Download What is Transcription?

What is Transcription?
Transcription is the process of creating
a complementary RNA copy of a
sequence of DNA. During transcription, a
DNA sequence is read by RNA
polymerase, which produces a
complementary, antiparallel RNA strand.
As opposed to DNA replication,
transcription results in an RNA
complement that includes uracil (U) in all
instances where thymine (T) would have
occurred in a DNA complement.
The Importance of Transcription
Transcription is the first step leading to gene
expression. The stretch of DNA transcribed into an
RNA molecule is called a transcription unit and
encodes at least one gene. If the gene transcribed
encodes a protein, the result of transcription is
messenger RNA (mRNA), which will then be used
to create that protein via the process of translation.
Alternatively, the transcribed gene may encode for
either ribosomal RNA (rRNA) or transfer RNA (tRNA), other
components of the protein-assembly process, or other
ribosome.
Translation process and outcome
In translation, messenger RNA (mRNA) produced by
transcription is decoded by the ribosome to produce a
specific amino acid chain, or polypeptide, that will later fold
into an active protein. In Bacteria, translation occurs in the
cell's cytoplasm. In Eukaryotes, translation occurs across
the membrane of the endoplasmic reticulum. The ribosome
facilitates decoding by inducing the binding of tRNAs with
complementary anticodon sequences to that of the mRNA.
The tRNAs carry specific amino acids that are chained
together into a polypeptide as the mRNA passes through
and is "read" by the ribosome.
Translation Illustration
The Role of DNA and RNA during
Transcription Process
In DNA replication, DNA is read from 3' → 5' during
transcription. Meanwhile, the complementary RNA is
created from the 5' → 3' direction. This means its 5' end
is created first in base pairing. Although DNA is arranged
as two antiparallel strands in a double helix, only one of
the two DNA strands, called the template strand, is used
for transcription. This is because RNA is only singlestranded, as opposed to double-stranded DNA. The
other DNA strand is called the coding strand, because its
sequence is the same as the newly created RNA
transcript (except for the substitution of uracil for
thymine).
Transcription process
1) Helicase unwinds/"unzips" the DNA by breaking the hydrogen
bonds between complementary nucleotides.
GGCCGCTTGACAAAAGTGTTAAATTG
coding DNA
CCGGCGAACTGTTTTCACAATTTAAC
DNA template
Transcription Process
2) RNA nucleotides are paired with
complementary DNA bases from the 5’ to the 3’
end when the RNA Polymerase-Promoter
Complex binds to the promoter gene in the
DNA.
RNA
5’ GGCCGCUUGACAAAAGUGUUAAA
3’ CCGGCGAA CTGTTTTCACAATTT 5’
Complementary DNA
Transcription Process
3) sugar-phosphate backbone forms with
assistance from RNA polymerase.
Transcription Process
If the cell has a nucleus, the RNA is further
processed (addition of a 3' poly-A tail
and a 5' cap) and exits through to the
cytoplasm through the nuclear pore
complex.
Transcription Process
4) Hydrogen bonds of the untwisted
RNA+DNA helix break, freeing the newly
synthesized RNA strand.
Newly synthesized RNA strand
5’ GGCCGCUUGACAAAAGUGUUAAA
Transcription Steps Review
1) Pre-initiation-RNA polymerase binds to core promoters on DNA in the presence
of various specific transcription factors.
2) Initiation-The completed assembly of transcription factors and RNA
polymerase bind to the promoter, forming a transcription initiation complex.
3) Promoter clearance-After the first bond is synthesized, the RNA polymerase
must clear the promoter. During this time there is a tendency to release the
RNA transcript and produce truncated transcripts.
4) Elongation-RNA polymerase traverses the DNA template strand from 3' → 5',
and uses base pairing complementarity with the DNA template to create an
RNA copy. This produces an RNA molecule from 5' → 3', an exact copy of the
coding strand DNA (except that thymines are replaced with uracils, and the
nucleotides are composed of a ribose (5-carbon) sugar where DNA has
deoxyribose (one less oxygen atom) in its sugar-phosphate backbone).
5) Termination-in eukaryotes termination of transcription involves cleavage of the
new transcript followed by template-independent addition.
Illistration of Transcription Steps
2) Initiation
4) Elongation
5) Termination
QUIZ
1) Transcription is
a) A process creating a complimentary DNA
strand from a RNA template strand.
b) A process creating a complimentary RNA
strand from a DNA template strand.
c) A process creating a complimentary RNA
strand from a RNA template strand.
d) A process creating a complimentary DNA
strand from a DNA template strand.
QUIZ
2) The result of transcription is
a) Gene expression leads to desired protein
synthesis.
b) Protein synthesis leads to gene
expression.
c) Formation of mRNA, tRNA and rRNA.
d) Only a & c
QUIZ
3) Translation occurs
a)
b)
c)
d)
During the process of transcription.
Before the process of transcription.
After the process of transcription.
Before and after the process of
transcription.
QUIZ
4) Protein is made up of subunits of
a)
b)
c)
d)
Nucleic acids
Amino acids
Fatty acids
Nuclear base pairs
QUIZ
5) During transcription, the RNA strand is
always read from
a)
b)
c)
d)
3’ to 5’ direction
5’ to 3’ direction
Either direction
Anywhere on the DNA strand.
QUIZ
6) In Eukaryote, the process of the
transcription occurs in
a)
b)
c)
d)
The nucleus
The cytoplasm
The endoplasmic Reticulum
The ribosome
QUIZ
7) The five steps of transcription are in the order of
a) Pre initiation, Initiation, elongation, promoter
clearance, termination
b) Pre initiation, Initiation, promoter clearance,
elongation, termination.
c) Pre initiation, Initiation, elongation, promoter
clearance, termination.
d) Pre initiation, Initiation, elongation, termination,
promoter clearance.
QUIZ
8) During elongation process of
transcription, the Base Thymine is
replaced with
a)
b)
c)
d)
Adenine
Cytosine
Uracil
Guanine
QUIZ
9) The difference(s) between DNA and RNA
is(are)
a)
b)
c)
d)
One of the base
Form of sugar
Single versus double strands
All of the above
QUIZ
10) Translation in Eukaryote takes place in
a)
b)
c)
d)
The nucleus
The cytoplasm
The Endoplasmic Reticulum
The ribosome
Challenge Question
Why is transcription process so important?
If a mistake happens during the transcription
process, what will happen?
QUIZ Answer – Transcription
1) b
2) b
3) c
4) b
5) b
6) c
7) b
8) c
9) d
10) d
Answer to Challenge Question
Transcription is so important because the
genetic coding needs to be transcribed
correctly to produce the necessary proteins
that the body needs. If a mistake happens
during transcription, non functional proteins
might be produced and leads to deficiency in
the body functions.