RNA is usually synthesized from DNA. The synthesis usually requires one or more enzymes like RNA polymerase. The DNA strand is used as a template or guide on which the RNA is formed. Since RNA forms the proteins, this is the way the DNA maintains the blue print for all proteins without leaving the nucleus.
Using the DNA strand as template, a long chain of nucleotides are formed. This is called transcription. Initiation of transcription begins with the binding of the enzyme to a promoter sequence in the DNA. This region controls the reading of the DNA and formation of the RNA strand.
The DNA is a double helix and two strands are wound tightly and the whole thing is twisted over itself. As a first step the DNA double helix is unwound by the helicase activity of the enzyme.
The DNA strand is then read from the 3’ to 5’ direction and a complementary RNA is formed with elongation occurring in the 5’ to 3’ direction. The DNA sequence also dictates where termination of RNA synthesis will occur.
There are several thousands of ribonucleotides ready before the transcription process occurs and the messenger RNA or mRNA is formed. Each of the 100,000 or so proteins in the human body is synthesized from a different mRNA that has been transcribed from a specific gene on DNA.
It may be asked why the mRNA is needed in the first place since the DNA holds the information for the protein. The answer lies in the fact that the DNA needs to be preserved. If DNA is damaged in any way, then the coding sequence is changed and a mutation could result which could greatly affect the cell or even the whole organism. This makes the DNA vulnerable if it comes out in the cytoplasm to the ribosomes for protein synthesis. The mRNA brings in the necessary information from the DNA into the cytoplasm to the ribosomes for protein synthesis.
Once the transcription is over the RNA strand is modified by enzymes. For example, a poly and a 5' cap are added to eukaryotic pre-mRNA and introns are removed by the spliceosome.
Thus the mRNA is formed from the DNA is a process of transcription that is similar to DNA replication. However, in transcription only one strand – the antisense strand – is copied to form the mRNA. Since the template strand and the informational strand are complementary, and since the template strand and the mRNA molecule are also complementary and the mRNA is a copy of the DNA informational strand.
Some RNA polymerases use the DNA as template for copying strands of RNA (as described above). There are also numerous RNA-dependent RNA polymerases that use RNA as their template for synthesis of a new strand of RNA. For instance, a number of RNA viruses (such as poliovirus) use this type of enzyme to replicate their genetic material.
The polymerase needs to know where to begin copying the DNA. This is recognized by the promoter site. These sites are recognized by a factor called "SIGMA". This tells the DNA dependent RNA polymerase where to begin transcription. Once the RNA polymerase has been directed to the start point of the gene by sigma, the sigma factor is released and the RNA polymerase carries out the process of transcription.
Similarly another factor called "RHO" aids in terminating the process of transcription. Towards the end the rho factor binds to the mRNA and interacts with the RNA polymerase. This detaches the RNA polymerase and stops the transcription.