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Transcription Process| NMDCAT 2021
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This is the process in which an RNA copy of the DNA sequence encoding the gene is
produced with the help of an enzyme, RNA polymerase. Only one of the two strands
of DNA are transcribed. This strand is called template strand or the antisense strand.
Thq opposite strand is called coding strand or the sense strand. The RNA polymerase
enzymes synthesize RNA from 5’ —» 3’ direction. There is only one type of RN A
polymerase in prokaryote which is responsible for the synthesis of all the three types
of RNAs viz. rRNA, mRNA and tRNA. On the other hand there are three types of RNA
polymerases in eukaryotes namely RNA polymerase I, which synthsize rRNA, RNA
polymerase II, which synthesizes mRNA and RNA polymerase III which synthesizes
tRNA.
Transcription starts at the RNA polymerase binding site called promoter on the DNA
template strand. In prokaryotes within promoter there are two binding sites TTGACA
also called -35 sequence and TAT A AT sequence also called -10 sequence, which have
ainity for the RNA polymerase. In eukaryotes these sites are at -75 and -25 sites,
respectively
The binding of RNA polymerase to the promoter is the irst step in gene transcription.
One of the subunits of RNA polymerase sigma factor, is responsible for correct initiation
of transcription process. Once the transcription has started the sigma factor is released
and the remaining part of the enzyme (core enzymes) moves over template; strand
and completes the transcription of the gene. The DNA strands open up at the place
where enzyme is attached to the templete strand forming transcription bubble. The
transcription bubble moves down the DNA, leaving the growing strand protruding
from the bubble. The stop sequences at the end of the gene terminate the
synthesis of mRNA. The simplest stop signal is a series of GC base pairs followed by a
series of AT base pairs. The RNA formed in this region forms a GC hairpin
followed by four or more U ribonucleotides. The hairpin causes RNA polymerase to
stop synthesis.
In bacteria the newly synthesized mRNA is directly released into the pytoplasm, when it
is converted into polypeptide chain. In eukaryotes however, it has to travel long distance
from inside the nucleus to ribosomes outside in the cytoplasm. The eukaryotic mRNA
is therefore modiied in several ways to aid this journey. A cap and a tail is added so
that the molecule may remain stable during long journey to ribosome. The cap is in the
form of 7 methyl GTP, which is linked 5’ to 5’ with the irst nucleotide, whereas tail is in
the form of poly A tail linked to 3’ end of the RNA. These caps and tails save the mRNA
from variety of nucleases and phosphatases.
This is the process in which an RNA copy of the DNA sequence encoding the gene is
produced with the help of an enzyme, RNA polymerase. Only one of the two strands
of DNA are transcribed. This strand is called template strand or the antisense strand.
Thq opposite strand is called coding strand or the sense strand. The RNA polymerase
enzymes synthesize RNA from 5’ —» 3’ direction. There is only one type of RN A
polymerase in prokaryote which is responsible for the synthesis of all the three types
of RNAs viz. rRNA, mRNA and tRNA. On the other hand there are three types of RNA
polymerases in eukaryotes namely RNA polymerase I, which synthsize rRNA, RNA
polymerase II, which synthesizes mRNA and RNA polymerase III which synthesizes
tRNA.
Transcription starts at the RNA polymerase binding site called promoter on the DNA
template strand. In prokaryotes within promoter there are two binding sites TTGACA
also called -35 sequence and TAT A AT sequence also called -10 sequence, which have
ainity for the RNA polymerase. In eukaryotes these sites are at -75 and -25 sites,
respectively
The binding of RNA polymerase to the promoter is the irst step in gene transcription.
One of the subunits of RNA polymerase sigma factor, is responsible for correct initiation
of transcription process. Once the transcription has started the sigma factor is released
and the remaining part of the enzyme (core enzymes) moves over template; strand
and completes the transcription of the gene. The DNA strands open up at the place
where enzyme is attached to the templete strand forming transcription bubble. The
transcription bubble moves down the DNA, leaving the growing strand protruding
from the bubble. The stop sequences at the end of the gene terminate the
synthesis of mRNA. The simplest stop signal is a series of GC base pairs followed by a
series of AT base pairs. The RNA formed in this region forms a GC hairpin
followed by four or more U ribonucleotides. The hairpin causes RNA polymerase to
stop synthesis.
In bacteria the newly synthesized mRNA is directly released into the pytoplasm, when it
is converted into polypeptide chain. In eukaryotes however, it has to travel long distance
from inside the nucleus to ribosomes outside in the cytoplasm. The eukaryotic mRNA
is therefore modiied in several ways to aid this journey. A cap and a tail is added so
that the molecule may remain stable during long journey to ribosome. The cap is in the
form of 7 methyl GTP, which is linked 5’ to 5’ with the irst nucleotide, whereas tail is in
the form of poly A tail linked to 3’ end of the RNA. These caps and tails save the mRNA
from variety of nucleases and phosphatases.
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