TRANSCRIPTION

TRANSCRIPTION:

‘The process of copying genetic information from one strand of the DNA into RNA is termed as transcription’.
Transcription vs. Replication:

·         Principle of complementarity governs the process of transcription except Adenosine of DNA forms base pair with the Uracil instead of thymine. During replication Adenine pairs with thymine instead of uracil.

·         During replication once started the whole DNA is duplicated, where as transcription takes place only a segment of DNA.

·         In replication both strand acts as template, where as in transcription only one strand is acts as template to synthesize RNA.

·         In replication DNA copied from a DNA, where as in transcription RNA copied from the DNA.

Why both strands of DNA not copied during transcription:

·         If both strand of DNA acts as template, they would translated into two RNA of different sequences and in turn if they code for proteins, the sequence of amino acids in the protein would be different. Hence one segment of DNA would be coding for two different proteins.

·         The two RNA molecules if produced from simultaneously would be complementary to each other, hence will form double stranded RNA. This would prevent RNA translation into protein.

Transcription unit:

·         A transcription unit in DNA consists of three regions:

·         A promoter

·         The structural gene

·         A terminator.

·         DNA dependent RNA polymerase catalyses the polymerization in only one direction that is 5’→3’.

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Structural gene:

·         The DNA strand having polarity 3’→5’ is called template strand for transcription.

·         The other strand of DNA having polarity 5’→3’ is called coding strand.

·         The sequences of nitrogen base in the RNA transcribed from the template strand are same as the coding strand of DNA except having Thymine in place of Uracil.

·         All the reference point defining a transcription unit is made with the coding strand only, not the template strand.

 Promoter:

·         Promoter and Terminator present on either side of structural gene.

·         The promoter located towards 5’ end (upstream) of the structural gene.

·         It is a short sequence of DNA that provides binding site for RNA polymerase. (mostly TATA , Commonly called TATA box)

·         Presence of the promoter defines the template and coding strands.

·         If the position of promoter is changed with terminator the definition of coding and template strand will be reversed.

Terminator:

·         The terminator located towards 3’ end (down stream) of coding strand.

·         It terminates the process of transcription.

·         It is also a short segment of DNA which recognizes the termination factor. (ρ-factor)

Transcription unit and the gene:

·         Gene is defined as the functional unit of inheritance.

·         Genes are located on the DNA.

·         The DNA sequence coding for tRNA and rRNA molecule also define a gene.

·         Cistron: a segment of DNA (structural gene) coding for a polypeptide.

·         Monocistronic: most of eukaryotic structural gene codes for single polypeptide.

·         Polycistronic: Most prokaryotic structural gene code for more than one polypeptides.

·         In eukaryotes the monocistronic  structural gens have interrupted coding sequences, the genes are said to be split gene:

o    The coding sequences or expressed sequences are called Exons.

o    Exons are interrupted by Introns.

·         Exons are said to be those sequences that appear in mature or processed mRNA.

·         Introns never appear in mature of processed mRNA. They are spliced out.

Types of RNA:

·         In prokaryotes there are three major types of RNAs: mRNA (messenger), tRNA (transfer), and rRNA(ribosomal).

·         All three RNAs are required to synthesize protein in a cell.

·         The mRNA provides the template and having genetic information in the form of genetic code.

·         The tRNA brings the amino acids and read the genetic code of mRNA.

·         The rRNA is the structural part of the ribosome and also as catalytic role during process of translation.

Process of transcription: prokaryotes.

·         There is a single DNA dependent RNA polymerase that catalyses transcription or synthesis of all three types of RNAs in prokaryotes.

·         The process of transcription completed in three steps:

Initiation:

·         RNA polymerase binds to the specific site of DNA called promoter.

·         Promoter of the DNA is recognized by initiation factor or sigma (σ).

·         RNA polymerase along with initiation factor binds to the promoter.

 

  Elongation:

  • RNA polymerase unzipped the DNA double helix and forms an open loop.
  • It uses ribonucleoside triphosphates as substrate and polymerizes in a DNA template following the rule of complementarity.
  • Only a short stretch of polymerized RNA remains binds with the enzyme.
  • The process of polymerization continued till the enzyme reaches the terminator gene.

Termination:

  • RNA polymerase recognizes the terminator gene by a termination-factor called rho (ρ) factor.
  • The RNA polymerase separated from the DNA and also the transcribed RNA.

Additional complexities in eukaryotes:

  • There are three different types of RNA polymerases in the nucleus:
    • RNA polymerase I transcribes rRNA (28S, 18S, and 5.8S)
    • RNA polymerase II transcribes heterogeneous nuclear RNA (hnRNA).
    • RNA polymerase III transcribes tRNA, 5srRNA and snRNA.
  • Post transcriptional processing: (occurs inside the nucleus)

(a) Splicing:

  • The primary transcript (hn RNA) contain both exons and introns and required to be processed before translationally active (mRNA).
  • The introns are removed and exons are joined in a defined order.
  • This process is catalyzed by SnRNP, introns removed as spliceosome.

(b) Capping: an unusual nucleotide called methyl guanosine triphosphate is added to the 5’ end of hnRNA.

(c) Tailing: Adenylate residues (200-300) are added at 3’ end of hnRNA in a template independent manner.

The processed hnRNA is now called mRNA and transported out of the nucleus for translation.

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