Difference between revisions of "Gene expression"

From Conservapedia
Jump to navigation Jump to search
Line 9: Line 9:
  
 
===Prokaryotic Transcription===
 
===Prokaryotic Transcription===
−
[[Image:Bacterial_rna_polymerase.png‎|Left|thumb|Prokaryotic RNA polymerase]]
+
[[Image:Bacterial_rna_polymerase.png‎|left|thumb|Prokaryotic RNA polymerase]]
−
[[Image:Image009.gif|right|thumb|RNA synthesis]]
 
  
 
Bacterial RNA polymerases are composed of multiple subunits.  The core enzyme consists of 2 α, β and β′ subunits.  This enzyme contains the catalytic activity for RNA synthesis.  Association of the core enzyme with the σ subunit is required for initiation of transcription.  This complex is referred to as the holoenzyme.  The holoenzyme binds to DNA nonspecifically and scans for a promoter sequence.  In E. coli, the most common type of promoter contains two conserved sequences centered at -10 and – 35.  The σ subunit interacts specifically with these sequences in the double stranded DNA, positioning the active site at +1 (transcription start site).  This complex is referred to as the closed complex.
 
Bacterial RNA polymerases are composed of multiple subunits.  The core enzyme consists of 2 α, β and β′ subunits.  This enzyme contains the catalytic activity for RNA synthesis.  Association of the core enzyme with the σ subunit is required for initiation of transcription.  This complex is referred to as the holoenzyme.  The holoenzyme binds to DNA nonspecifically and scans for a promoter sequence.  In E. coli, the most common type of promoter contains two conserved sequences centered at -10 and – 35.  The σ subunit interacts specifically with these sequences in the double stranded DNA, positioning the active site at +1 (transcription start site).  This complex is referred to as the closed complex.
 
<br />The second step in transcription initiation involves the unwinding of the DNA between the -10 region and the start site of transcription.  This is referred to as the open complex.
 
<br />The second step in transcription initiation involves the unwinding of the DNA between the -10 region and the start site of transcription.  This is referred to as the open complex.
−
 
+
[[Image:Image009.gif|left|thumb|RNA synthesis]]
 
[[Image:Image010.gif|right|thumb|Termination of Prokaryotic Transcription]]
 
[[Image:Image010.gif|right|thumb|Termination of Prokaryotic Transcription]]
  
Line 26: Line 25:
  
 
===Eukaryotic Transcription===
 
===Eukaryotic Transcription===
−
 
+
[[Image:Eukaryotic_rna_polymerase.png‎|Left|thumb|Eukaryotic Polymerase]]
 
The basic features of RNA synthesis are shared between prokaryotes and eukaryotes; however, transcription in eukaryotes differs in that it is significantly more complex. First, rather than having a single RNA polymerase, eukaryotes have three different RNA polymerases, each of which transcribes a different set of genes. RNA polymerase I transcribes three types of rRNA (the 18S, 5.8S, and 28S species), RNA polymerase II transcribes mRNA, and RNA polymerase III transcribes tRNA and the smallest rRNA (the 5S species). The eukaryotic RNA polymerases consist of between eight and fourteen subunits, with two of them corresponding to the β and β′ subunits of prokaryotic RNA polymerases.
 
The basic features of RNA synthesis are shared between prokaryotes and eukaryotes; however, transcription in eukaryotes differs in that it is significantly more complex. First, rather than having a single RNA polymerase, eukaryotes have three different RNA polymerases, each of which transcribes a different set of genes. RNA polymerase I transcribes three types of rRNA (the 18S, 5.8S, and 28S species), RNA polymerase II transcribes mRNA, and RNA polymerase III transcribes tRNA and the smallest rRNA (the 5S species). The eukaryotic RNA polymerases consist of between eight and fourteen subunits, with two of them corresponding to the β and β′ subunits of prokaryotic RNA polymerases.
  
 
All three eukaryotic polymerases have some homology to the E. coli core polymerase (α2ββ′), but also contain additional subunits not found in prokaryotes.  In total, approximately 12-17 subunits (varies with the species) are required for activity in vivo, making eukaryotic polymerases much more complex than prokaryotic polymerases.
 
All three eukaryotic polymerases have some homology to the E. coli core polymerase (α2ββ′), but also contain additional subunits not found in prokaryotes.  In total, approximately 12-17 subunits (varies with the species) are required for activity in vivo, making eukaryotic polymerases much more complex than prokaryotic polymerases.
−
 
+
[[Image:Rna_polymerase_ii.png‎|right|thumb|Eukaryotic Polymerase II (PolII)]]
 
'''RNA Polymerase II'''
 
'''RNA Polymerase II'''
  
 
Near the carboxy end of Pol II’s largest subunit a unique region is found (CTD, carboxy terminal domain).  This region contains a stretch of 7 amino acids that is repeated between 26 and 52 times (differences in the number of repeats occur in a species specific manner).  During transcription initiation several amino acids in the repeat becomes phosphorylated.
 
