Introduction
DNA Transcription is the process of synthesis of RNA from DNA using various DNA Transcription enzymes. As we know DNA shows the semiconservative mode of transfer that is it believes in equal sharing of genetic material. It is a double helical structure and the structural details we have already seen in the previous blogs.
RNA is the first genetic material on the earth then how it can get synthesize from DNA? So, the answer behind it is the evolution and the natural selection of DNA over RNA. In addition to that stability and processivity of the DNA take it towards selection. The process of transcription is carried out in the cytosol of the cell. But the twist is RNA is synthesizing in the nucleolus.
Before starting the exact process of transcription let’s clear some basics required for the transcription. DNA as genetic material is get synthesized and get stored in the nucleus. RNA also gets synthesized in the nucleus but not stored in it. It occurs in various forms namely t-RNA, rRNA, and mRNA. These are some types of RNA found in the cell which we will discuss in detail further.
We know prokaryotic cell lacks nucleus hence the process of RNA and DNA synthesis takes place in cytosol only. On the other hand, this process of synthesis in eukaryotes occurs in the nucleus. Despite different synthesis locations, the DNA Transcription takes place in the cytosol for both eukaryotic and prokaryotic cells.
Transcription is a unidirectional process, unlike replication which is a bidirectional process. Various DNA transcription enzymes involve in this process but the primary enzyme is RNA Polymerase.
Essentials of DNA Transcription
DNA to RNA synthesis is known as transcription. Now we all know the structure of DNA which is double helical and this structure has some conserved sequences in it which provide the starting point and the endpoint for some processes like transcription, replication, and translation. This starting and endpoints are also made up of various nucleotides but with specific numbers and pairing.
These conserved sequences provide a binding site for specific enzymes only and it is known as a promoter, Operator, Initiator. Every DNA has its Promoter, Operator, Initiator, etc. According to the synthesizing ability of these conserved sequences, the cell DNA is get differentiated. If the DNA can synthesize one gene from one promoter it is named Mono Cistronic (cistron- gene) and if many genes synthesize from one promoter it is known as Poly Cistronic.
Eukaryotes – Mono Cistronic
Prokaryotes – Poly Cistronic
The transcription enzyme used for the DNA to RNA is RNA polymerase. Process of transcription and translation occurs together in the case of prokaryotes as it lacks a nucleus. And takes place separately in eukaryotes because it contains a nucleus. Unlike DNA polymerase in replication RNA polymerase does not show a special requirement of free 3’–OH and primer.
The conserved sequence that provides the start point for transcription we have discussed above is known as Pribnow Box in prokaryotes. And Goldberg Hogness Box in eukaryotes. Direction of RNA synthesis from DNA is 5’→3’. Eukaryotic RNA is protected by a 5’ cap and 3’ poly-A tail immediately after synthesis.
RNA Polymerase
An enzyme uses for the DNA transcription is looks like a Crab Claw. RNA polymerase is primer independent. It uses nucleotides for the polymerization but in RNA synthesis uracil(U) is incorporated instead of thymine(T). On the promoter, a conserved sequence of ‘TATAAT’ is specific for the binding of polymerase enzyme. It is known as the ‘TATA box’. And the protein that binds on it or recognizes it for binding is TBP (TATA-binding protein). TBP is a universal transcription factor.
Core RNA polymerase is made up of a total of five subunits beta(β), beta’(β’), 2 alpha(α), and omega(Ѡ).
β subunit – it catalyzes the polymerization
β’ subunit – it binds and open DNA template
2α subunit – find out DNA to be transcribed
Ѡ subunit – provide protection
Different RNA polymerase enzymes prepare different RNAs. The types of RNA polymerase enzymes change according to the cell type. For example, for prokaryotic cells, there is only one type of RNA polymerase which synthesizes all three RNAs (mRNA, rRNA, and tRNA). For eukaryotes, there are three types of RNA polymerase enzymes namely.
RNA Pol – I → synthesize rRNA (ribosomal RNA)
RNA Pol – II → synthesizes mRNA (messenger RNA)
RNA Pol – III → synthesizes tRNA (transfer RNA)
The process of transcription is the same for all eukaryotes and prokaryotes but the enzymes and promoters are different for every gene.
Steps of DNA Transcription
DNA transcription requires various enzymes for processing. The RNA synthesize after transcription is known as ‘transcript’. This transcript is further going for the translation that is RNA to protein synthesis. The transcription process is divided into three steps viz.
Initiation – here RNA polymerase binds on the promoter sequence of the DNA which is present near the gene to be transcribed. On the binding, there is a formation of bubble on the DNA strand which separates the double-strand. Each gene present on the helix has its promoter. The separation of two strands provides the template strand for the synthesis of RNA.
Elongation – template strand synthesize in the initiation process is read by the RNA pol base by base and synthesize RNA. The addition of one-by-one nucleotide bases elongates the synthesizing strand in a 5’→3’ direction. This transcribed RNA shows sequence similarity with the coding that is non-template strand and complementary pairing with the non-coding that is template strand. Transcribed RNA has uracil(U) instead of thymine(T) as it is RNA.
Termination – after finishing the desired length of RNA it has to get separated from the assembly of parent DNA and polymerase enzyme. The transcription endpoint or terminator sequence signals that the transcription is complete. The separated transcript further went for the processing that is protein synthesis.
Proofreading of the Transcript
Now suppose we have transcribed RNA then what is the proof of it’s not correctly sequenced or damaged?
So, to check all these things in transcript RNA polymerase enzyme has proofreading activity. It has two ways to check the RNA one is by pyro phosphorylating editing and the second is hydrolytic editing.
Pyro Phosphorylating Editing – when enzyme uses its active site to catalyze the removal of incorrectly added nucleotide triphosphate (NTPs).
Hydrolytic Editing – here polymerase enzyme backtracks the RNA by one or more wrongly added nucleotide and cut the RNA, remove error sequence and go ahead.
It is like you are walking straight on steps and by mistake miss one step, so you come on the missed step without turning back. Follow that step and start your walk again from the missed step.
DNA polymerase proofreading activity is more efficient than RNA proofreading due to exonuclease activity which is not present in RNA pol.
The story after the RNA transcript formation we will discuss in detail in the upcoming article till the time stay connected with us.
