DNA is a large biological chemical which is a polymer found in chromosomes and genes and stores our hereditary information.
A sub-unit of DNA is called a nucleotide which consists of a phosphate group, a five-carbon sugar called deoxyribose and one of four nitrogenous bases. These are called adenine, guanine, cytosine and thymine.
DNA exists as a double-helix which is a double stranded molecule resembling a twisted ladder. Two nucleotides are held together by their nitrogenous bases in the center by hydrogen bonds. Bases are complementary and pair according to the following rules- adenine pairs to thymine and cytosine to guanine.
DNA can separate into two strands and either can serve as a template for replication into more DNA or can be transcribed into messenger RNA. This nucleic acid can carry the DNA code for a specific gene to a ribosome in the cytoplasm which is a place where protein synthesis can occur. When a protein is made according to the genetic code, this is called gene expression.
When DNA is copied into mRNA, base pair rules apply. There is one exception-RNA lacks the base thymine so the base uracil is substituted.
For example, if the DNA code reads: TAC GGC, the complementary RNA code would be: AUG CCG.
Every three bases of RNA is actually a codon--a triplet which is the code for a particular amino acid to be added to a growing polypeptide chain at the ribosome. The first triplet is AUG which is a start codon and also codes for an amino acid called methionine. As each triplet is translated, and amino acids are added to the growing polypeptide chain with peptide bonds, eventually a stop codon is reached. This can be UAA, UGA or UAG. Now, the polypeptide is released from the ribosome and can fold into a functional protein.
To summarize, a section of DNA is called a gene. Once DNA is copied into mRNA, the code can be translated into a polypeptide at the ribosome and later fold into a functional protein. This is called gene expression.