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IX: DNA Function: Protein Synthesis
IX: DNA Function: Protein Synthesis
A. Overview:
1. The central dogma of genetics:
unidirectional flow of information
IX: DNA Function: Protein Synthesis
A. Overview:
1. The central dogma of genetics:
unidirectional flow of information
2. Why a two-step process?
a. Historical contingency…
That’s how it evolved from an RNA Protein
system…
IX: DNA Function: Protein Synthesis
A. Overview:
1. The central dogma of genetics:
unidirectional flow of information
2. Why a two-step process?
- it evolved that way….
- because it is more productive….
tRNA
IX: DNA Function: Protein Synthesis
A. Overview:
1. The central dogma of genetics:
unidirectional flow of information
2. The code is:
3. Why a two-step process?
4. Players:
ds-DNA; GENE (recipe)
tRNA
RNA polymerases make:
m-RNA (gene transcript)
r-RNA (reader in ribosome)
t-RNA (AA carrier)
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
1. The DNA template:
‘sense’ strand
5’
…C A T…
5’
…G T A…
‘anti-sense’ strand
In a given region (gene), only one strand is transcribed; only one
strand carries a message that makes ‘sense’. The sequence on the other strand is
limited to being complementary to the first strand.
3’
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
1. The DNA template:
‘sense’ strand
5’
…C A T…
5’
…G T A…
‘anti-sense’ strand
In a given region (gene), only one strand is transcribed; only one
strand carries a message that makes ‘sense’. The sequence on the other strand is
limited to being complementary to the first strand.
There is a region ‘upstream’ from the gene called the PROMOTER.
This is where the RNA Polymerase binds. The polymerase is attracted to particular
sequences. Many are consensus sequences found upstream from different genes, and
across many many species.
3’
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
1. The DNA template:
‘sense’ strand
5’
…C A T…
5’
…G T A…
‘anti-sense’ strand
In a given region (gene), only one strand is transcribed; only one
strand carries a message that makes ‘sense’. The sequence on the other strand is
limited to being complementary to the first strand.
There is a region ‘upstream’ from the gene called the PROMOTER.
This is where the RNA Polymerase binds. The polymerase is attracted to particular
sequences. Many are consensus sequences found upstream from different genes, and
across many many species. In all bacteria, the sequence TATAAT lies 10 bases upstream
from all bacterial genes, and TTGACA lies 35 bases upstream. Two binding sites create a
directionality.
Promoters can be 40 bases long. Frequency of binding is affected by variation in the
rest of the sequence….
3’
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
1. The DNA template:
‘sense’ strand
5’
…C A T…
5’
…G T A…
‘anti-sense’ strand
In a given region (gene), only one strand is transcribed; only one
strand carries a message that makes ‘sense’. The sequence on the other strand is
limited to being complementary to the first strand.
In all eukaryotes, there is a consensus sequence of TATA in all
promoters at -35, and a CAAT box at -80. There are also enhancer regions that can
modulate binding, and Transcription Factors that bind to the promoter and
increase/decrease the efficacy of polymerase binding.
3’
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
1. The DNA template:
‘sense’ strand
5’
…C A T…
5’
…G T A…
‘anti-sense’ strand
In a given region (gene), only one strand is transcribed; only one
strand carries a message that makes ‘sense’. The sequence on the other strand is
limited to being complementary to the first strand.
In all eukaryotes, there is a consensus sequence of TATA in all
promoters at -35, and a CAAT box at -80. There are also enhancer regions that can
modulate binding, and Transcription Factors that bind to the promoter and
increase/decrease the efficacy of polymerase binding.
Why is there consensus in promoter sequences across all life? What does that say
about how well mutations are tolerated in these regions?
3’
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
‘sense’ strand
5’
…C A T…
5’
1. The DNA template:
…G T A…
‘anti-sense’ strand
In a given region (gene), only one strand is transcribed; only one
strand carries a message that makes ‘sense’. The sequence on the other strand is
limited to being complementary to the first strand.
There is region downstream called a TERMINATOR (40 bases long).
3’
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
1. The DNA template:
‘sense’ strand
5’
…C A T…
5’
…G T A…
3’
‘anti-sense’ strand
In a given region (gene), only one strand is transcribed; only one
strand carries a message that makes ‘sense’. The sequence on the other strand is
limited to being complementary to the first strand.
