Download RNA and transcription

RNA (Ribonucleic acid)
Structure: Similar to that of DNA except:
1- it is single stranded polunucleotide chain.
2- Sugar is ribose
3- Uracil is instead of thymine
There are 3 types of RNA:
1- Ribosomal RNA (rRNA)
2- Messenger RNA (mRNA)
3- Transfer RNA (tRNA)
RNA are copies from DNA sequences formed by a process called “
transcription”. After transcription some modifications occur to obtain
the three types of RNA.
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1- Ribosomal RNA (rRNA):
• 80 % of total RNA in the cells are rRNA.
• rRNA are found in combination with several proteins ( about 82
proteins) as component of the ribosome
Which is the site of protein synthesis.
• In Eucaryotic ( mammals). There are
4 size types of rRNA (5S, 5.8S, 18Ss and 28S)
representing 2/3 particle mass of the ribosome.
N.B In procaryotics ( bacteria), there
are 3 size species of rRNA.
2- Messenger RNA (mRNA):
• comprised only 5% of total cellular RNA.
• Function: Carry genetic information from DNA in the nucleus to
ribosomes (in cystol) where it is used as template for protein
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biosynthesis.
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3- Transfer RNA (tRNA): tRNA represents 15% of total RNA in the
cell.
Structure:
1- amino acid attachment site or amino acid acceptor: which terminates
with the triplet CCA.
2- Anticodon loop or anticodon triplet
3- D loop and T loop: contain unusual bases e.g. dihydrouracil,
ribothymidine or methyl guanine
Functions of tRNA:
1- transport amino acids to ribosome for protein synthesis. Each tRNA
carry only one amino acid. The specific amino acid is attached
enzymatically to 3' end of tRNA.
2- recognize the specified codon on mRNA to ensure the insertion of
the correct amino acid in the growing polypeptide chain. This
function is due to anticodon triplet which binds to codon on mRNA
by base pairing.
NB: Three nucleotide bases on mRNA form a codon which is then
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translated into specific amino acid.
Structure of tRNA
Funtions of tRNA
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Transcription
RNA is a complementary copy from DNA template except uracil is
instead of thymine.
The process of RNA synthesis from DNA template is called:
transcription and occurs in nucleus.
The enzyme responsible for transcription is RNA polymerase
Steps in RNA synthesis: (Watch Movie)
1) Initiation:
In contrast to DNA polymerase, RNA polymerase does not need a
primer to begin synthesis. Here the transcription is initiated by the
binding of RNA polymerase to a specific region of DNA double helix.
This site is called promoter site or promoter region. This region is
recognized by sigma factor (subunit) of RNA polymerase.
When RNA polymerase recognizes this region, it binds to it leading to a
local unwinding (separation) of the promoter region into 2 single
strands:
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a- DNA strand that is transcripted into mRNA and called template
strand or antisense strand.
b- The other strand is coding strand or sense strand that contains gene
to be translated ( This strand not transcripted, not used)
Direction of transcription:
RNA polymerase will read the information sequence on DNA template
from 3′ → 5′ direction, so RNA is synthesized antiparallel to DNA
template i.e. from 5′ → 3′ direction.
2) RNA elongation:
Once RNA polymerase recognizes promoter region, it begins to
synthesize a transcript (copy) of DNA template.
3) Termination: Process of elongation of RNA continues until reach
what is called : termination region which is recognized by rho factor
(subunit in RNA polymerase) resulting in release of the enzyme, and the
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synthesized RNA
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Notes:
1- The synthesized RNA will have the sequence of the sense strand
except for U instead of T.
2- In prokaryotic (bacteria) all types of RNA are synthesized by only
one species of RNA polymerase.
3- In Eukaryotic ( mammaians), there are 3 classes of RNA polymerase
a- RNA polymerase I: synthesizes the precursor of rRNA named : pre
rRNA
b- RNA polymerase II: synthesizes pre mRNA
c- RNA polymerase III: synthesizes pre tRNA
All these enzymes synthesize what is called primary transcript or
immature RNAs (pre form) which by some modifications occur after
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transcription, will give the mature rRNA, mRNA and tRNA.
Examples on post-transcriptional modifications of RNA:
1-Transcritptinal modifications of mRNA:
After transcription, the formed immature mRNA will undergo the
following modifications to be mature and functioning:
1) 5′-capping: 5′- end in the first nucleotide is blocked by 7-methyl
guanosine triphosphate (7 methyl-GTP).
Role of cap:
a- help to stabilize mRNA
b- Permit initiation of translation (specifies, where translation should
begin).
2) Poly A tail: chain of about 20-200 AMP is attached to 3′- end is added
after transcription
Role of poly A: Help to stabilize mRNA and facilitate its exit from the
nucleus.
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2- In rRNA:
pre rRNA is a long RNA that is cleaved into the normal sized
rRNA species ( 5.8s, 18s and 28s). 5s species is synthesized
separately. See hand out.
3- In tRNA:
a- addition of CCA sequence to 3′ end of pre tRNA
b- modification of bases to give unusual base
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