monomers). Each nucleotide contains:

• Nitrogenous base:
- purine (Adenine/Guanine)
- pyrimidine (Cytosine/Thymine)

• Sugar : Deoxyribose (DNA)

• Phosphate group (PO4)

 Figure 2: Nucleotide

There are four different types of nucleotides in DNA (A), (G), (C) and (T):
• A: adenosine
• C: cytosine
• G: guanine
• T: thymine
The proportion of A always equals that of T (A=T). The proportion of G always equals that of C (G=C).
For those of you who like equations: A + G = T + C

These four fundamental bases are all that is required to write a code that describes our entire body plan; this complete code is called The Genome. How is it possible to have so much variation then?

Think of it that way: you are limited to a total of 4 letters, but you can write them in a sentence that has limitless length. In other words, there are so many ways that you can connect these 4 letters that the number of total sequences you would have is endless.
The human DNA code is called the "human genome", and is about 3 million bases in total. DNA molecules are arranged in a linear shape however are twisted in a double-helix, with a start and an end, and do not contain any cycles.

Where is it found?

The DNA is stored in the chromosomes which are found in the nucleus of each cell. In humans, each person gets 2 pairs of chromosomes, 23 from each parent, to total 46 chromosomes in each human cell. Usually hundreds of millions of base pairs are found in each chromosome. Considering the large amount of base pairs available the chromosomes themselves are physically large enough to be seen on high power microscopes.

Figure 3: Human Chromosomes

The Human Genome

The Human Genome Project (HGP) was a 13-year project coordinated by the U.S. Department of Energy and the National Institutes of Health., and was completed in 2003. If you find this topic interesting, make sure you visit the following site for more information:
http://www.ornl.gov/sci/techresources/Human_Genome/home.shtml

DNA Structure

The DNA Molecule consist of two strands held together in an antiparallel (glossary) manner by intermolecular forces which are called hydrogen bonds, they are formed between only between these base pairs (Adenine-Thymine) and (Guanine-Cytosine). Therefore, the base sequence (code) in one strand determines the code of the other strands (complementary bases).

Figure 4: DNA Structure