Deoxyribonucleic acid, or DNA, is the molecule that holds the genetic material for all organisms. It is found in most viruses, all bacteria and all plant and animal cells. DNA is found within the nucleus, or control center, of the cell. It can also be found within mitochondria and in chloroplasts, in those cells that have them, such as plants.
A deoxyribonucleic acid molecule is made up of a long chain of nucleotides. Nucleotide molecules are formed when a five carbon sugar molecule, a pentose sugar, joins to a phosphate group and an organic base. Within a DNA molecule, the sugar is deoxyribose and the organic bases can be either adenine, thymine, cytosine or guanine.
DNA molecules are macromolecules. They are made up of many subunits that are either similar or identical to each other. The subunits, nucleotides, are joined together end to end, similar to a pearl necklace. Deoxyribonucleic acid is a polynucleotide, but is usually referred to as a nucleic acid.
Single nucleotides are joined together between the phosphate group of one and the sugar molecule of another. A condensation reaction takes place to join the two nucleotides together and also produces water. The sugar and phosphate group are the same through the length of the chain, but any of the four bases may be attached to the sugar.
A DNA molecule is made up of two complementary strands of nucleotides that are joined together by hydrogen bonds between the bases. Each base can only bond with one other base. Specifically, adenine always pairs with thymine and cytosine always pairs with guanine. Between adenine and thymine, there are two hydrogen bonds, and between cytosine and guanine, there are three bonds. The two nucleotide strands spiral around each other to form an alpha double helix structure.
The specific sequence of the nucleotides along the deoxyribonucleic acid molecule is what determines the genetic code for any cell. Genes are made up of lengths of nucleotides along a DNA molecule. They are sequences of triplets of the nucleotides along the deoxyribonucleic acid molecule. These triplets determine which amino acids will be used to create proteins. Specific base triplets code for different amino acids.
The genetic code of the DNA molecule determines what proteins must be formed in a cell. Different chemical reactions take place within different cells depending on the type of cell and its function. These chemical reactions are controlled by enzymes, which are proteins. Therefore DNA determines a cell’s structure and function based on the fact that the sequence of nucleotides specifies what proteins must be formed and when.