2. contents
❑ Introduction
❑ Features of transcription
❑ Site of transcription
❑ Transcription unit
❑ Stages of transcription
❑ Difference between prokaryotic and Eukaryotic
❑ Conclusion
3. Introduction
Transcription is the process of copying a segment of DNA into RNA .
Transcription is performed by enzymes called RNA polymerases ,
which link nucleotides to form a RNA strand (using a DNA strand as
a template).
4. Features of transcription
Synthesis of All types of RNA .
Only one strand of DNA participates.
Ribonucleotides are used in RNA synthesis .
RNA synthesis occurs in 5’-3’ direction. DNA template is read
from 3’-5’ direction.
Synthesis follows waston-crick base pairing rules –A to U, G to C .
Enzyme - DNA dependent RNA polymerase or RNA polymerase.
5. RNA polymerase
The main enzyme involved in transcription is RNA polymerase,
which uses a single-stranded DNA template to synthesize a
complementary strand of RNA .
Specifically, RNA polymerase builds an RNA strand in the 5’ to 3’
end of the strand.
6. Site of transcription
❖ Prokaryotes - cytoplasm (all RNAs)
❖ Eukaryotes – Nucleus & mitochondria
a) Nucleolus – rRNA
b) Nucleoplasm – tRNA & mRNA
7. Transcription unit
Promoter
Site at which RNA polymerase binds .found at 5’-end (up
stream) of structural gene .
Structural gene
Main part which code for RNA .
Terminator
Site at which transcription stops.
Found at 3’ – end(down stream) of structural gene.
9. a) Initiation
RNA polymerase binds to a sequence of DNA called the
promoter , found near the beginning of a gene .( each gene
has its own promoter)
Once bound , RNA polymerase separates the DNA strands ,
providing the single stranded template needed for
transcription .
11. b) Elongation
One strand of DNA , the template strand act as a template for
RNA polymerase .
As it “reads” this template one base at a time , the polymerase
builds an RNA molecule out of complementary nucleotides ,
making chain that grows from 5’ to 3’.
The RNA transcript carries the same information as the non-
template(coding) strand of DNA , but it contains the base
uracil(U) instead of Thymine(T).
13. c) Termination
Sequence called terminators signal that the RNA transcript is
complete . Once they are transcribed , they cause the
transcript to be released from the RNA polymerase .
14. Difference between prokaryotic &
eukaryotic transcription
Prokaryotic transcription
▪ Occurs in the cytoplasm.
▪ There is no definite phase for
its occurrence .
▪ A single RNA polymerase
synthesizes all the three type of
RNA(mRNA, tRNA, rRNA).
▪ Coupled transcription
translation is the rule.
▪ RNAs are released and
processed in the cytoplasm.
Eukaryotic transcription
▪ Occurs in the nucleus.
▪ Takes place in the G1 and G2
phases of cell cycle.
▪ Three RNA polymerases I,II and
III synthesize rRNA ,mRNA,
tRNA respectively.
▪ Coupled transcription
translation is not possible .
▪ RNAs are released and
processed in the nucleus.
16. Conclusion
Transcription is the first step in gene expression , in
which information from a gene is used to construct a
functional product such as protein .
The goal of transcription is to make a RNA copy of
gene’s DNA sequence .
