DNA Transcription Process: Steps And Detailed Facts

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The process in which the mRNA molecule is synthesized from a DNA strand, it is called Transcription.

DNA transcription process is a complicated mechanism in which a DNA portion is converted or copied into a RNA molecule. The central dogma is a complicated process in which the DNA is transformed into a messenger RNA molecule and the messenger RNA is translated into a protein molecule.

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What is the DNA transcription Process?

Every cell in every organism needs to be repaired and rejuvenated at one point of time.

DNA transcription is a process in which a segment of the DNA molecule is transcribed or converted or copied into a RNA molecule to be more precise the DNA molecule get copied to a messenger RNA molecule.

This is called the DNA transcription process.

DNA (segment) gets converted to a messenger RNA molecule.

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dna transcription process
DNA transcription Process
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What is the DNA transcription process in prokaryotic organisms?

The DNA transcription process in prokaryotes is quite simple and it is of 3 stages or steps.

DNA transcription process is a complicated mechanism in which a DNA portion is converted or copied into a RNA molecule-Messenger RNA with the help of an enzyme called RNA polymerase.

Initiation of the DNA transcription process:

  • Initiation is a process in which the process starts.
  • This process is initiated at a particular segment of DNA sequence which is commonly called the promoter region.
  • The enzyme RNA polymerase incorporates Ribo- nucleotides to the template DNA strand.

Initiation consists have 3 types

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RNA Polymerase
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Closed complex

  • Closed complex is a stage in which the RNA polymerase adheres or bind to the promoter region of a DNA sequence.
  • Like in DNA replication which uses the enzyme DNA polymerase, RNA polymerase does not need any enzyme like primase to initiate its desired process.

Open complex

  • Open complex is the stage when the enzyme that is held together to the DNA sequence unwinds or release the double stranded DNA into a single strand at a certain or particular portion, this is called the open complex.
  • The incorporation of rNTPs or the ribo-nucleotides to the template DNA will be done.
  • The final product will be short stretches of RNA strand produced complementary to the template DNA.

Abortive initiation

Elongation of the DNA transcription process:

  • The sigma factor in the RNA polymerase will be freed from the complex and the elongation of the RNA strand complementary to the template DNA strand will be performed.
  • Elongationis a process in which the biosynthesis of an RNA strand happens,the strand will be elongating by the addition of rNTPs or ribo- nucleotides.

Termination of the DNA transcription process:

There has to be an end point to end this transcription process and this is called the termination process or the end process.

DNA transcription
Transcription
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The prokaryotic termination process will be of 2 types.

Rho independent:

  • The other name of the Rho independent is  intrinsic terminators.
  • This kind of termination  does not need any external proteins.

This consist of 2 sequence elements, they are

Short inverted repeats

Here are few examples of inverted repeats

Example – I

– – – – – – – – G T A G C A T T C G G – – – – – – – – – C C G A A T G C T A C – – – – – –

– – – – – – – – C A T C G T A A G C C – – – – – – – – – G G C T  T A C G A T G – – – – – – –

Example – II

– – – – – – – – T A G C A T T C G G T- – – – – – – – A C C G A A T G C T A- – – – – – –

– – – – – – – – A T C G T A A G C C A- – – – – – – – T G G C T T A C G A T- – – – – – –

Example – III

– – – – – – – – G T A G C A T T C G G T- – – – – – – – A C C G A A T G C T A C- – – – – – –

– – – – – – – – C A T C G T A A G C C A- – – – – – – -T G G C T T A C G A T G- – – – – – –

So from the above examples, we can see that in the 2 strands of the DNA the nitrogenous bases are repeated in the inversion form on its complementary strand with minimal gaps (having other nitrogenous bases).