So, the polymerase binds at the promoter, transcribes the whole gene, and decouples at
the terminator.
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
1. The DNA template:
‘sense’ strand
5’
Continuous recipe for a protein
5’
3’
‘anti-sense’ strand
In a given region (gene), only one strand is transcribed; only one
strand carries a message that makes ‘sense’. The sequence on the other strand is
limited to being complementary to the first strand.
In bacteria, the gene is a continuous coding sequence….
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
1. The DNA template:
Disconti
‘sense’ strand
nuous recipe for
5’
a protein
5’
3’
‘anti-sense’ strand
In a given region (gene), only one strand is transcribed; only one
strand carries a message that makes ‘sense’. The sequence on the other strand is
limited to being complementary to the first strand.
In bacteria, the gene is a continuous coding sequence….
In eukaryotes, genes contain non-coding, intervening sequences called
introns; the coding sequences are called exons.
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
1. The DNA template:
Disconti
‘sense’ strand
nuous recipe for
5’
a protein
5’
3’
‘anti-sense’ strand
In a given region (gene), only one strand is transcribed; only one
strand carries a message that makes ‘sense’. The sequence on the other strand is
limited to being complementary to the first strand.
In bacteria, the gene is a continuous coding sequence….
In eukaryotes, genes contain non-coding, intervening sequences called
introns; the coding sequences are called exons. Although transcription is continuous and
every base is transcribed, RNA processing is required to splice out the non-coding
introns and create a continuous reading frame for translation.
There are two alternate hypotheses for the evolution of introns:
Eubacteria (no
introns at all)
Archaea (introns in r-RNA
and t-RNA genes)
Eukarya (introns)
There are two alternate hypotheses for the evolution of introns:
“Introns Early”: The ancestral structure was a split gene structure, favored because
evolution could proceed rapidly by the shuffling of functional exons to create new
genes (exon shuffling hypothesis).
Eubacteria (no
introns at all)
Archaea (introns in r-RNA
and t-RNA genes)
Eukarya (introns)
There are two alternate hypotheses for the evolution of introns:
“Introns Early”: The ancestral structure was a split gene structure, favored because
evolution could proceed rapidly by the shuffling of functional exons to create new
genes (exon shuffling hypothesis).
Eubacteria (no
introns at all)
Introns were lost from
prokaryotes because of the
extreme selective
advantage for rapid division.
Archaea (introns in r-RNA
and t-RNA genes)
Eukarya (introns)
There are two alternate hypotheses for the evolution of introns:
“Introns Late”: The ancestral structure was a continuous gene structure. Introns
evolved as “transposeable elements” and inserted themselves and multiplied only in
eukaryotic ancestors.
Eubacteria (no
introns at all)
Archaea (introns in r-RNA
and t-RNA genes)
Eukarya (introns)
The split-gene structure of eukaryotes was discovered in 1977:
Philip Sharp and coworkers found that viral genes in eukaryotes and initial
transcripts were longer than the functional m-RNA or proteins.
The split-gene structure of eukaryotes was discovered in 1977:
Philip Sharp and coworkers found that viral genes in eukaryotes and initial
transcripts were longer than the functional m-RNA or proteins.
1978: Walter Gilbert coins the terms ‘intron’ and ‘exon’.
Heteroduplex analyses of DNA
and m-RNA show “loops” of
RNA that have no complement
in the DNA template strand:
All eukaryotic genes except those coding for histones have introns; some make
up the vast majority of the ‘gene’, itself:
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
‘sense’ strand
5’
…C A T…
5’
…G T A…
‘anti-sense’ strand
1. The DNA template:
2. RNA Polymerase:
In bacteria, there is only one enzyme; consisting of a ‘core enzyme’
(responsible for polymerization), and subunits that affect different functions. For
example, the sigma subunit is responsible for initiation, and the rho subunit stimulates
termination.
3’
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
‘sense’ strand
5’
…C A T…
5’
…G T A…
‘anti-sense’ strand
1. The DNA template:
2. RNA Polymerase:
In bacteria, there is only one enzyme; consisting of a ‘core enzyme’
(responsible for polymerization) with two polypeptides, and subunits that affect
different functions. For example, the sigma subunit is responsible for initiation, and the
rho subunit stimulates termination. There are different sigma subunits that affect
polymerase binding; complementing the variations in promoters.
