Transcription of RNA according to curriculum of Medical Biochemistry of MBBS course

1. TRANSCRIPTION
Dr. Farhana Atia
Associate Professor
Department of Biochemistry
Nilphamari Medical College, Nilphamari

2. FLOW OF GENETIC INFORMATION
The genetic information flows from DNA to mRNA
and then to the protein synthesizing machinery

3. TEMPLATE STRAND
With the exception of T for U changes, coding strand
corresponds exactly to the sequence of the RNA
primary transcript
The information in the template strand is read out in
3’-5’ direction

4. The strand that is transcribed or copied into an RNA
molecule is referred to as the template strand of the DNA.
The other DNA strand is referred to as the coding strand
of that gene.
The template strand for each gene will not necessarily be
the same strand of the DNA
Thus, a double-stranded DNA molecule will serve as the
template strand for some genes and the coding strand of
other genes.

5. TRANSCRIPTION UNIT
• A transcription unit is defined as that region of DNA
that includes the signals for transcription initiation,
elongation, and termination
• DNA-dependent RNA polymerase (RNA-P) is the
enzyme responsible for the polymerization of
ribonucleotides
• The enzyme attaches at a specific site on the template
strand- the promoter

6. TRANSCRIPTION UNIT
Promoters are located a short distance upstream of
the NT where transcription starts

7. PRIMARY TRANSCRIPT
• The RNA product, which is synthesized in the 5' to 3'
direction, is the primary transcript
• The starting point of transcription corresponds to the
5' nucleotide of the mature RNA

8. PRIMARY TRANSCRIPT
• This is designated as position +1, number increases as the
sequence proceeds downstream
• The nucleotide in the promoter adjacent to the
transcription initiation site is designated -1
• These negative numbers increase as the sequence
proceeds upstream, away from the initiation site

9. • RNA Pol I: Synthesizes rRNA
(28s, 18s, 5.8s)
• RNA Pol II: Synthesizes mRNA
and miRNA
• RNA Pol III: Synthesizes tRNA,
rRNA (5s), snRNA

10. Functions:
• Search for promoter sequence
• Unwinding of DNA
• Detection of terminator & ending of
transcription
• Interaction of activator/ repressor
protein to regulate transcription

11. TRANSCRIPTION
• The synthesis of an RNA molecule from DNA
• Requirements-
a) Activated ribonucleotide tri-phosphate
b) DNA template
c) Initiating nucleotide- Tri-phosphate variety of
Adenine/ Guanine.
d) Enzyme- DNA dependent RNA polymerase
e) Transcription factor- A large number of protein

12. GENERAL FEATURES OF
TRANSCRIPTION
• Conservative process- Entire template conserved
• Asymmetric process- Both strand of DNA can not act
as template
• Highly selective- Selective part of DNA strand act as
template
• Initiating nucleotide is required. No need of primer.

13. GENERAL FEATURES OF TRANSCRIPTION
• Template will be copied/read from 3΄5΄ & RNA
synthesis occur from 5΄3΄ direction
• At the end of transcription, nascent product/
primary transcript undergoes post transcription
modification
• RNA-P has no proof reading capacity. It is a process
of less fidelity.
• Involve 3 steps- Initiation, Elongation, Termination.

14. I) INITIATION
• An RNA polymerase along with general transcription
factors binds to the promoter region of the gene to form a
closed complex called the pre-initiation complex (PIC)
• Different classes of sequence elements are found in the
promoters such as TATA box, CAAT, GC box

15. I) INITIATION
• TATA box is usually located 25–30 bp upstream from
the transcription start site.
• General transcription factors for RNA-P II include
TFIID, TFIIA, TFIIB, TFIIF, TFIIE, and TFIIH

16. I) INITIATION
• The TATA box is bound by TATA binding protein (TBP)
• TFIID binds to the TATA box via the TBP, five more
transcription factors and RNA-P combine around the
TATA box in a series of stages to form a pre-initiation
complex (PIC)
• Enhancer binds with activator, combines with PIC,
RNA-P form an initiation complex

17. Enhancers and Repressors
• DNA sequence (located up/downstream)
• that can either increase or decrease the rate of
transcription
• are called either enhancers or repressors (or silencers),
depending on which effect they have
• Enhancers act by binding specific protein factor called
activators

18. II) ELONGATION
• RNA polymerase progresses along the DNA molecule
• Local unwinding of DNA by RNA-P
• Synthesis of 5΄ 3΄ RNA transcript coded by DNA
template

19. II) ELONGATION
• The enzyme add RNT to 3΄ end of growing RNA molecule
& release PP
• Base pairing rule is followed during the incorporation of
ribonucleotides

21. III) TERMINATION
• Termination occurs when a termination signal sequence
appear
• Primary transcript is released
• Most eukaryotic RNAs are synthesized as precursors
that contain excess sequences which are removed prior
to the generation of mature, functional RNA

22. POST TRANSCRIPTIONAL MODIFICATION
Most eukaryotic RNAs are synthesized as precursors that
must be modified to mature functional molecules.
Modifications include-
• Removal of extra nucleotide
• Base modification
• Addition of nucleotide
• Separation of different RNA sequence by the action of
specific nuclease
Finally RNA must be exported from the nucleus

23. Post Transcriptional Modification of rRNA
• rRNA are synthesized from long precursor molecules
called pre-ribosomal RNA (45s).
• These precursors are cleaved by ribonuclease to yield
intermediate sized pieces of RNA
• These are further trimmed to produce required rRNA
species

24. Post Transcriptional Modification of tRNA
• Intron is to be removed from anticodon loop
• Sequence from both 5΄ & 3΄ end is to be trimmed
• Addition of a -CCA sequence by nucleotidyl transferase
to the 3΄terminal end of tRNA
• Modification of bases at specific position to produce
unusual base

25. Post Transcriptional Modification of mRNA
• All primary transcripts synthesized by RNA-P II (known
as hnRNA) are extensively modified after transcription
1. 5' capping: The cap is a 7-methyl
guanosine attached backward by
a tri phosphate linkage to the 5'
terminal end of mRNA. The cap
facilitates the initiation of
translation & stabilize the mRNA
from 5' exonuclease activity.

26. Post Transcriptional Modification of mRNA
2. Addition of poly-A tail at 3‘ end:
• The poly-A tail is a long chain of adenine nucleotides
that is not transcribed from DNA. After transcription it
is added by the nuclear enzyme poly-A polymerase.
• This end helps stabilize the mRNA from 3‘- exonuclease
& facilitates exit of mRNA from nucleus

27. Post Transcriptional Modification of mRNA
3. Removal of introns, exons are spliced
together
Maturation of mRNA involves -
• Removal of RNA sequence (introns/
intervening sequences) that do not
code for protein from the primary
transcript. The remaining coding
sequences, exons are spliced together

28. Post Transcriptional Modification of mRNA
• snRNA & protein form snRNPs.
It facilitates splicing of exon
segments by forming base pair
with each end of introns
• This binding brings neighboring
exons into the correct alignment
for splicing and introns are
excised