• Transcription is the process of the synthesis
• Promoter region
• Transcribed region
• And termination region.
• Which is certain sequence of base located at
the beginning of stretch. It is important for
initiation for transcription to occur and can be
recognized by RNA polymerase.
• 1. The pribno box :stretch of 6 nucleotides
(TATAAT) located about 10 bases to the left of
the transcription. Initiation site.
• 2. A second nucleotide stretch (TGTTGACA)
that located about 35 bases to the left of the
transcription initiation site.
• 3. 19 bases (nucleotide) in between two
• Both pribno box and TGTTGACA are region can
be recognized by RNA polymerase.
• 1. The Hoges or TATA box: stretch of
nucleotide that is almost similar to
prokaryotes,located almost 25 nucleotide to
the left of the trancrition initiation site.
• 2. CAAT box : located about 70 to 80
nucleotides to the left of the trasncription
• 3. 40 bases between the two stretches.
• Transcription region: which is stretch of DNA
that is to be transcribed into RNA molecules.
• Termination region: which is a stretch of DNA
located at the end of DNA sequence to be
Requirement for RNA synthesis
• Transcription unit: transcription occurs on one
of the two strands of the DNA template and
never on both complementary strand
• Four ribonucleotide triphosphate;ATP,GTP,UTP
• RNA poymerase ,RNA(=DNA –dependent RNA
Prokaryotes RNA polymerase.
• 1.E.g E.coli ,enzyme synthesize all types of
RNA molecules on DNA template.
• 2. the holoenzyme of the RNA polymerase
consist of a core molecule and specific protein
factor(sigma [σ] factor.
• A. The core enzyme molecule : 4 subunits: 2 of
them are identical (alpha subunits.the other
2(β and β’) are similar but not identical .RNAP
consist of 2 zinc molecules.
• Enables the polymerase to recognize
promotor regions on DNA .
• Helps the core enzyme to attach more tightly
to the promotor site.
• Some regions on DNA that signal the
termination of transcription are recognized by
the RNA polymerase itself. Others are
recognized by specific termination factor e.g
rho factor of E.coli.
• 1. RNA polymerase holoenzyme binds with
the promotor area:
• A . Sigma subunit ,enables polymerase
polymerase to recognize promotor region on
• B. Β̒subuint to the template.
• C. B subunit binds to the nucleotide substrate.
• 2. binding of RNA poymearse DNA template
leads to local separation (unwinding)of the
DNA double helix into sense and antisense
• 3. first nucleotide of RNA transcript at the
initiation site is almost always purine.
• 1.At the sense formation of RNA molecule begins
at the 5̒end of core enzyme with release of sigma
• 2. Then elongation of the RNA molecule occurs
from 5 ̒ to 3̒ end ,antiparallel to its template.
• 3. nucleotide building blocks are 5̒ ribonucleotide
triphosphate(ATP ,GTP,CTP,and UTP). They are
inserted in the RNA = pairing rule.pyrophosphate
is released when each new nucleotide is added to
growing chain . GTPGMP + PPi
• 4.RNA polymerase forms a phosphodiester
bond between the ‘3” OH of one ribose sugar
and 5’ OH of the next ribose.
• 5. the process of elongation of RNA chain
continues until a termination region reached.
• Termination region on DNA template can be
recognized by :
• A. RNA – polymerase enzyme itself (rho
• B. Rho [ρ] factor which may be required for
the release of both RNA strand and RNA
polymerase (rho dependant termination).
Termination result from either
• A. Binding of rho factor polymerase enzyme :
when RNA – polymerase enzyme reaches the
termination site,rho factor binds with it
• Slowing down of RNA – polymerase at the
• 1. termination site on DNA is charaterized by
the presence of palidromes.
• Palidromes is a region of a double stranded
DNA in which each of the two strands haas the
same sequence when read in the same
direction. E.g in the 5’ to 3’direction.
• 2.The RNA transcript of the DNA palidrome
can form a stable hairpin structure which is
self –complementary structure. This hairpin
structure causes slowing of RNA – poylmerase
at the termination site.
• Rifampcin: binds to the beta subunit .
Treatment of leprosy and TB patients.
• Actinomycin D: binds to DNA template
,prevent movement of RNA – polymerase
along the DNA.
• Alpha –amanitin: toxin from mushroom ,
inactivates RNA – polymerase II of
Post transcriptional modification of
• A. eukaryotes mRNA : primary mRNAs are
called heterogeneous nuclear RNA (hnRNA)
.hnRNA is modified into mature mRNA in the
nucleus by capping additon of nucleotides
• The 5’end of the RNA requires a cap which is 7
methyl-guanosine triphosphate. It is attached
by a 5’to 5’triphosphate linkage.
• A. reaction needs an enzyme called guanyl
• B. function of this cap is to facilitate the
initiation of translation, and protects the
5’end of mRNA from attack by 5’to
Addition of poly (A) tail.
• mRNA require almost 40 to 200 adenine
nucleotides added at 3’terminus to form a
poly adenine (A) tail:
• poly A polymerase . Enzyme for the reaction.
• This tail protects the 3’end of mRNA form 3’to
5’ exonuclease attack.
Splicing (removal of introns)
• hnRNA is formed many pieces of some of
them (exons) will be translated into amino
• Others (introns) will be not be translated into
amino acids and must be removed before
translation takes place.
• Responsible for removal of introns from the
hnRNA , and splicing (ligation) of both ends of
exons to form mature mRNA.
• It consist of primary hnRNA ,4 small nuclear
RNA’s (U1,U2,U5, and U4/U6) and an
undetermined number of proteins.
• Smaller nuclear RNA’s (snRNA) is to bind each
end of the introns by forming base pair with
• Spliceosomes facilitate also the transport of
mature mRNA from the nucleus to the
• Defect in the process of spilicing may lead to
disease e.g at least one from of beta
thalassemia , a disease in which there is
absent synthesis of beta chain of hemoglobin
appears to result from a nucleotide change at
an exon-introns junction.
Transfer RNA(t RNA)
• 1.It serves as a adapter mol. For the
translation of mRNA into protein sequence.
• 2. primary tRNA transcript are subjected to
• A. tRNA mol are transcribed as larger
precusors. These precursors are reduced in
size by specific class of ribonucleases.
• B. Attachment of the characteristics C.C.A
terminus at the 3’end of the molecules.
• The tRNAs contain many modification of the
standard bases A,U,G and C .Some bases are
methylated, alkylated, or attached to
carbohydrate residue by glucosidic bonds.
• some tRNA contain near to the anticodon
loop a single intron 10-40 nucleotides long.
These introns are removed with splicing ofr
exons to produce an active tRNA for protein
• 1.In mammals cells ;rRNA is transcribed as a
single larger precursor molecules called 45s.
• 2.In the nucleus ,45s is methylated and
cleaved by specific endonucleases and
exonucleases to give four kind of r RNA :
5SrRNA,5.8SrRNA,18SrRNA and 28SrRNA.
• 3. The four kind rRNA combine with a number
proteins to form a ribosomes.
• 1. ribosomes are cytoplasmic nucleoproteins
composed of 4 rRNAs plus a number of
• 2. These rRNAs and proteins are distributed
specifically between the two smaller and
larger ribosomal subuints.
• 3. The smaller subunits is called 40S .It
contains one 18S rRNA and 33 proteins.
• 4. The larger subunit is called 60S . It contains
the remaining r RNAs (28S,5.8S and 5S) and 45
• 5. Both smaller (40S) subuint form the whole
Function of ribosomes
• They are site of protein synthesis within cells.