3’
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
‘sense’ strand
5’
…C A T…
5’
…G T A…
‘anti-sense’ strand
1. The DNA template:
2. RNA Polymerase:
In bacteria, there is only one enzyme; consisting of a ‘core enzyme’
(responsible for polymerization) with two polypeptides, and subunits that affect
different functions
In eukaryotes, there are three RNA Polymerases:
3’
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
5’
…C A T…
5’
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
A
‘sense’ strand
U
…G T A…
3’
‘anti-sense’ strand
A
U
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
4. Process in Bacteria
‘sense’ strand
5’
…C A T…
5’
…G T A…
‘anti-sense’ strand
3’
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
‘sense’ strand
5’
…C A T…
5’
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
4. Process in Bacteria
…G T A…
‘anti-sense’ strand
a. Polymerase (with sigma) lands on ds-DNA at promoter
3’
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
‘sense’ strand
5’
C C C A G T C A T G G G T….
GGG
5’
3’
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
4. Process in Bacteria
a. Polymerase (with sigma) lands on ds-DNA at promoter
b. Helicases separate strands; first bases linked 5’  3’
5’
3’
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
‘sense’ strand
5’
C C C A G T C A T G G G T….
G G G UC A GU A C C C A…
5’
3’
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
4. Process in Bacteria
a. Polymerase (with sigma) lands on ds-DNA at promoter
b. Helicases separate strands; first bases linked 5’  3’
c. Sigma subunit dissociates, polymerization continues
5’
3’
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
‘sense’ strand
5’
C C C C G C A A G C G G G G A A T T….
G G G GC G UU C G C C C C U U A A…
3’
5’
3’
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
4. Process in Bacteria
a. Polymerase (with sigma) lands on ds-DNA at promoter
b. Helicases separate strands; first bases linked 5’  3’
c. Sigma subunit dissociates, polymerization continues
d. Termination:
sequences rich in C’s and G’s, followed by A’s and T’s
IX: DNA Function: Protein Synthesis
3’
‘sense’ strand
A. Overview:
B. Transcription:
5’
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
4. Process in Bacteria
G G G GC G U
5’
UCGCCCC
C C C C G C A A G C G G G G A A T T….
3’
a. Polymerase (with sigma) lands on ds-DNA at promoter
b. Helicases separate strands; first bases linked 5’  3’
c. Sigma subunit dissociates, polymerization continues
d. Termination:
sequences rich in C’s and G’s, followed by A’s and T’s
- Rho Independent:
- the C’s and G’s in the m-RNA for a ‘stem-loop’ structure;
and binds to a protein bound to the Polymerase (nusA)
3’
IX: DNA Function: Protein Synthesis
3’
‘sense’ strand
A. Overview:
B. Transcription:
5’
UCGCCCC
C C C C G C A A G C G G G G A A T T….
G G G GC G U
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
4. Process in Bacteria
5’
3’
a. Polymerase (with sigma) lands on ds-DNA at promoter
b. Helicases separate strands; first bases linked 5’  3’
c. Sigma subunit dissociates, polymerization continues
d. Termination:
sequences rich in C’s and G’s, followed by A’s and T’s
- Rho Independent:
- the C’s and G’s in the m-RNA for a ‘stem-loop’ structure;
and binds to a protein bound to the Polymerase (nusA)
- this causes the polymerase to pause, just as it is reading
the area rich in A’s… which have fewer h-bonds. The pausing and the destabilization of
the polymerase caused by the ‘stem-loop’ causes the m-RNA/polymerase to detach.
3’
IX: DNA Function: Protein Synthesis
3’
‘sense’ strand
A. Overview:
B. Transcription:
G G G GC G U
UCGCCCC
C C C C G C A A G C G G G G A A T T….
5’
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
4. Process in Bacteria
5’
3’
a. Polymerase (with sigma) lands on ds-DNA at promoter
b. Helicases separate strands; first bases linked 5’  3’
c. Sigma subunit dissociates, polymerization continues
d. Termination:
sequences rich in C’s and G’s, followed by A’s and T’s
- Rho Independent:
- recent research has identified certain RNA’s that loop
and bind a small protein produced BY their own code. So, if the product concentration
is high, the m-RNA binds the protein, loops, and shuts down transcription (downregulating the gene). These RNA’s that turn off their own gene are “riboswitches”
3’
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
‘sense’ strand
5’
C C C C G C A A G C G G G G A A T T….
