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Lecture 4
Transcription
Dr. M. A. Fouad
Gene Expression
Protein synthesis involves two
processes: transcription and
translation.
 In transcription the DNA
message is converted into
an mRNA molecule.
 In translation the mRNA
message is used to
assemble amino acids into a
protein chain.
Transcription: Definition
gene
• Transcription: Is the process in which genetic
information is transferred from DNA to RNA.
DNA
(nucleus)
Transcription
• Transcription: is the process of synthesis a
single strand of messenger RNA (mRNA)
with the same sequence as the original DNA
strand.
• Transcription is the first step of gene
expression
messenger
RNA
(cytoplasm)
Transcription: RNA Polymerase
Enzyme
• Transcription is catalyzed by the
enzyme RNA polymerase.
• RNA polymerase catalyzes the
synthesis of RNA on a DNA
template strand.
Transcription: The template strand
• In any particular gene only one
DNA strand of the double helix
acts as (template strand).
• The transcribed mRNA is a copy
of the complementary strand
(the sense strand) of the DNA
double helix.
• The template strand is
sometimes called the antisense
strand.
Sense strand
(Anti-sense
strand)
Sense strand
(Anti-sense
strand)
The transcription Process
• RNA polymerase recognizes the start of a gene
(promoter) and unwinds a segment of it.
start site
transcription complex
nucleotides
The Transcription Process
• RNA polymerase begins
moving down the DNA
template strand in the 3' to
5' direction.
• As it does so, it synthesize
complementary mRNA in
the 5' to 3' direction.
– Once the gene is transcribed, the RNA strand
is released and RNA polymerase detaches from
the DNA.
RNA
Complementary RNA pairs
• The base-pairing during
transcription is the same as
when DNA replicates, except
that RNA has uracil instead of
thymine:
 the base U in RNA pairs
with A in DNA.
.
RNA
DNA
Complementary RNA pairs
Template Strand
Quizzes
1. The enzyme required for transcription is
A.
B.
C.
D.
Ligase
DNA polymerase
RNA polymerase
RNAase
2. The process involved in the mRNA formation
on the DNA template is:
a)
b)
c)
d)
Transcription
Translation
Replication
Transformation
3. Which is found in RNA but not in DNA?
A.
B.
C.
D.
E.
Adenine
Cytosine
Guanine
Uracil
phosphate groups
4. Transcription is the transfer of genetic
information from
A.
B.
C.
D.
DNA to rRNA
tRNA to mRNA
DNA to mRNA
mRNA to tRNA
5. The DNA chain acting as template for RNA synthesis has the
following order of bases, AGCTTCGA. What will be the order of
bases in mRNA?
A.
B.
C.
D.
TCGAAGCT
UGCUAGCT
TCGAUCGU
UCGAAGCU
RNA Processing
Post-transcription modification
• The transcription results initially in a premessenger RNA (pre-mRNA) molecule.
• pre-mRNA must undergoes a number of
modifications, or what is known as
“RNA processing”, before it leaves the
nucleus.
RNA processing
Post-transcription modification include
1. mRNA splicing
2. 5’ capping
3. 3’ Polyadenylation
1- mRNA splicing
• Most eukaryotic protein-coding genes contain
non-coding segments called introns, which break up
the coding sequence into segments called exons.
• The coding regions of the RNA transcript are called exons.
1- mRNA splicing
Splicing is an RNA-processing step in
which:
• The Introns (non-coding
sequences) are removed.
• The exons (coding sequences) are
re-joined again.
RNA Splicing
2- RNA processing  5’capping
• A cap structure is added to the 5’ end of the transcribed
RNA.
• 5’ capping: is the process of adding methylated guanine
nucleotide to the 5’ end of the RNA molecule by an unusual
5 ' to 5' phosphodiester bond.
Cap
structure at
the 5 end of
a eukaryotic
mRNA
RNA processing  5’capping
5’ capping process in important because:
• Facilitate the transport of mRNA from nucleus to the
cytoplasm.
• Facilitate the attachment of mRNA to the ribosome.
• Protect mRNA from degradation by endogenous cellular
exonuclease.
RNA processing-Polyadenylation
• Polyadenylation is the process of adding 200 adenylate
molecules to the 3’ end of mRNA to form what is called as
poly(A) tail.
• This process is important for the transport of RNA from
nucleus to cytoplasm and translation.
Mature mRNA structure
Quizzes
6. The process of removing introns and
joining exons is known as:
A.
B.
C.
D.
mRNA splicing
5’ capping
Polyadenylation
Transcription
7. What is added to the 3'-end mRNAs during
mRNA processing?
A. Introns
B. Poly A tail
C. A cap structure, consisting of a
methylated G nucleotide
D. Exons
8. The non-coding segments that are removed
during mRNA processing are called?
A.
B.
C.
D.
Introns
Exons
Codons
Anticodon
9. What is added to the 5'-end mRNAs
during mRNA processing?
A. Introns
B. a poly A tail
C. a cap structure, consisting of
a methylated G nucleotide
D. exons