• It is the process of production of RNA copy
from a specific region along the length of DNA
• The structure of DNA is not altered as a result
of this process, and it continues to store
• Genes are defined as DNA sequences that are
transcribed into RNA
• Each gene has a promoter and one or more
– The promoter “promotes” transcription
– The regulatory sequences control when and where (in
what cell type) the gene will be expressed
• The promoter and the regulatory sequences are
DNA sequences that are not part of the
transcript. They are recognized and bound by
4. Transcription unit
• The transcription unit is a sequence (stretch)
of DNA, which is formed of the gene proper,
the transcription initiation and basal rate
controlling sequence (the promoter), the
regulatory gene response sequences
(enhancer/silencer) and the transcription
• The gene enhancer/silencer, are the gene
response sequences that control the regulated
rate of gene expression (more than the basal
• Transcribed region or gene proper, the DNA
sequence that is copied as hnRNA or other types
• Termination region, a regulatory DNA sequence
down-stream the gene proper of some genes, at
which the RNA polymerase disassembles from
the DNA template.
• Are DNA sequences that “promote” gene
• More precisely, they direct the exact location for
the initiation of transcription
• Promoters are typically located just “upstream”
(5’) of the site where transcription of a gene
• The promoter attracts RNA polymerase, the
enzyme responsible for transcribing RNA, to the
gene. Without a promoter, a gene sequence
would not be transcribed.
• 1. Template
• RNA is fundamentally single-stranded and
therefore only one strand of the DNA is actually
copied into RNA during transcription.
• The strand that is actually being copied is termed
the template strand.
• The RNA transcript will have the opposite polarity
and the complementary sequence to this strand
• The opposite strand is called the coding strand
• The base sequence of this strand is identical in
polarity and sequence to the RNA transcript
–Except for the substitution of uracil in RNA
for thymine in DNA
• 3. Enzyme
• RNA polymerase(DNA dependent RNA
• In Prokaryotes
• A single type is responsible for synthesis of all
types of RNA
• Products of RNA polymerase require slight or
no modification after transcription.
• The holoenzyme is formed of five subunits:
two identical subunits, two similar but not
identical ' subunits and a regulatory
• The core part of the enzyme is formed of the
four 2' subunits.
• In Eukaryotes
• There are three types; each is specific for
synthesis of a specific type of RNA.
• Mostly require extensive post-transcriptional
modifications particularly mRNA.
• They are much complex in structure and are
formed of up to 16 subunits.
• The three types of RNA polymerases are:
• RNA polymerase I: Responsible for synthesis of
the large RNA molecules (rRNA).
• RNA polymerase II: Responsible for synthesis
• RNA polymerase III: Responsible for synthesis
of small RNA molecules, i.e., tRNA and 5S
15. Stages of replication
• I- Initiation:
• Initiation occur on a single strand of a
transcription unit (a gene) that is called template
(non-coding) strand that is complementary to the
RNA, and it never occurs in the other strand,
coding strand. Coding strand is similar to mRNA
sequence except for U/T. The template strand is
read in 3'5' so that the synthesized RNA will be
formed in 5'3'.
• Which DNA strand is the template and which is
the coding, differs from one gene to the other but
is always the strand that contain the promoter
sequence read in 3'5' direction.
• RNA polymerase recognizes the promoter region
by the help of the (sigma) factor.
• Then, the core enzyme binds tightly to the DNA.
Once the core molecule binds to the DNA, it
unwinds 17 nucleotides to separate the two
• The binding of the enzyme to DNA occurs in a
sequential process, i.e., factor then core
enzyme (2') binds and it searches for the
transcription initiation site (an open reading
frame starting at TAC).
• The synthesized RNA always starts with a purine
that enters at the initiation site of the enzyme.
This purine ribonucleotide stays in prokaryotic
mature mRNA; whereas, it is removed in mature
eukaryotic mRNA due to the post-transcriptional
processing before capping.
• The tight binding of the RNA polymerase to
the promoter forms what is called the closed
• Then, the open complex is formed when RNA
polymerase denatures the double-stranded
DNA in the AT-rich Pribnow Box
• Next, the RNA polymerase makes a short RNA
strand copy of the template strand within the
– The sigma factor is released at this point
– This marks the end of initiation
– Note that RNA polymerase, unlike DNA
polymerase, is a “smart enzyme”! It can start an
RNA strand all on its own.
• The core enzyme now slides down the DNA to
synthesize the transcript
21. 2. Elongation
• The RNA transcript is synthesized during the
• The open complex formed by the action of RNA
polymerase is about 17 bases long and remains
that size as the polymerase moves along the DNA
• Behind the open complex, the DNA rewinds back
into the double helix
• On average, the rate of RNA synthesis is about 43
nucleotides per second
• Factor is released before elongation starts.
• The four ribonucleotides triphosphate; ATP,
GTP, CTP and UTP continue to enter into the
polymerization (or elongation) site of unit
with the release of a pyrophosphate (PPi) each
time a new nucleotide is added to the growing
• The RNA polymerase continues transcription
from 3' towards 5' end of the template strand
according to the base pairing role in an anti-
parallel manner so that A-U, G-C, T-A and C-G
• Elongation continues till the termination
point is reached.
