2. CONTENT
Introduction
Basic principle of transcription
Organization of transcriptional unit in prokaryotes
Promoters
RNA coding sequence
Termination
Reference
3. INTRODUCTION
Transcription is the first step in gene expression. It involves copying a
gene's DNA sequence to make an RNA molecule.
Transcription is performed by enzymes called RNA polymerases, which
link nucleotides to form an RNA strand (using a DNA strand as a
template).
Transcription has three stages: initiation, elongation, and termination.
In eukaryotes, RNA molecules must be processed after transcription:
they are spliced and have a 5' cap and poly-A tail put on their ends.
Transcription is controlled separately for each gene in your genome.
4. BASIC PRINCIPLE OF TRANSCRIPTION
RNA is the intermediate between gene and the proteins for which they code.
Transcription is the synthesis of RNA under the direction of DNA .
Transcription produces messenger RNA (mRNA) .
In prokaryotes, mRNA produced by transcription is immediately translated without
more processing.
In eukaryotes, the nuclear envelope separates transcription from translation.
Eukaryotic RNA transcripts are modified through RNA processing to yield finished
mRNA.
A primary transcript is the initial RNA transcript from any gene.
Eukaryotic RNA transcripts are modified through RNA processing to yield finished
mRNA.
5.
6.
7. ORGANIZATION OF TRANSCRIPTIONAL
UNIT IN PROKARYOTES
There are basically three transcriptional units in prokaryotic organisms and
which are as follows:
1. Promoters
2. RNA coding sequence
3. Terminator
8. Promoters
A promoter is a regulatory region of DNA located upstream (towards the 5' region)
of a gene, providing a control point for regulated gene transcription.
The promoter contains specific DNA sequences that are recognized by proteins
known as transcription factors. These factors bind to the promoter sequences,
recruiting RNA polymerase, the enzyme that synthesizes the RNA from the coding
region of the gene.
Prokaryotic promoters
In prokaryotes, the promoter consists of two short sequences at -10 and -35
positions upstream from the transcription start site.
The sequence at -10 is called the Pribnow box, or the -10 element, and usually
consists of the six nucleotides TATAAT. The Pribnow box is absolutely essential to
start transcription in prokaryotes.
The other sequence at -35 (the -35 element) usually consists of the six nucleotides
TTGACA. Its presence allows a very high transcription rate.
9.
10. RNA coding sequence
DNA is the RNA coding sequence and along with promoter region the
RNA polymerase carries on the activity of producing mRNA.
11. Termination
There are two major termination strategies found in bacteria: Rho-
dependent and Rho-independent.
In Rho-dependent termination, the RNA contains a binding site for a
protein called Rho factor. Rho factor binds to this sequence and starts
"climbing" up the transcript towards RNA polymerase.
When it catches up with the polymerase at the transcription bubble,
Rho pulls the RNA transcript and the template DNA strand apart,
releasing the RNA molecule and ending transcription. Another
sequence found later in the DNA, called the transcription stop point,
causes RNA polymerase to pause and thus helps Rho catch up.
12.
13. Rho-independent termination depends on specific sequences in the
DNA template strand. As the RNA polymerase approaches the end of the
gene being transcribed, it hits a region rich in C and G nucleotides. The
RNA transcribed from this region folds back on itself, and the
complementary C and G nucleotides bind together. The result is a stable
hairpin that causes the polymerase to stall.
In a terminator, the hairpin is followed by a stretch of U nucleotides in the
RNA, which match up with A nucleotides in the template DNA. The
complementary U-A region of the RNA transcript forms only a weak
interaction with the template DNA. This, coupled with the stalled
polymerase, produces enough instability for the enzyme to fall off and
liberate the new RNA transcript.
