Transcription and post-transcriptional modification

by Abhishek Dahal

rRNA: ribosomal RNA Major component of ribosomes

tRNA: transfer RNA Carries amino acids to mRNA An interpreter in translation

mRNA: messenger RNA Encodes genetic info from DNA and convey it to the ribosomes, where the info is translated into amino acid sequences

Transcription Process in which RNA is synthesized from DNA. DNA expresses through RNA 1st product of the RNA formed is called

TRANSCRIPT which is further altered to produce functionally active RNA molecules.

Synthesis of RNA occurs in 5´→3´ direction Nucleotides ATP,GTP,CTP,UTP are necessary Each nucleotide pairs with the complementary

nucleotide on the DNA template.

Stages of Transcription

Promoter Recognition Chain Initiation Chain Elongation Chain Termination

promoter recognition

Transcription factors bind to promoter sequences and recruit RNA polymerase.

Then, RNA polymerase denatures a small segment of the DNA.

Promoter region which consist of "TATA box" and "-35 sequence" where Transcription (sigma ) factor of polymerase enzyme act.

Transcription: chain initiation

Chain initiation: RNA polymerase locally denatures the DNA. RNA polymerase does not require a primer. The first 8 or 9 bases of the transcript are

linked. Transcription factors(sigma) are released, and the polymerase leaves the promoter region.

Transcription: chain elongation

Chain elongation: RNA polymerase moves along the

transcription unit of DNA strand. The new RNA molecule (primary

transcript) forms a short RNA-DNA hybrid molecule with the DNA template.

Transcription: chain termination

Rho(Ƥ) dependent termination : here Rho factor binds to the RNA or Weakly to DNA causing Polymerase to release and mRNA is formed.

Rho(Ƥ) independent termination via a molecule called as Philandromes that can form a hairpin loop and release of mRNA and polymerase occurs.

All have the same mechanism of action.

Recognize different type of promoters.

Eukaryote cells

Polymerase I produces most of the rRNAs

Polymerase II produces most of the mRNAs

Polymerase III produces small RNAs such as tRNA and 5 S RNA.

Post transcriptional modification

Prokaryotes: mRNA transcribed directly from DNA template and used immediately in protein synthesis

Eukaryotes: primary transcript (hnRNA) must be processed to produce the mRNA (active form).

It occurs in nucleas of cell.

4 process End modification:

5´-a 7 methyl guanosine cap is added

3´- a poly adenosine(poly A) tail is added

These capping and tail addition stabilises the RNA which is also required for translation.

Splicing Noncoding sequences (introns) are

removed Coding sequences (exons) spliced

together The removal is achieved by Small

nuclear ribonuclear protein (SnRNP).

Mutations at the splice sites can cause disease

Thalassemia • Breast cancer (BRCA 1)

mRNA is edited by splicing

Cutting:- here pieces of RNA are cut which are not required( trimming).

Chemical Modification:- new chemical group are added and RNA is modified into active form.

Modification of hnRNA

A cap is added at the 5’ end (cap sidt)Poly-A tail added to the 3” end.Introns are removed by a process called splicing to produce the mature mRNA.

Eukaryotic transcription and translation are separated in space and time

Prokaryotes Eukaryotes

exons intronsnucleus

cytosol

translation

translation

transcription

DNA

transcription

nuclear export

splicing

pre-mRNA

mRNA

processing