Transcription genetics is associated with the studies related to the transcription process in genetics. Transcription is the process of transfer of information from DNA to RNA. Thus, gene transcription definition can be given as a process of generating RNA copies from the genetic information stored in the DNA. This copy of RNA is known as the messenger RNA or the mRNA.
The process of transcription is the first step in the central dogma of molecular biology which states the information passes from DNA to RNA via the process of transcription and from RNA to proteins via translation. Several of the transcription factors, specialized proteins, control the transcription process. An example of this is the end-product of the myc gene, which produces a transcription factor responsible for regulating many pro-proliferative genes.
Biological Significance of Transcription
The central dogma of molecular biology states that a piece of information encoded in the genetic material is passed from DNA (Deoxyribonucleic Acid) to RNA (Ribonucleic Acid) through transcription and from RNA to protein through translation. According to this, transcription genetics is a field related to the transfer of information from DNA to RNA. Genetic information is the information vital for the survival of a cell or an organism made up of an organized structure of cells, as it contains all the molecular knowledge of the working and functional mechanisms. And it is encoded in a specific sequence of nucleic acids (usually the DNA) known as the gene.
The gene is the basic molecular unit of heredity. Different genes contain information regarding different functions and processes to be carried out by the cell for survival. The gene consists of three basic parts. Two untranslated regions or UTR parts flank the coding sequence present in between. It is this particular sequence of nucleic acids in the coding region of the gene that is used to create or make the RNA. Thus, transcription is the process of copying a gene to create RNA from DNA. It is noteworthy that transcription is different from replication as replication is making a copy of DNA whereas in transcription the information is passed into a molecularly different nucleic acid. This difference is clearly shown between the RNA sequences produced from the DNA sequences by the nucleotide uracil (U) which replaces the nucleotide thymine (T) present in the DNA. The other three nucleotides, adenine (A), guanine (G), and cytosine (C) remain the same in both DNA and RNA.
Several proteins are involved in making the process of transcription successful. RNA polymerase is the enzyme responsible for making RNA. This enzyme attaches itself to the DNA and reads the sequence information from the 3’ - 5’ of the antisense strand of DNA, as it can only add nucleotides at the 3’ end of a growing RNA chain. This in turn produces a 5’ - 3’ RNA which is also known as the messenger RNA (mRNA). This mRNA fragment generated contains only the information of creating the whole protein and the sequence matches the sense strand of the DNA with the only change of thymine to uracil. Comparing DNA replication and transcription, it can also be pointed out that because of the given process of working of RNA polymerase, it does not require a DNA primer or the creation of Okazaki fragments which are fundamental to DNA replication. Once generated, the mRNA is then sent to the ribosomes for protein synthesis.
A very simple diagram showing mRNA synthesis is given below:
[Image will be uploaded soon]
Molecular Machinery Involved in Transcription
It is clear from above that protein machinery is required to transcribe and translate a gene from DNA from RNA. It is not only the coding sequence in a gene that is responsible for the transcription. It also contains two additional regions for the start and end of transcription. Together with those, the gene is known as a transcriptional unit. The three parts of the transcriptional unit and the gene are the promoters, the structural gene, and the terminator. The promoter region or the promoter gene is the one responsible for the initiation of transcription. The structural gene is the segment that is transcribed into RNA and the terminator is the portion that stops the process of transcription. The process is further controlled and aided by transcription factors obtained as byproducts of specialized genes such as the myc gene. The myc gene contains the sequence required for activation of the expression of several pro-proliferative genes via binding with the enhancers. Thus, after the initiation, the elongation of the mRNA nucleotide chain and termination of the mRNA from the transcription site occurs.
But these processes are highly regulated to control gene expression. There is a complete classification of genes depending on their expression requirements, of which some are expressed regularly while others only when required. Amongst the different classes of genes - repressed genes are the ones that have reduced or inhibited expression. One of the examples of the repressed genes is the lac repressor in bacteria. Bacteria always utilize glucose as the main source of food. In absence of glucose, another source of energy is lactose which is broken down by the bacteria. Once glucose is available, the lac gene expressing genes for the necessary proteins gets repressed again as the repressor protein binds to the promoter. Thus, repressed genes are the ones that are switched off, different from silencer genes as the silenced genes are the ones that cannot be accessed for transcription. The silencer genes halt the transcription of DNA as the genes get tight wrapped around the histone proteins thus becoming inaccessible to the polymerases.
Although there has been severe research in transcription genetics there are some aspects of eukaryotic gene transcription that are still unclear. The termination of transcription by the RNA polymerases in eukaryotic cells is still under research and is ongoing.