• 3. Termination of transcription:
• Termination may be rho () factor-dependent or
• rho-dependent termination,
• rho factor recognizes and binds the termination
sequence in the template DNA, then; it
disassembles the enzyme/RNA/DNA complex to
release RNA polymerase and the synthesized RNA
molecule from DNA template strand
• rho-independent termination
• RNA polymerase stops transcription when the
synthesized complete RNA molecule takes its
three-dimensional form with the formation of
specific secondary structures such as a hairpin
• This leads to pausing of RNA polymerase and
hence disassembly of the transcription
27. Antibiotic inhibitors of transcription:
• Rifampin: Binds to the core enzyme (β sub
unit) occupying substrate binding site and
inhibiting the incoming nucleotides from
binding to the initiation site.
• Actinomycin D: Binds to DNA template
inhibiting its transcription by preventing
movement of RNA polymerase along DNA.
28. Post-transcriptional modification of
• I. Processing of mRNA:
• Eukaryotic crude transcript of mRNA produced
in the nucleus is called heterogeneous nuclear
RNA (hnRNA), i.e., the blue script.
• Post-transcriptional Processing of mRNA
include decreasing its size, 5'-capping and 3'-
tailing along with the post-maturation mRNA
• Intron removal (Decrease in size):
• It is due to the splicing or removal of internal
non-translatable sequences, i.e., Introns from
the translatable sequences, i.e., Exons, by
• Removal of introns facilitates the transport of
mature mRNA from the nucleus to the
cytoplasm otherwise it will be degraded in the
• Addition of 7-methyl-guanylate 5'-capping:
• It is the addition of 7-methyl guanosine
triphosphate to the 5'-end of mRNA by
specific enzyme called guanylatetransferase.
• The cap is attached by 5' to 5' triphosphate
linkage. It enhances the translation of mRNA
and protects mRNA from the action of 5'3'
exonucleases and phosphatases.
• Addition of polyadenylate 3'-tailing:
• Addition of 20 - 250 polyadenylate tail at the
3'-end by the action of poly-A-polymerase
enzyme. It is specified by a special sequence
near the 3'-end of the mRNA (AAUAAA).
• The tail protects 3'-end of mRNA from 3'5'
exonuclease and facilitates mRNA transport
36. II- Processing of tRNA:
• Primary tRNA transcript is a large precursor
containing more than one tRNA. It is
processed by the action of specific class of
ribonucleases that also include removal of
introns and cleavage of a 5'-leader sequence.
• The tRNA nucleotides are modified by
methylation, deamination, alkylation,
reduction and glycosylation.
• Finally, replacement of the 3'-terminal UU by
the characteristic amino acceptor CCA
terminus at the 3' ends by
• This end of tRNA function as an acceptor arm
for binding amino acids.
38. III- Processing of ribosomal RNA:
• A large 45S precursor intron-less molecule is
cleaved by specific endonuclease and
exonuclease into 5.8S, 18S, and 28S rRNAs.
The 5S rRNA is a separate gene.
• The 5S, 5.8S and 28S together with 50
proteins form the 60S sub unit and the 18S
together with 33 proteins form the 40s sub
unit of the rRNA.
39. Genetic code
• It is the collection of codons
• Codon is a three base sequence in DNA that is
copied in mRNA (with U/T) which determines the
type and site of amino acid in the translated
• It is usually read in the 5'3' direction in the
• It is identified according to base complementarity
in an anti-parallel manner by the anticodon,
present in tRNA (3'5' direction).
• All probable triple alternative combinations of
the four bases enter in DNA or RNA structure give
64 triple codons or possible amino acid names.
• Among the 64 codons there are three codons
(UAA, UAG, UGA) that are called non-sense (i.e.,
meaning-less) or Termination codons that is
because they do not code for an amino acid but
they signify termination of translation of mRNA.
Characteristics of the genetic code:
• Specific or unambiguous since each codon is
specific for a single amino acid, e.g., UUU
encodes phenylalanine only.
• Degenerate, since there are 61 amino acid
encoding codons, each amino acid may be
encoded for by several codons. The 61 codons
are not equally divided on the 20 amino acids
entering in protein structure.
• Therefore, arginine has 6 codons (synonyms or
nicknames, CGU, CGC, CGA, CGG, AGA and
AGG), whereas, methionine has only one
(AUG). This is why codon is said to be
degenerate, see the table.
• Universality: The genetic code is universal,
i.e., specify the same amino acid in all living
organisms from viruses, bacteria, plants,
insects to mammals.
• Non-overlapping: The mRNA codons are read
in a continuous manner in the 5'3' direction
without interruption in three-base sequence,
i.e., no base functions as a common member
of two consecutive codons. Therefore,
addition or removal of a base leads to shifting
of the reading frame producing totally
different amino acid sequence.