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3. INTRODUCTION
The process of copying genetic information from one strand of DNA into RNA is
called transcription. In other words, transcription is the process by which a single-
stranded RNA is formed from a single strand of DNA. The process is similar to that
of replication of DNA and involves the following sequence of events:
1. Uncoiling of two strands of DNA in a specific region. It exposes the bases of the
DNA strands.
2. One strand remains dormant (not participate) and the other one acts as a
template for the formation of the new RNA strand that means only one of the
strands is copied into RNA.
3. The building block-the free ribonucleotides (A, U, C And G) align themselves and
form the complementary RNA according to the base pairing rule, i.e., Adenine (A)
of DNA pairs with uracil (U) of RNA and so on. The reaction is catalyzed by the
enzyme RNA polymerase.
4. RNA POLYMERASE
RNA polymerases are enzymes that transcribe DNA into RNA. Using a DNA template,
RNA polymerase builds a new RNA molecule through base pairing. For instance, if
there is a G in the DNA template, RNA polymerase will add a C to the new, growing
RNA strand. RNA polymerase always builds a new RNA strand in the 5’ to 3’ direction.
That is, it can only add RNA nucleotides (A, U, C or G) to the 3' end of the strand.
In eukaryotes, there are three different RNA polymerase that catalyze the synthesis of
three type of RNA
• RNA polymerase i for ribosomal RNA or rRNA,
• RNA polymerase ii for messenger RNA or mRNA and
• RNA polymerase iii for transfer RNA or tRNA.
Prokaryotes contain only one RNA polymerase that has different subunits that can
perform different functions and catalyze for different kinds of RNA.
5. TRANSCRIPTION UNIT
A TRANSCRIPTION UNIT OF DNA MAINLY CONSISTS OF THREE
REGIONS. THESE ARE:
THE STRUCTURAL GENE IS PRESENT IN THE CENTER WHILE THE
PROMOTER AND THE TERMINATOR REGIONS ARE PRESENT AT THE
TWO ENDS OF THE STRUCTURAL GENE.
A Promoter
The Structural
gene
A Terminator
6. It is present at the beginning towards the 5'- end (upstream) of
the structural gene. (reference is with respect to the coding
strand).
• it is a DNA sequence that provides the binding site for RNA
polymerase.
• It is the presence of a promoter that defines the template and
the coding strands(of structural gene).
• By switching its position with the terminator, the definition of
the template and coding strand would be reversed.
Promoter
7. All though only one strand of DNA is used in transcription, by
convention, the structural gene is defined by both the strands of
DNA.
• As the two strands of DNA have opposite polarity, one is in 5'→
3' direction and the other is in 3' → 5' direction and the DNA
dependent RNA polymerase catalyzes the polymerization only
in 5'→ 3' direction, so the strand that has the polarity 3' → 5'
acts as a template. It is called the template strand.
• The other strand of DNA that actually does not participate and
has the polarity 5'→ 3' is referred as coding strand.
• The Coding strand is displaced during transcription.
• It is called coding strand but actually it does not code for
anything in transcription process. However all reference points
while defining a transcription unit is made with coding strand
Structural gene
8. The terminator is present at the end of the structural gene,
towards the 3'-end(downstream) with respect to the coding
strand.
• Terminator is at the opposite end of the promoter.
• It normally defines the end of the process of transcription.
There may be some additional proteins or regulatory sequences
that may present before (upstream) or after (downstream) to the
promoter.
Terminator
10. Initiation
RNA polymerase
binds to the
promoter and
initiates
transcription does
by associating
transiently with
initiation factor,
Sigma
Termination
When RNA polymerase reaches the
terminator region of DNA, the
newly formed RNA (nascent RNA)
separates and falls off and RNA
polymerase dissociates from DNA.
Since terminator region does not
code for any codon, it results in
termination of transcription.
Actually RNA polymerase binds with
the termination factor, Rho
factor(p) to terminate the
transcription. Though RNA
polymerase is able to catalyse all
three steps given above, iactually
catalyses only the process of
elongation. It associates with sigma
factor or initiation factor to initiate,
and with Rho factor (p) or
termination factor to terminate the
process.
Elongation
It then uses nucleoside
triphosphates (like
adenosine triphosphate
guanosine triphosphate
etc.) and polymerases
forming a template of
RNA following the rule of
complementarity of base
pairing. RNA polymerase
also facilitates opening of
the DNA helix and
continueselongation. As
RNA polymerase
continues elongation, only
a short stretch of RNA
formed remains bound to
the enzyme
11.
12. RNA PROCESSING/FORMATION OF
FUNCTIONAL RNA
IN EUKARYOYES, THE TRANSCRIBED RNA IS KNOWN AS THE
PRE-MRNA. IT IS PROCESSED FURTHER TO CONVERT IT
INTO MATURE RNA. RNA PROCESSING INCLUDE:
Capping Polyadenylation Splicing
13. • A methylated
guanine cap is added
to protect the mRNA.
It involves:
• Addition of
methylated guanine
• It occurs at 5′ end of
mRNA transcript
• It protects the mRNA
from degradation
Capping
• The poly-A tail also
protects the mRNA
from degradation. It
involves:
• The endonucleases
cleave the mRNA at a
specific sequence.
• The enzyme poly A
polymerase
facilitates the
addition of several
adenine nucleotides.
Polyadenylatio
n The non-coding
sequences, i.e., the
introns are removed by
spliceosome excision.
The coding sequences
or the exons join
together by ligation.
Thus several proteins
can be made from a
single pre-mRNA. A
mature mRNA is
obtained at the end of
transcription.
Splicing
