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RNA &
Transcription
RNA (Ribonucleic Acid) Journal
For all your RNA news!
Comparison of DNA & RNA
What are the types of RNA & their
functions?
1) Messenger RNA (mRNA) – brings DNA code
for proteins, from the nucleus to cytoplasm
ribosomes, and directs amino acid sequence.
It is a single strand without loops. It contains
nucleotide sequences called codons.
mRNA
2) Transfer RNA (tRNA) binds to amino acids
and carries them to the proper locations within the
growing polypeptide chain.
It has a looped structure & contains
an amino acid attachment site,
as well as a sequence of
nucleotides known as the
anticodon.
3) Ribosomal RNA (rRNA) combines with
protein to form ribosomes that are the site
of protein synthesis.
snRNA – the 4th RNA
snRNAs - “small nuclear RNAs” They combine
with proteins to form ribosome-like structures
known as snRNPs, also called “snurps.” Snurps
play a role in the editing of mRNA.
Transcription: RNA synthesis
In Eukaryotic cells, the DNA does not
leave the nucleus to direct the building of
proteins in the cytosol.
All Ribonucleic Acid , mRNA, tRNA, rRNA,
even snRNA, are assembled according to
instructions in the nuclear DNA.
Then, the RNA can leave the nucleus to fulfill
its function in the cytosol.
Transcription may be divided into three
stages:
• Initiation
• Elongation
• Termination
Stage 1: Initiation
1) An enzyme called RNA polymerase which
binds to the DNA strand at the beginning of a
gene called the promoter region .
The DNA is transcribed from the template of the
the coding strand. It is used to direct
synthesis of RNA.
2) The DNA strands separate as DNA
“unwinds” or “unzips”.
RNA synthesis starts with the unwinding of
DNA exposing the template of the coding
strand.
Stage 2: Elongation
3) As the RNA polymerase leaves the promoter
region and moves along the template of DNA
coding strand RNA nucleotides, matching the
corresponding DNA nucleotides bond to form an
RNA strand.
A single complimentary strand is synthesized
becoming longer as the RNA polymerase moves
along the template of the coding strand.
Stage Three: Termination
4) The RNA is complete when the RNA polymerase
reaches the terminator region at the end of the
DNA strand. (termination). Transcription is complete.
The RNA is released, & the RNA polymerase falls off.
• At the end of RNA transcription, the DNA
molecule “rewinds” as the strands
“rebond” once again.
Transcription Animation
mRNA processing:
The newly synthesized RNA is called
the primary RNA transcript (preRNA).
It may average 200,000 nucleotides (the avg. for
human cells is only 1000.) Studies show that
RNA molecules undergo extensive processing
before leaving the nucleus.
Processing involves the addition and
chemical modification of certain nucleotides
as well as the removal of some segments
from RNA.
When transcription ends editing begins.
5) RNA IS EDITED: sections removed are called Introns
while the parts that stay are called exons.
The parts of the primary transcript called introns are cut
out. Introns appear to match noncoding regions of DNA.
In order for this to happen, “Snurps” (snRNA & Protein
complexes) bind to form spliceosome to cut out the
introns,
and bond the remaining portions called exons.
A methyl-guanine or (mG cap) is placed on the
starting end of the mRNA.
Then 100 to 200 adenine nucleotides are added
to the other end to form a “poly-Adenine-tail”
(poly-A tail)
The cap and tail protect the mRNA from
enzymes in the cytosol.
Tail may aid in transport through the
nuclear pore.