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As we are entering the twenty-first century, we are learning more and more about whom we truly are and what makes us that. We now know that we are made out of cells that are controlled by deoxyribonucleic acid or DNA; they are the blueprints of what we are today. DNA make up genes, which are the hereditary molecules. A mistake in the DNA is called a mutation, which makes us each special. Mutations create variations, which is ultimately the key to evolution. Perhaps through the years, we will learn even more about the fascinating mystery of the human body.

DNA are large molecules that are made of four different units that are called nucleotides. Each nucleotide is made up of three elements. They are phosphate, deoxyribose sugar, and a nitrogenous base. Each of the nucleotides have it's own distinctive base, they are cytosine (C), adenine (A), thymine (T), or guanine (G). A phosphodiester bond links the nucleotides. Nucleotides are linked from one phosphate to the deoxyribose sugar, which is linked to a nitrogenous base. So the nucleotides are linked in a series, from a phosphate to a deoxyribose sugar to another phosphate to another deoxyribose sugar, etc. The order of the nucleotides change, and information is coded into the DNA sequence (Aronson 93).

The DNA molecules are shaped like a twisted ladder: a double helix. Alternating deoxyribose sugar and phosphate molecules form the uprights of the ladder while complementary pairs of nitrogen bases form the rungs. The adenine pairs with the thymine and the cytosine pairs with the guanine. There are ten nucleotides per helical repeat. Therefore, the helices are antiparallel to each other (Belsie 99).

But how does the information travel to the proteins? Ribonucleic acids or RNA take this information and pass it to the amino acids. While DNA is found inside the nucleus of a cell, RNA is found mostly inside the cytoplasm. Unlike DNA, RNA has ribose sugar instead of a deoxyribose sugar and it uses the nitrogen base uracil instead of thymine. Uracil can bond with adenine just like thymine does. (Dolan) However, RNA is a single helix while DNA is a double. Finally, there are different types of RNA. A ribosome is made out of proteins and RNA. First, messenger RNA, or mRNA copies the DNA code inside the nucleus and takes it to the ribosome. The transfer RNA, or tRNA are the adaptors. There are twenty of them, one for each amino acid (amino acids are the twenty subunits that protein is made out of). They pair up with their amino acid and take it to the ribosome for protein synthesis. The RNA inside the ribosome (ribosomes are made out of protein and RNA) or the rRNA is waiting (Dolan).

Three nucleotides form a "codon" or a word that specifies one amino acid. Translation begins with the code: AUG - the only unique codon (Dolan). Translation stops with the code: UAA, UAG, or UGA. So, within the nucleus, the DNA code is transcribed into a complementary mRNA molecule. The molecule enters the cytoplasm where it associates with a ribosome. The mRNA code is then translates into a polypeptide chain. The codon AUG signals the start of the translation. An activated tRNA takes the amino acid to the ribosome and the tRNA binds to the AUG codon on the mRNA. The mRNA shifts, and the next codon is read by another tRNA. As the two amino acids are held next to each other, a peptide bond is formed between them. The second tRNA accepts the growing protein chain and the first tRNA is released. This process goes on until they reach a stop codon. Stop codons don't have matching tRNAs, and the peptide chain is released (Dolan).

New DNA is made when a cell divides. Each daughter cell receives a copy of the mother cell's DNA. DNA replication occurs at the end of Interphase, the cells resting and growth period of the cell cycle. The DNA molecule unzips along its hydrogen bases and becomes two different strands. The two strands halves become two whole DNA molecules because the floating nitrogen bases, phosphate, and deoxyribose acid make up the missing halves (Kreitman). This is called the semi-conservative replication because each of the daughter molecules has one old strand and one new strand. The enzyme that synthesizes DNA is DNA polymerase. There are actually three different DNA polymerases, and the one that is used for replication is DNA polymerase III (Young 90).

Mutations, or mistakes in the DNA usually occur during replication. Three different types of mutations are possible. Addition of a nitrogen base is detrimental only if the mutation occurred near the beginning. This is because all codons after the addition are changed. Subtraction of a nitrogen base is the most detrimental. This is because not only are all the codons after the subtraction are changed, but also because not enough amino acids will be produced. The last type, substitution of a nitrogen base, is the least detrimental. This is because only the substituted codon is changed (Kreitman).