3.2 Gene Expression

The sequence of bases in a DNA molecule represents the information content of a gene, which is known as the genetic code. These bases are arranged in a random fashion. The type, number and sequence of these four bases determine the type of the gene.  Any change of these bases will lead to a change in gene function, i.e. forming a mutation. The total sum of DNA molecules found within every cell of the human body is known as the human genome. Nearly all the genome is found in the nucleus, although, mitochondria also contain essential genetic information.      

This DNA contains all the information needed for the development of the zygote to form the adult. However, only about 3% of the total DNA of the human genome represent the functional genes. The rest of the DNA is non-functional or has a functional role in regulating and promoting gene expression.

It is now established that the gene functions through its control to the synthesis of protein. It involves two basic steps, the first is the synthesis of different types of RNA from DNA, by a process known as transcription. The information in the RNA can then be translated to a polypeptide chain by the process of translation.

Different types of RNA polymerase catalyze the process of transcription.  Three types of RNA are formed namely the messenger RNA (mRNA) that contains the genetic code for the functional genes, transfer RNA (tRNA), that carries the amino acids to the site of protein synthesis and ribosomal RNA (tRNA), that combine with proteins to form the ribosomes.  All types of RNA polymerase require the action of transcription factors to bind DNA and promote transcription.

The process of transcription uses one strand only of DNA as a template by the help of RNA polymerase. A newly synthesized RNA strand is formed that is an exact copy of the DNA strand. By this way the information content of the DNA molecule is maintained during transcription into a mRNA molecule (Fig 5).

Most eukaryotic genes including man are interrupted by stretches of DNA that are not translated into proteins. These are known as intervening sequences or entrons, whereas the translated sequences are known as exons. When DNA is transcribed into RNA, the primary product contains both entrons and exons.  Entrons are then spliced out and the exons are unitedtogether to form the mature mRNA that will be translated to the different amino acids. Other modification to the mRNA is carried out, that is adding at one end a cap made of 7 methyl

Figure 5 : The process of transcription and translatation

guanosine and the other end a tail composed of polyadenine. The capping and tailing of RNA is necessary to export the RNA from the nucleus (Fig 6).    

Figure 6 : splicing out of intron from precursor to form the maure RNA       

Translation occurs on the ribosomes in the cytoplasm. The mRNA functions as a template for the synthesis of protein. The process is complex, but it depends on the genetic information in the mature mRNA that determines the correct amino acid sequence in the polypeptide chain. The sequence of the bases present in the mRNA represent the genetic code. The code is triplets code i.e. every three-consequative bases represent a codon, which specifies the particular amino acid.