Transcription is the process by which the information in DNA is copied to make RiboNucleic Acid (RNA). RNA in turn synthesizes proteins through a process called translation. These two steps are together the Central Dogma of Molecular Biology.
There are mainly three kinds of RNA that are produced by transcription.
- the messenger RNA (mRNA)
- ribosomal RNA (rRNA)
- transfer RNA (tRNA)
RNA are single stranded molecules that are synthesized in the nucleus, but are mainly found in the cytoplasm and ribosomes in all the cells of every living organism.
RNA has a chemical make-up similar to DNA, but is less stable than DNA. RNA, like DNA is composed of nucleotides, with the same phosphate group, but the sugar is a five-carbon compound. The two complementary pairs of bases are adenine and uracil (instead of thymine), and guanine and cytosine.
Process of Transcription
The process is basically the same in eukaryotes and prokaryotes. One of the DNA strands is used as the template by RNA polymerase to form the RNA strand that is complementary to the template strand. Thus the base sequence in the RNA strand is the same as the non template DNA strand which is called coding strand or sense strand. The template DNA strand is also called anti-sense strand.
In prokaryotes, there is only one RNA polymerase that produces all the three types of RNA. In eukaryotes, there are three groups. RNA polymerase I transcribes rRNA, RNA polymerase II transcribes mRNA, while RNA polymerase III transcribes tRNA.
During transcription, only sections of the DNA with the gene to be expressed are involved. There can be different genes that are transcribed at any given time. This process is slower than DNA replication, with only 50-100 bases added per second. Both strands can act as template for different genes.
Transcription requires definite initiation and termination. The cis phase regulates transcription through promoters which determine initiation or the site on DNA where transcription should start. The promoter is a definite sequence of bases located on the 5' side of the gene. Another group of sequences called enhancers also determine initiation site, but unlike promoters, have no fixed position or orientation to the gene.
In prokaryotes, a sigma factor associates itself with the RNA polymerase and guides it to the relevant promoter. The sigma factor disassociates from the polymerase once the RNA polymerase binds itself to the promoter site. Since there is only one type of RNA polymerase in prokaryotes, the different sigma factors have important roles in control its functions to produce the three kinds of RNA, or different necessary mRNA.
In eukaryotes, TATA box and CAAT box are the promoter sites and do not have to be found at the site of transcription. They are found 25-100 and 70-150 bases respectively, upstream on the DNA strand. Many protein-transcription factors bind themselves to the promoter site and help attract RNA polymerase. The promoter and the polymerase together start to unwind the double stranded DNA.
The RNA polymerase moves and reads the template strand in a 3' to 5' direction, assembling ribonucleotides and forming the complementary RNA strand in a 5' to 3' direction. The bases added are complementary to the bases in the DNA. The only difference being that instead of thymine, uracil is used as the complement to adenine. As the RNA polymerase proceeds and the RNA strand elongates, the earlier sections of DNA rejoin and rewind.
When the RNA polymerase reaches a particular sequence that is the transcription terminator, the RNA synthesizes stops and the polymerase and new preliminary RNA strand disassociate from the DNA. Elongation also releases the sigma factor or transcription factors from the promoter, so that a new cycle of RNA production can start. The polymerase phase is also the trans regulation phase of transcription.
RNA formed in prokaryotes is ready for translation at this stage. However, RNA in eukaryotes have to undergo many other steps before being functional and ready to produce proteins.
Importance of transcription
Though all cells have the same DNA, different regions of the DNA are used during transcription to give different mRNAs, which gives different proteins that then direct the functions of the cell. This is the how different cells in eukaryotes get specialized to perform different tasks. Depending on their environment, even unicellular prokaryotes make changes to their functions, all of which is possible due to transcription of different mRNA.
Transcription is the process where genetic information in the DNA is used to produce different kinds of single stranded RiboNucleic Acid (RNA). Messenger RNA has sequences of codes needed to synthesize proteins. Transcription is more complex in eukaryotes and includes many more enzymes.