G G G GC G UU C G C C C C U U A A…
3’
5’
3’
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
4. Process in Bacteria
a. Polymerase (with sigma) lands on ds-DNA at promoter
b. Helicases separate strands; first bases linked 5’  3’
c. Sigma subunit dissociates, polymerization continues
d. Termination:
sequences rich in C’s and G’s, followed by A’s and T’s
- Rho-dependent:
Rho – a circular hexamer, binds the m-RNA in C-G rich regions
IX: DNA Function: Protein Synthesis
3’
A. Overview:
B. Transcription:
‘sense’ strand
5’
C C C C G C A A G C G G G G A A T T….
G G G GC G UU C G C C C C U U A A…
3’
5’
3’
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
4. Process in Bacteria
a. Polymerase (with sigma) lands on ds-DNA at promoter
b. Helicases separate strands; first bases linked 5’  3’
c. Sigma subunit dissociates, polymerization continues
d. Termination:
sequences rich in C’s and G’s, followed by A’s and T’s
- Rho-dependent:
Rho – a circular hexamer, binds the m-RNA in C-G rich regions
it slides up the strand; decoupling the polymerase from the
m-RNA at A-T rich sites.
IX: DNA Function: Protein Synthesis
Three genes ‘read’ as a single unit
A. Overview:
B. Transcription:
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
4. Process in Bacteria
a. Polymerase (with sigma) lands on ds-DNA at promoter
b. Helicases separate strands; first bases linked 5’  3’
c. Sigma subunit dissociates, polymerization continues
d. Termination:
e. Polycistronic DNA:
In bacteria, proteins involved in the same metabolic process
are often encoded by neighboring genes that are read as a UNIT (operon),
IX: DNA Function: Protein Synthesis
Three genes ‘read’ as a single unit
A. Overview:
B. Transcription:
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
4. Process in Bacteria
a. Polymerase (with sigma) lands on ds-DNA at promoter
b. Helicases separate strands; first bases linked 5’  3’
c. Sigma subunit dissociates, polymerization continues
d. Termination:
e. Polycistronic DNA:
In bacteria, proteins involved in the same metabolic process
are often encoded by neighboring genes that are read as a UNIT (operon), producing one
m-RNA that has the transcript of all three genes…
IX: DNA Function: Protein Synthesis
Three genes ‘read’ as a single unit
A. Overview:
B. Transcription:
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
4. Process in Bacteria
a. Polymerase (with sigma) lands on ds-DNA at promoter
b. Helicases separate strands; first bases linked 5’  3’
c. Sigma subunit dissociates, polymerization continues
d. Termination:
e. Polycistronic DNA:
In bacteria, proteins involved in the same metabolic process
are often encoded by neighboring genes that are read as a UNIT (operon), producing one
m-RNA that has the transcript of all three genes… then, in TRANSLATION, ribosomes attach
at ‘start codons’ along the strand, synthesizing all proteins simultaneously.
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
4. Process in Bacteria:
Translation of m-RNA by ribosomes occurs even before m-RNA is complete!
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
4. Process in Bacteria:
5. Process in Eukaryotes:
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
4. Process in Bacteria:
5. Process in Eukaryotes:
- The chromatin must be unwound = chromatin remodeling
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
4. Process in Bacteria:
5. Process in Eukaryotes:
- The chromatin must be unwound = chromatin remodeling
- Initiation is regulated by ‘enhancer sequences’ upstream and
downstream from the gene, and transcription factor binding.
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
4. Process in Bacteria:
5. Process in Eukaryotes:
- The chromatin must be unwound = chromatin remodeling
- Initiation is regulated by ‘enhancer sequences’ upstream and
downstream from the gene, and transcription factor binding.
The enhancer sequences are “cis-acting regulatory elements” (CRE’s), because
they are on the same chromosome as the gene. Since the transcription factors are proteins
encoded elsewhere in the genome, even on different chromosomes, they are called “transacting regulatory elements”.
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
4. Process in Bacteria:
5. Process in Eukaryotes:
- The chromatin must be unwound = chromatin remodeling
- Initiation is regulated by ‘enhancer sequences’ upstream and
downstream from the gene, and transcription factor binding.
- Because the m-RNA is bound in the nucleus—separated from the
ribosomes—translation does not take place immediately.
