The contents of this textbook are distributed on eleven chapters, which containing more than 250 figures. In which, the chemical structure of nucleic acids, the way of their packaging, the concept of the genes that formed as a result of the specialized arrangements of these nucleotides were discussed. Then, the mode of genes replication, transcription, translation, and the control of their expression were explained as well. Moreover, the means through which the genetic material can be transferred naturally among bacterial cells was discussed and made as an introduction to know the scientific basics that human beings based on their direct intervening on gene transfer form one organism to the other counterpart, and hence its considered as basics for gene cloning experiments. Eventually, the most common tools used in dealing with genes are reviewed to pave the way for the reader to know the practical applications of this tiny interesting world.