Process of transcription:
The process in which genetic material is transferred from DNA to RNA is called transcription.in this process DNA strand is used as a template and mRNA is synthesized. The enzyme that take part in transcription is RNA polymerase. During cell division the whole genome of living organism is replicated a lot of time, but transcription take place only of short portion of genome. The region that is able to be transcribed into RNA is called gene.
Difference between replication and transcription:
- In the process of replication both strands of DNA are needed while in the process of transcription single strand involve.
- In replication whole genome is replicated while during transcription only small portion of genome is transcribed
- In replication primer is involve while during transcription no primer is involve.
- DNA polymerase take part in the process of replication while during transcription RNA polymerase plays its role.
- The end product of replication is double stranded DNA while the end product of transcription is single stranded RNA.
- In the process of replication base Pair formation of adenine and thymine, and guanine and cytosine take place while during transcription adenine pairs with uracil ,thymine with adenine and guanine with cytosine.
- During replication proof reading take place while during transcription there is no proof reading.
DNA strand can act either as template strand or coding strand:
Template strand:
It is also known as antisense strand. And this is the actual strand from which RNA will be transcribed. This strand run in 3’ to 5’ direction.
Coding strand:
It is called as sense strand. Its has base pairs which will specifies the sequence of amino acid during protein synthesis. This strand run in 5’ to 3’ direction.
Asymmetrical transcription:
Usually only template strand is used for process of transcription and the coding strand don’t take part in transcription. But according to the idea of asymmetrical transcription it Can happen as that rarely both the strand may be used as a template strand for the transcription of different genes. So this happened as that the transcription that is taking place on different strand will take place in opposite direction.
RNA polymerase enzyme that take part in transcription:
RNA polymerase is the enzyme that is itself encoded by the genes of the DNA and responsible for the RNA synthesis.
Prokaryotic RNA polymerase:
- Prokaryotic organisms have only one type of RNA polymerase enzyme. Which is composed of several multiple sub-units of approximately 480kDa protein
- So in E.coli which prokaryote the core RNA polymerase enzyme is composed of 5 subunits which are α,α,β, β’,ω and σ which make this enzyme holoenzyme.
Function of the sub-units of RNA polymerase:
- Determination of DNA which is to be transcribed is take place by the α, α sub-units.
- β sub-unit catalyze polymerization
- binding and opening of DNA strand take place by the β’ sun-unit
- ω its function is not know till now
- promotor site which is initiator region is recognized by the σ sub-unit.
Eukaryotic RNA polymerase:
- Eukaryotic organisms have total 5 types of enzymes RNA polymerase. Each type have multiple subunit and each type is responsible for the transcription of different type of RNA like mRNA, tRNA, rRNA, SnRNA and pre-mRNA.
Function of types of RNA polymerase
- In the synthesis of pre-mRNA polymerase one type take part and pre-mRNA than mature into different sizes like (5.8s, 18s, and 28s)
- Synthesis of small nuclear RNA and microRNA and pre-mRNA is take place by the polymerase type two
- Synthesis of transfer RNA and ribosomal RNA of the size of 5S take place by the RNA polymerase type 3
- Synthesis of the small interfering RNA in the plants take place by the type 4 of RNA polymerase.
- in plants during heterochromatin formation RNA is involved this RNA synthesis take place by RNA polymerase type 5.
Recognition of origin of transcription:
Every region which is able to transcribe is called operon. And operon include genes and the up-stream regions which are actually regulatory region these collectively called promotor region.
- In prokaryotes polycistronic transcript are commonly present like one promotor is sufficient For genes.
- In eukaryotes mRNA are commonly monocistronic like one promotor require for one gene.
In prokaryotes origin of transcription:
- The binding site of RNA polymerase and the recognition region RNA polymerase is -35 region of TTGACA.
- A stable complex of DNA and RNA polymerase formation take place at the -10 region of TATAAT
In eukaryotes origin of transcription:
- For proper binding of RNA polymerase there are specific DNA sequence on the promotor site.
- There are several transcriptional factors which are proteins that produce RNA polymerase also attach to the specific promotor region.
- In eukaryotic organism there are several transcriptional factors which have their specific repressor and activator which are also attach to the promotor part and involve in the regulation gene expression.
Three stages of transcription:
- Initiation stage
- Elongation stage
- Termination stage
- In prokaryotic organism for RNA polymerase to bind do not require co-factor it directly bind to the DNA template.
- In eukaryotic organism RNA polymerase require specific co-factor for binding with the DNA template in the process of transcription.
Transcription process in prokaryotes:
Initiation stage:
this stage involve in the recognition of promotor region and followed by the start of transcription process. Purine triphosphate is the first nucleotide on the transcript of RNA
elongation stage:
in this process free nucleotides triphosphate keep on adding in the growing RNA strand exactly according to the sequence, in the transcription bubble. Nucleotides keep on adding at the rate of 50 nucleotide per second this is for bacterial RNA polymerase.
Termination stage:
In this stage RNA polymerase enzyme stop its movement along the DNA template strand and than the newly manufactured RNA move out of the transcription bubble and it is now separated from DNA template.
Transcription process in eukaryotes:
Initiation:
this phase require promotor region and the other upstream regulatory region this is called cis-element
cis-element:
- Enhancer region
- It has GC box
- It has CAAT box
- And TATA box which is also called hogness box
Elongation:
this stage is similar to the prokaryotic elongation phase.
But the contrast is that unlike prokaryotes, in eukaryotes transcription and translation process take place in separate compartment which is separated by nuclear membrane.
Termination:
process of transcription stop in eukaryotes when the sequence of AAUAAA is reached and following is then the poly A tail on the 3’ end of nascent RNA strand.
mRNA splicing:
this is the process of removing intron region from primary mRNA and leaving behind only exon region. This process makes mature mRNA which is shorter in size than the actual DNA strand.
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