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
1. The DNA template:
2. RNA Polymerase:
3. RNA triphosphate precursors:
4. Process in Bacteria:
5. Process in Eukaryotes:
- The chromatin must be unwound = chromatin remodeling
- Initiation is regulated by ‘enhancer sequences’ upstream and
downstream from the gene, and transcription factor binding.
- Because the m-RNA is bound in the nucleus—separated from the
ribosomes—translation does not take place immediately.
- Most significantly, the initial RNA product is PROCESSED.
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
C. RNA Processing:
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
C. RNA Processing:
- In prokaryotes, only r-RNA genes have introns; protein-encoding genes have a
continuous coding sequence.
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
C. RNA Processing:
- In prokaryotes, only r-RNA genes have introns; protein-encoding genes have a
continuous coding sequence. As such, the m-RNA can be translated as soon as it is made.
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
C. RNA Processing:
- In eukaryotes, the initial RNA
transcripts contain introns that are spliced
out.
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
C. RNA Processing:
- In eukaryotes, the initial RNA
transcripts contain introns that are spliced
out.
- In addition a 7-mG cap and
poly-A tail are added to the processed
RNA; probably to reduce the rate of
exonuclease activity in the cytoplasm.
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
C. RNA Processing:
Free guanine nucleoside
- In eukaryotes, the initial RNA
transcripts contain introns that are spliced
out.
- In addition a 7-mG cap and
poly-A tail are added to the processed
RNA; probably to reduce the rate of
exonuclease activity in the cytoplasm).
- Group I introns splice
themselves out of r-RNA; they have autocatalytic function. These were the first
RNA molecules found that had enzyme-like
catalytic properties = ribozymes (Cech,
1982)
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
C. RNA Processing:
- Group I introns splice
themselves out of r-RNA; they have autocatalytic function. These were the first
RNA molecules found that had enzyme-like
catalytic properties = ribozymes (Cech,
1982)
- Group II introns are
autocatalytic, too, and occur in
mitochondria and chloroplast m,t-RNA.
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
C. RNA Processing:
- Group I introns
- Group II introns
- nuclear introns:
m-RNA transcripts in the nucleus are very large, and
processing is more complicated
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
C. RNA Processing:
- Group I introns
- Group II introns
- nuclear introns:
m-RNA transcripts in the nucleus are very large, and
processing is more complicated. Terminal ‘GU-AG’
sequences are recognized by snRP’s (‘snurps’) that
have sn-RNA’s rich in Uracil (‘U’ designations).
sn-RNA = short, nuclear RNA… also “U-RNA”
snRP’s = short, nuclear, riboproteins
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
C. RNA Processing:
- Group I introns
- Group II introns
- nuclear introns:
m-RNA transcripts in the nucleus are very large, and
processing is more complicated. Terminal ‘GU-AG’
sequences are recognized by snRNP’s (‘snurps’) that
have sn-RNA’s rich in Uracil (‘U’ designations). Binding
of complementary snRP’s creates the spliceosome,
which creates a lariat structure in the RNA.
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
C. RNA Processing:
- Group I introns
- Group II introns
- nuclear introns:
m-RNA transcripts in the nucleus are very large, and
processing is more complicated. Terminal ‘GU-AG’
sequences are recognized by snRNP’s (‘snurps’) that
have sn-RNA’s rich in Uracil (‘U’ designations). Binding
of complementary snRP’s creates the spliceosome,
which creates a lariat structure in the RNA. The intron
is cleaved, and the exons are ligated together.
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
C. RNA Processing:
- Group I introns
- Group II introns
- nuclear introns:
m-RNA transcripts in the nucleus are very large, and
processing is more complicated. Terminal ‘GU-AG’
sequences are recognized by snRNP’s (‘snurps’) that
have sn-RNA’s rich in Uracil (‘U’ designations). Binding
of complementary snRPS’s creates the spliceosome,
which creates a lariat structure in the RNA. The intron
is cleaved, and the exons are ligated together. This
splicing can vary, such that a single m-RNA can be
spliced at different places and produce different
proteins…ultimately from the same gene!
IX: DNA Function: Protein Synthesis
A. Overview:
B. Transcription:
C. RNA Processing:
So, the final product has a 7mG cap, polyA tail, and a continuous message.
This ‘mature’ m-RNA leaves the nucleus
and enters the cytoplasm, where it will be
translated.