small introduction

1. A part of post trancriptional
modification & elongation and
termination
Vimal priya subramanian
1st M.sc Botany
18MBO027

2. Activator
• A transcriptional activator is a protein that
increases gene transcription
• Most activators are DNA binding proteins that
bind to enhancers or promoter proximal
elements.
• The DNA site bound by the activator is
referred to as an activator site
• Eg; catabolite activator protein(CAP) in lac
operon concept

3. REPRESSOR
• In molecular genetics, a repressor is a
DNA/RNA binding protein that inhibits the
expression of one or more gene by binding to
the operator or associated silencers.
• A DNA binding repressor blocks the
attachment of RNA polymerase to the
promoter, thus preventing transcription of the
messenger RNA.
• Eg; Negative regulation in lac operon concept

4. RNA polymerase
• RNA polymerase unwinds about 15 bases of
DNA around the initiation site to form an open
promoter-DNA complex and provides single-
strand of DNA to act as template for
transcription.
• RNA polymerase basically catalyses the
formation of phosphodiesster bonds between
successive nucletides of a polynucleotide
chain during synthesis of both DNA and RNA.

5. RNA Polymerase
• RNA polymerase binds to the promoter,
transcription begins at the start point,
progresses along the length of coding
sequence and terminates at the terminator
sequence or at temination codon. The
transcription of mRNA starts in 5’--->3’
direction on 3’-->5’ strand of DNA.

6. RNA capping
• The 5’ end of primary mRNA transcript is
modified by the removal of a phosphate from
the triphosphate functional group.
• A guanosine monophosphate (GMO)gets
attached to this diphosphate at 5’ end
resulting in the formation of an unusual 5’-5’
triphosphate covalent linkage.
• The guanine residue at the end of this linkage
is methylated at N7.

7. RNA capping
• The methylated guanosine forms a cap at 5’ end
of mRNA (cap.0)
• Additional capping may occur by another
methylation of hydroxyl groups of ribose of first
one or two nucleotides in some higher
eukaryotes(cap1/cap2).
• Only cap 0is formed in lower eukaryotes.
• Capping of RNA is probably necessary for marking
mRNA and to protect it against cleavage by
exonucleases.

8. Elongation of RNA polynucleotide
chain
• Once the first two nucleotide residues have been
joined, chain elongation proceeds rapidly with
transcription taking place in the 5’- 3’ direction,
antiparallel to the 3’strand of the template DNA .
• During elongation a segment of the newly formed RNA
formed RNA forms a transient complementary hybrid
RNA-DNA duplex.
• Once transcription beings and about 10 nucleotide
residues are added, the Ω factor dissociates from the
holoenzyme to yield he core polymerase which
proceeds to complete the process for transcription.

9. Cont..
• The free Ω factor then becomes available for
initiating a new RNA chain with another
molecule of core polymerase. The elongation
of the RNA chain continues along the DNA
templates until the core polymerase gets a
signal for termination.

10. Termination
• Specific termination signals are necessary
because the DNA under transcription has
many genes and is often circular. Without a
termination signal, the RNA would be
transcribed indefinitely.
• Moreover , tRNAs and rRNAS have specific
chain lengths. Therefore, at the end of a gene
a sequence of bases signal the completion of
transcription.

11. •Thanking you