In order for genetic information in the DNA to be used, it must first be transcribed to form an RNA molecules. DNA sequence should be converted in RNA, no DNA code directly convert in protein. Synthesis of RNA from DNA is known as transcription.
As known DNA of eukaryotic cell is double stranded, but only one strand is used to read at the time of synthesis of RNA, this strand is known as template strand. Other complementary strand that have the same nucleotide sequence as mRNA (except DNA bases) is called as complementing strand.
Because the mRNA is made from the 5’ to 3’ end, the polarity of the DNA template strand is 3’ to 5’. Therefore we can conclude that beginning of the gene is at the 3’ end of the template strand. An important site, the promoter, is located at the start of the gene. Promoter is neither transcribed nor translated.
The promoter is a recognition / binding site for RNA polymerase, the enzyme that synthesize RNA.
The transcription start site represent the first nucleotide in the mRNA synthesized from the gene. However, the initially transcribed protein of the gene does not necessarily code for amino acids. Instead it is a leader sequence that is transcribed into mRNA, but is not translated into amino acid. The leader sequence includes a region called the Shine-Dalgarno sequence that is important in the initiation of translation. It is also some time involved in regulation of transcription and translation.
Immediately next to the leader is the most important part of the gene, the coding region.
In genes that direct the synthesis of proteins, the coding region typically began with the template DNA sequence 3’-TAC-5’. This produces the codon 5’-AUG-3’, which in bacteria codes for N-formylmethionine, a specially modified amino acid used to initiate protein synthesis. After initiation there is sequence of condones those code for a amino acid for that particular protein. The coding region ends with a specific codon, which is known as stop codon, this codon (stop codon) signals the end of protein and stops the ribosome during translation.