Download Transcription is the process of creating a complementary RNA copy

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.
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.
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.
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 single-stranded, 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.
2) RNA nucleotides are paired with complementary DNA bases from the 5’ to the 3’
end.
3) Sugar-phosphate backbone forms with assistance from RNA polymerase.
4) Hydrogen bonds of the untwisted RNA+DNA helix break, freeing the newly
synthesized RNA strand.
5) 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 Steps
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 complementarily 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 sugarphosphate backbone).
5) Termination-in eukaryotes termination of transcription involves cleavage of the new
transcript followed by template-independent addition.
QUIZ- Transcription
1) Transcription is
a)
b)
c)
d)
A process creating a complimentary DNA strand from a RNA template strand.
A process creating a complimentary RNA strand from a DNA template strand.
A process creating a complimentary RNA strand from a RNA template strand.
A process creating a complimentary DNA strand from a DNA template strand.
2) The result of transcription is
a)
b)
c)
d)
Gene expression leads to desired protein synthesis.
Protein synthesis leads to gene expression.
Formation of mRNA, tRNA and rRNA.
Only a & c
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.
4) Protein is made up of subunits of
a)
b)
c)
d)
Nucleic acids
Amino acids
Fatty acids
Nuclear base pairs
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.
7) The five steps of transcription are in the order of
a)
b)
c)
d)
Pre initiation, Initiation, elongation, promoter clearance, termination
Pre initiation, Initiation, promoter clearance, elongation, termination.
Pre initiation, Initiation, elongation, promoter clearance, termination.
Pre initiation, Initiation, elongation, termination, promoter clearance.
8) During elongation process of transcription, the Base Thymine is replaced with
a)
b)
c)
d)
Adenine
Cytosine
Uracil
Guanine
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
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) d
3) c
4) b
5) b
6) c
7) b
8) c
9) d
10) d
Answer to the 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.
QUIZ Answer – Transcription
11) b
12) d
13) c
14) b
15) b
16) c
17) b
18) c
19) d
20) d