This document discusses post-transcriptional modification of RNA in eukaryotic cells. It involves three major steps: capping, tailing, and splicing. Capping adds a modified guanine to the 5' end of mRNA and is necessary for protein synthesis. Tailing adds a polyadenine tail to the 3' end for stability. Splicing removes introns in the nucleus and joins exons to form mature mRNA using a spliceosome complex. Post-transcriptional modification chemically alters the primary RNA transcript to produce a functional molecule that can leave the nucleus.
2. TRANSCRIPTION
• Transcriptionis the first step in the process of using the genetic code in DNA to synthesize or build
all the different proteinsin body
• One problem with synthesizing these proteinsis that the instructionsfor making them are in DNA which
is located inside the nucleus
• DNA to ribosome DNA uses mRNA to carry the genetic code from the nucleus to ribosomes
• The process of buildingthis mRNA is called transcription
3. DIFFERENCE BETWEEN PROKARYOTES & EUKARYOTES
TRANSCRIPTION
• Prokaryotic transcriptionoccur in the cytoplasm
• There is only one type of RNA polymerase
enzyme in prokaryotic transcription and it helps
to synthesise all the other types of RNA in the
cells ( mRNA, tRNA, and rRNA).
• Sigma factor & Rho factor is required
in prokayotictranscription
• Eukaryotictranscriptionoccur in the nucleus.
• Eukaryotictranscriptioninvolvesthree types of
RNA.
• There is RNA Polymerase I that helpsin the
rRNA synthesis.
• RNA Polymerase II for mRNA.
• RNA Polymerase III that aidsin the synthesis
of tRNA.
4.
5.
6. POST TRANSCRIPTIONAL MODIFICATION
• Post-transcriptional modification is a set of biological processes common to most eukaryotic cells by
which an primary RNA transcript is chemically altered following transcription from a gene to produce a
mature, functional RNA molecule that can then leave the nucleus and perform any of a variety of
different functions in the cell.
• There are 3 majorsteps involvein Post-transcriptionalmodificationthat significantlymodify the
chemical structure of the RNA molecule: capping, tailing & splicing .
7. CAPPING
• Capping occur at 5′ end and modifed guanosine, 7- methyl guanosine is added to the 5′ end . The cap
is added by the enzyme guanyltransferase. This enzyme catalyzes the reaction between the 5′ end of
the RNA transcript and a guanine triphosphate(GTP) molecule.
• Additionof an extra nucleotideat the 5′ end of the mRNA.
• Methylationto the base (at position7. so, 7-methyl) in the newly added neucleotide.
• Methylation(may be) to the 2’–OH group of the sugar of one or more nucleotides at the 5′ end (second
and third Sugars
• It is necessary for the mRNA to bind with the ribosome to begin protein synthesis (Cap bindingproteins
first identify the cap and attach to it; a ribosome then bindsto these proteinsand moves downstream
along the mRNA until the start codon is reached and translation begins).
8. TAILING
• Tailing occur at 3' end and adenosine mono phosphate is added for stability and to form poly
tail A
• The poly(A) tail confers stability on manymRNAs, increasing the time during which the mRNA
remains intact and available for translation before it is degraded by cellular enzymes.
• The addition of the adenines is catalyzed by the enzyme poly (A) polymerase.
9. SPLICING
• RNA splicing is the process by which introns, regions of RNA that do not code for proteins, are removed
from the pre-mRNA and the remaining exons connected to re-form a single continuousmolecule.
• Splicing occurs in the nucleus following transcriptionbut before the RNA moves to the cytoplasm.
• RNA splicing takes place in nuclearparticles known as spliceosomes. These abundantparticles are
composed of protein and several types of specialized small nuclearRNA (snRNA) molecules
• These intronsare removed from the primary transcript in the nucleus, exons (coding sequences) are
ligatedto form the mRNA molecule, and the mRNA molecule is transported to the cytoplasm.
• The molecularmachine that accomplishes the task of splicing is known as the spliceosome.
