WELCOME TO PRESENTATION
PRESENTED BY
Dr. Md. Ariful Karim
MD (Biochemistry)
Phase- A Resident, Year- ii
Sir Salimullah Medical College Mitfort Hospital

TOPIC: RNA SYNTHESIS,
PROCESSING
AND
MODIFICATION

Gene is the functional unit of DNA,
composed of coding region and it’s
regulatory sequence that carry genetic
information encoded within the base
sequence of coding region.

Coding region of gene
Coding region of a gene is a mosaic of exons and
introns.
 Exons are discrete unit of DNA within the coding
segments which are expressed.
 Introns are interposed between exons within the
coding segments of DNA that are transcribed but
not expressed.

Regulatory Sequences
Coding region of a gene is flanked by
regulatory sequence.
 Promoter sequence
 Terminator sequence
 Enhancer
 Silencer

Regulatory sequences of gene
 Promoter:
It consist of TATA box, CAT box, GC box.
It lies towards the 5’ end of gene.
It initiates transcription.
 Terminator:
It lies towards the 3’ end of gene.
It terminates the transcription.

Regulatory sequences of gene
 Enhancer
It lies in upstream or downstream or within
the coding region.
It accelerates the transcription.
 Silencer:
It lies in upstream or downstream or within
the coding region.
It suppresses the transcription process.

Transcription

Transcription
 Synthesis of RNA according to the base
sequence of template DNA.
 Transfer of genetic information from DNA
to RNA.
 All types of RNAs are copied from DNA by
transcription.

TRANSCRIPTION

Requirements
 Activated nucleotide triphosphate.
 DNA template
 RNA polymerase
 Initiating nucleotide
 RNAP associated proteins and enzymes
 Magnesium ion ,manganese.

 TATA, or Hogness box and different core
promoter elements such as Inr (initiator) or
DPE(downstream promoter element) are
present as promoter sequences.


 These sequences serve as binding sites for
proteins known as general transcription
factors (GTFs). Which in turn interact with
each other and with RNA pol II.

General transcription factors
 Recognition of the promoter
 Recruitment of RNA pol II to the promoter
 Initiation of transcription

General transcription factors
 TFIID,
TATA-binding protein and TATA-associated
factors,
recognizes and binds the TATA box (and other
core promoter elements).
 TFIIF,another GTF, brings the polymerase to
the promoter.
 The helicase activity of TFIIH melts the DNA,
and its kinase activity phosphorylates
polymerase

Criteria of transcription
 Totally conservative process
 Asymmetric
 Highly selective process
 No primer is needed.
 DNA template is read from 3’ to 5’ direction
and synthesized from 5’ to 3’ direction.

 RNA polymerase has no proof reading
property.
 It is a process of less fidelity.
 Primary transcript needs extensive post
transcriptional modification.
Criteria of transcription

Types of RNA polymerase
 RNA polymerase I :trascribes rRNA
(28s,18s,5.8s)
 RNA polymerase II :hn mRNA,Sn RNA,
Sc RNA
 RNA polymeraseIII :tRNA, 5s rRNA

 Transcription unit
It is the segment of gene between
promoter sequence and terminator
sequence.
Transcription unit includes signal for
transcription initiation, elongation,
termination.

 Primary transcript
Initial linear copy of a transcription unit.
Primary transcript of all RNA are modified
to mature RNA.

 Transcription bubble
It is the transiently melted DNA that exposes
the single strand template for bonding of
RNAP.
As RNAP moves, transcription bubble also
moves.

 TSS(Transcription start site)
it is the first nucleotide of a gene to be transcribed. It
is designated as +1.

Initiation of transcription
 TFIID containing TBP and TATA-associated
factors
recognizes and binds the TATA box
 TFIIF brings the polymerase to the promoter
 TFIIH melts the DNA, and its kinase activity
phosphorylates polymerase
 Binding of initiating nucleotide with with RNAP

Elongation
5’
3’
5;
3’
Ribonucleotides

Termination
 Rho-independent termination
forming a hairpin like loop rich
in C-G facilitating separation of
newly synthesized RNA.
 Rho dependent termination
ATP dependent helicase activity
of
Rho-protein separates RNA-
DNA hybrid helix.

Post transcriptional modification
Primary transcript is the initial linear RNA
copy of a transcription unit. The primary
transcripts of tRNA and rRNA are post
transcriptionally modified by
cleavage of the original transcripts by
ribonucleases. tRNAs are then further modified
to give it’s unique identity.

Post transcriptional modification of
rRNA
 rRNAs of eukaryotic cells are generated from
a single long precursor molecules called pre-
rRNAs.28S, 18S, and 5.8S rRNA of eukaryotes
are produced from a single pre-rRNA
molecule.Eukaryotic 5S rRNA is synthesized
by RNA pol III.

Post transcriptional modification
of rRNA

 Sequences at both ends of the molecule are
removed.
 Intron is removed from the anticodon loop by
nucleases.
 addition of a –CCA sequence to the 3’ end.
 modification of bases at specific positions to
produce the “unusual bases” characteristic of
tRNA
Post transcriptional modification of
tRNA

Post transcriptional modification of
tRNA

Post transcriptional modification
mRNA
 Primary transcripts synthesized in the nucleus
by RNA pol II is known as heterogeneous
nuclear RNA (hnRNA). The pre-mRNA
components of hnRNA undergo extensive co-
and post transcriptional modification in the
nucleus.

5’ “Capping”
 The cap is a 7-methylguanosine attached to the 5 -
terminal end of the mRNA.
 Addition of guanosine monophosphate by the
nuclear enzyme guanylyltransferase.
 Methylation of this terminal guanine occurs in the
cytosol and is catalyzed by
guanine-7 methyltransferase.
Importance:The addition of this 7-methylguanosine
cap helps stabilize the mRNA and permits efficient
initiation of translation

Addition of poly-A tail
 It is done by trimming at 3’ end followed by
addition of 40-250 Adenine nucleotide
catalyzed by poly-A polymerase.
Importance:
 Stabilize the mRNA
 Facilitates the exit from nucluus.
 And aid in translation.

Splicing
(Removal of introns and joining the
exons)
 Mature mRNA consist of exons only by grace
of splicing.

Mechanism of splicing
 SnRNP=Protein+SnRNA
 ScRNP=Protein+ScRNA
 SnRNP+ScRNP=Ribozyme
 Ribozyme+hn mRNA= Spliceosome
 Spliceosome removes the introns from hn
mRNA and joins the exons.

Importance of splicing
 Mature mRNA consist of exons only by grace of
splicing.
 Make uninterrupted genetic code.
 After splicing, mature mRNA molecules leave the
nucleus and pass into the cytosol.

Reverse Transcription
 Reverse transcription is the process in cells
by which an enzyme makes a copy of DNA
from RNA. The enzyme reverse transcriptase
is found in retroviruses, such as the human
immunodeficiency virus (HIV).

Thank you

Questions on RNA synthesis,
processing and modification
 Define transcription. Give the features of transcription.
 What is transcription factor?
 Write in short about post transcriptional modification.
 Briefly state the post transcriptional modification of
mRNA and tRNA.
 Briefly discuss the importance of post transcriptional
modification.
 Enumerate the regulatory sequence of a gene with
their functions. What is enhancer, promotor ?
 What is reverse transcription?