Basic objective of this lecture is to present on Genetic Code. Here briefly present on the structure of DNA. The genetic code is the set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins (amino acid sequences) by living cells. Genes are sections of DNA that contain coded information that determines the nature and development of organisms. This lecture describe the structure of DNA and explain the difference between an allele, a gene and homologous chromosomes.