A T rich sequence

– – G T A G C A T T C G G – – – – – – – – C C G A A T G C T A C – – – – A A A A

– – C A T C G T A A G C C – – – – – – – – G G C T  T A C G A T G – – – – T T T T 

  • A T rich sequence is a sequence in which continuously the strand has adenine that complements thymine or vice versa.
  • First the inverted repeated sequences will be present and then they are followed by AT rich sequences.
  • Now due to this inverted sequence and AT rich sequence at the terminal point, a hairpin or inverted U shape like structure will be produced by base pairing in the same strand which then stops the complete process.
  • Then A T rich sequence becomes A U rich sequence as in RNA thymine is replaced by uracil.
  • Among all the base pairs ( A – T ; T – A ; C – G ; G – C ; A – U; U – A), the weakest pair will be  A-U or U – A.
  • Due to this weak pairing element, this is the terminal point and the RNA will be released or freed and the process is terminated.

Rho dependent:

  • The Rho dependent termination process will need a Rho protein to end the process of transcription.
  • This Rho protein is a hexameric protein and needs Adenosine triphosphate molecules for its action.
  • This Rho protein adheres to the RNA sequence which has more number of cytosine or C, which are also known as Rho utilization sites or the RUT site.

– – – – – – – – C C C C C C – – – – – – – 

  • This will eventually become the Rho sensitive pause site and unwinds the region.
  • Now the final product will be a strand of RNA that is let out from the template DNA, this is the process of prokaryotic DNA transcription.

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What is the DNA transcription process in eukaryotic organisms?

The transcription process is quite complex in eukaryotic organisms.

The eukaryotic organism transcription has 3 steps: Initiation, elongation and termination.

Transcription bubble
Eukaryotic transcription
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Initiation of the eukaryotic transcription process:

  • In this process, 3 RNA polymerase (RNA polymerase I,RNA polymerase II,RNA polymerase III) is used. More importantly RNA polymerase II contributes more in the eukaryotic transcription process.
  • There are few transcription factors involved which are called the TF.

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The types of TFs are

The eukaryotic promoter is about 40 nucleotides long located in the upstream and downstream in the sequence.

Regulation of transcription in mammals
Transcription factors with their associated proteins
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TFIID :

  • Initially, the transcription factor TFIID binds to the TATA element of the promoter.
  • TFIID produces a protein called the TATA binding protein OR TBP and this protein binds to the TATA sequence resulting in the bending of the sequences to about 80 degree.
  • This bending helps in the adhesion of other factors like TFIIA and TFIIB.

TFIIA :

TFIIA aids in stabilizing the binding of TFIID into the promoter sequences.

TFIIB :

  • TFIIB interacts with TBP or the TATA binding proteins at the promoter region downstream to the sequence.
  • The TFIIB aids in the recruitment of the RNA polymerase II on the promoter as the RNA polymerase can not bind on its own.

TFIIF :

TFIIF aids in the binding of RNA polymerase II into the promoter sequence.

TFIIE :

TFIIE helps to start the preinitiation complex.

TFIIH

  • TFIIH is a large complex consisting of 9 subunits, 2 subunits have ATPase activity which provides energy and it acts like helicase and unwinds the DNA.
  • This results- From preinitiation complex to open complex.

The other 7 subunits acts as a kinase activity and binds to the RNA polymerase tail and moves to the next step elongation.

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Elongation of the eukaryotic transcription process:

  • The elongation factors are the components that aid in the elongation process.
  • TFEB is a kinase protein and undergoes phosphorylation which stimulates elongation and TFIIS is like a catalyst which aids in the fastening of the elongation process by not letting the RNA polymerase to pause.
  • 5′ capping is a process in which certain enzymes add and delete few compounds that aid in the initiation of the translation process.

Termination of the eukaryotic transcription process:

  • The RNA polymerase reaches the end point, 2 proteins called the CstF and CPSF interact with the transcription complex and cleaves the messenger RNA from the complex.
  • Every component from the complex dissociates.
  • CPSF then adds or recruits poly A polymerase which adds about 200 bases of adenine at the 3 ‘ end resulting in the poly A tail by utilizing ATP and the process stops.

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