PROTEIN SYNTHESIS
TRANSCRIPTION
Initiation:
The process begins when a protein called General Transcription Factors first binds to a specific region of
DNA called the promoter region, which is located near the start of the gene (TATA box).
The GTF helps recruit RNA polymerase II & position it at the promoter. The enzyme unwinds a small
section of the DNA double helix creating a transcription bubble.
RNA polymerase identifies the +1 site where RNA synthesis will begin.
Elongation:
RNA polymerase moves along the template strand of DNA in the 3' to 5' end adding complementary
RNA nucleotides to the growing RNA chain in the 5' to 3' direction according to the complementary base
pairing rule.
Termination:
When the enzyme reaches the end of the DNA gene it encounters a specific termination sequence in the
DNA which causes the enzyme & the newly formed RNA strand to break away from the DNA strand.
Post-transcriptional modifications
After transcription, the pre-mRNA undergoes processing to become mature:
1. 5' capping: Addition of a (7-methylguanosine cap) to the 5' end to (protect against degradation
of the mRNA).
2. 3' polyadenylation: Addition of a poly-A-tail at the 3' end by (enzyme poly-A-polymerase).
3. Splicing: Removal of introns (non-coding sequence) and joining of exons (coding sequences).
* Poly-A-tail: A long chain of Adenine nucleotides.
* Polyadenylation signal: AAUAAA (signal on the pre-mRNA that signals site for polyadenylation).
, Types of RNA
(a) pre-mRNA (precursor messenger RNA): large, immature single strand of RNA that is processed to
form mature mRNA.
(b) Small Nuclear RNA (snRNA): directs splicing of pre-mRNA to form mRNA.
(c) Messenger RNA (mRNA): carries the genetic code to the cytoplasm for controlling the type of protein
to be formed.
(d) Transfer RNA (tRNA): transports activated amino acids to the ribosomes to be used in assembling the
protein molecule.
(e) Ribosomal RNA (rRNA): forms ribosomes along with about 75 different proteins.
(f) Micro RNA (miRNA): 21–23 nucleotides that regulate gene transcription & translation.
* 5' capping
RNA triphosphatase enzyme removes one phosphate from the triphosphate group on the 5'
end.
Guanylyl transferase enzyme adds a GMP group on the 5' phosphate end.
Methyl transferase enzyme adds a methyl group to the GMP.
o All the added molecules = 7-methylguanosine.
o 5' capping also initiates translation.
* 3' polyadenylation
Also initiates transcription.
Prevents degradation by exonucleases.
Transport of the mRNA out of the nucleus.
The enzyme recognizes the AAU AAA sequence on the mRNA on the 3' end.
TRANSCRIPTION
Initiation:
The process begins when a protein called General Transcription Factors first binds to a specific region of
DNA called the promoter region, which is located near the start of the gene (TATA box).
The GTF helps recruit RNA polymerase II & position it at the promoter. The enzyme unwinds a small
section of the DNA double helix creating a transcription bubble.
RNA polymerase identifies the +1 site where RNA synthesis will begin.
Elongation:
RNA polymerase moves along the template strand of DNA in the 3' to 5' end adding complementary
RNA nucleotides to the growing RNA chain in the 5' to 3' direction according to the complementary base
pairing rule.
Termination:
When the enzyme reaches the end of the DNA gene it encounters a specific termination sequence in the
DNA which causes the enzyme & the newly formed RNA strand to break away from the DNA strand.
Post-transcriptional modifications
After transcription, the pre-mRNA undergoes processing to become mature:
1. 5' capping: Addition of a (7-methylguanosine cap) to the 5' end to (protect against degradation
of the mRNA).
2. 3' polyadenylation: Addition of a poly-A-tail at the 3' end by (enzyme poly-A-polymerase).
3. Splicing: Removal of introns (non-coding sequence) and joining of exons (coding sequences).
* Poly-A-tail: A long chain of Adenine nucleotides.
* Polyadenylation signal: AAUAAA (signal on the pre-mRNA that signals site for polyadenylation).
, Types of RNA
(a) pre-mRNA (precursor messenger RNA): large, immature single strand of RNA that is processed to
form mature mRNA.
(b) Small Nuclear RNA (snRNA): directs splicing of pre-mRNA to form mRNA.
(c) Messenger RNA (mRNA): carries the genetic code to the cytoplasm for controlling the type of protein
to be formed.
(d) Transfer RNA (tRNA): transports activated amino acids to the ribosomes to be used in assembling the
protein molecule.
(e) Ribosomal RNA (rRNA): forms ribosomes along with about 75 different proteins.
(f) Micro RNA (miRNA): 21–23 nucleotides that regulate gene transcription & translation.
* 5' capping
RNA triphosphatase enzyme removes one phosphate from the triphosphate group on the 5'
end.
Guanylyl transferase enzyme adds a GMP group on the 5' phosphate end.
Methyl transferase enzyme adds a methyl group to the GMP.
o All the added molecules = 7-methylguanosine.
o 5' capping also initiates translation.
* 3' polyadenylation
Also initiates transcription.
Prevents degradation by exonucleases.
Transport of the mRNA out of the nucleus.
The enzyme recognizes the AAU AAA sequence on the mRNA on the 3' end.
