Transcription is the process by which a DNA strand is copied into RNA. It involves three main phases - initiation, elongation, and termination. Initiation begins when RNA polymerase binds to the promoter region of DNA and separates the strands to form a transcription bubble. Elongation then occurs as RNA polymerase reads the template strand and builds a complementary RNA chain. Termination happens when a termination signal is reached, causing the RNA transcript to detach from the polymerase. The main enzyme involved is RNA polymerase, which synthesizes RNA using a DNA template.

1. Transcription
Presented by: Ahtisham Ali
Riphah International University

2. INTRODUCTION:
The copying process, during which a DNA strand serves
as a template for the synthesis of RNA, is called
transcription.
Transcription produces messenger RNAs (mRNAs) that
are translated into sequences of amino acids,
polypeptide chains or proteins.

3. PROCESS:
 The process of transcription of a typical gene can be
divided into three phases:
• Initiation
• Elongation
• Termination

4. 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'
direction, adding each new nucleotide to the 3' end of the strand.

5. Properties of RNA Polymerase:
 RNA Polymerase is a multi-subunit enzyme that recognizes
a nucleotide sequence (the promoter region) at the
beginning of a length of DNA that is to be transcribed.
 It next makes a complementary RNA copy of the DNA
template strand.
 Then recognizes the end of the DNA sequence to be
transcribed (the termination region).

6. “RNA polymerase binds to a
sequence of DNA called
the promoter region, found near
the beginning of a gene”.
INITIATION

7. Initiation Process: Each gene (or group of
co-transcribed genes, in
bacteria) has its own
promoter region.
 Once bound, RNA
polymerase separates
the DNA strands,
providing the single-
stranded template
needed for transcription.

8. Promotor Regions:
 –35 Sequence:
A consensus sequence (5'-TTGACA-3'),
centered about 35 bases to the left of
the transcription start site is the initial
point of contact for the holoenzyme
and a closed complex is formed.
 Pribnow Box:
The holoenzyme moves and
covers a second consensus
sequence (5'-TATAAT-3'),
centered at about –10,
which is the site of initial
DNA melting (unwinding).

9. “Melting of a short stretch
(about 14 bases) converts the
closed complex to an open
complex known as a
transcription bubble.”
ELONGATION

10.  One strand of DNA,
the template strand,
acts 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 a
chain that grows from
5' to 3’.

11. Elongation Process:
 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).
 As with replication, transcription is always in the 5'→3' direction.
 In contrast to DNA polymerase, RNA polymerase does not require a
primer and does not appear to have proofreading activity.

12. “The elongation of the single-
stranded RNA chain continues
until a termination signal is
reached.”
TERMINATION

13.  Sequences called terminators signal that the RNA transcript is
complete. Once they are transcribed, they cause the transcript to
be released from the RNA polymerase.
 An example of a termination mechanism involving formation of a
hairpin in the RNA is shown below:

14. Eukaryotic Modifications:
 In bacteria, RNA transcripts can act as messenger
RNAs (mRNAs) right away. In eukaryotes, the transcript of a
protein-coding gene is called a pre-mRNA and must go through
extra processing before it can direct translation.
 Eukaryotic pre-mRNAs must have their ends modified, by
addition of a 5' cap (at the beginning) and 3' poly-A tail (at the
end).
 Each eukaryotic RNA polymerase has its own promoters and TFs
that bind core promoter sequences.

15.  Many eukaryotic pre-mRNAs undergo splicing. In this
process, parts of the pre-mRNA (called introns) are chopped
out, and the remaining pieces (called exons) are stuck back
together.

16. Animation Demonstration:

18. References:
 Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., and Walter, P. (2002).
Posttranscriptional controls. In Molecular biology of the cell (4th ed.). New York, NY:
Garland Science. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK26890/.
 Berger, Shanna. (2006). Eukaryotic transcription. In Transcription and RNA polymerase II.
Retrieved
from http://www.chem.uwec.edu/Webpapers2006/sites/bergersl/pages/eukaryotic.html.
 Boundless (2016, January 8). Initiation of transcription in eukaryotes. In Boundless biology.
Retrieved from https://www.boundless.com/biology/textbooks/boundless-biology-
textbook/genes-and-proteins-15/eukaryotic-transcription-108/initiation-of-transcription-
in-eukaryotes-445-11670/.