Near the carboxy end of Pol II’s largest subunit a unique region is found (CTD, carboxy terminal domain).  This region contains a stretch of 7 amino acids that is repeated between 26 and 52 times (differences in the number of repeats occur in a species specific manner).  During transcription initiation several amino acids in the repeat becomes phosphorylated.
−
 
+
[[Image:Rna_polymerase_features.png‎|left|thumb|Transcription of DNA with PolII]]
 
'''Promoters'''
 
'''Promoters'''
  
Line 40: Line 39:
  
 
Although Pol II is very large and complex, it is unable to neither recognize specific promoter sequences on its own nor catalyze all the steps required for transcription initiation.  It requires the help of several factors known as general transcription factors (GTFs).  The factors required at most promoters are TFIIA, TFIID, TFIIE, TFIIF and TFIIH.
 
Although Pol II is very large and complex, it is unable to neither recognize specific promoter sequences on its own nor catalyze all the steps required for transcription initiation.  It requires the help of several factors known as general transcription factors (GTFs).  The factors required at most promoters are TFIIA, TFIID, TFIIE, TFIIF and TFIIH.
−
 
+
[[Image:RNAPII_holo.gif‎|left|thumb|Holoenzyme]]
 
'''Holoenzyme'''
 
'''Holoenzyme'''
  
Line 68: Line 67:
  
 
==Eukaryotic RNA Processing==
 
==Eukaryotic RNA Processing==
 +
 +
===Splicing===
 +
Transcripts in eukaryotes are c. 10 000 nt, but polypeptides are 400 aa = 1200 nt. 80% of transcript is not translated, i.e. pre-mRNA is generally 7800 bases longer than a typical 1200 bp mRNA. What happens to all this excess RNA? Eukaryotic genes (and some Archaeal gene) contain internal 'junk' (discovered in 1977). Exons (which are expressed, and exit the nucleus) are separated by introns (100 - 100 000 nt long), which are removed from the pre-mRNA and destroyed without ever leaving the nucleus. The size of introns is species-specific: yeasts have very few, some human genes can have 50 in a single gene! Typically there are four introns within five exons, but the introns (as mentioned above) are generally much larger than the exons. Introns must be spliced out from between the exons before the mRNA can leave the nucleus.
 +
 +
5′---E1---I1---E2---I2---E3---I3---E4---I4---E5---3′
 +
 +
As mRNA is transcribed, it is complexed by five small nuclear ribonucleoproteins (snRNPs, also known as 'U' particles).RNAse treatment of pre-mRNA degrades pre-mRNA into heterogeneous ribonucleoprotein fragments. These proteins have a common RNA binding motif (below). Some of these hnRNPs are U-proteins, directly involved in splicing out introns, others are important for labelling introns and exons.
 +
 +
H3N+---{KR}G{FY}{AG}{FY}VX{FY}---…---COO−
 +
 +
Sufferers of systemic lupus erythrematosus (SLE) produce autoimmune anti-snRNP antibodies, which can be used to reveal islands of snRNP in nucleus, which are know as Cajal bodies.
 +
 +
Splice sequences are quite variable across species, but in general:
 +
 +
Exon1 ends AG (the donor site).
 +
There must be internal A in branch site, which reacts chemically with the donor junction to effect the splice.
 +
Exon2 starts G (the acceptor site).
 +
5′  donor      branch  acceptor  3′
 +
 +
5′---AGGURAGU---CURAYY---YNCAGG---3′
 +
 +
5′---AGG---3′
 +
 +
The intron sequence is spliced out of the transcript to generate the processed mRNA. This is the human consensus sequence.
 +
 +
The U-particles combine with one another in a variety of ways to for spliceosomes. In the most typical splicing reaction, U1 binds the 5′ donor. U2 then binds the 3′ acceptor and bridge sequence. U4 and U6 act together to form a lariat, and then U5 ligates the exons together. Note that U2, U5 and U6 remain attached to the lariat in U-catalysed splicing. This helps mark the lariat for degradation.
 +
[[Image:Splicing.png|left|thumb|Eukaryotic RNA Processing]]
 +
Tetrahymena (a ciliate) and some Archaea have self-splicing introns. The snRNP system described above probably evolved from (type-II) self-splicing systems. Note that there are several other varieties of splicing, using the U particles whose existence you might be wondering about.
 +
[[Image:U_and_self_splicing.png|right|thumb|Self Splicing]]
 +
 +
The donor and acceptor splice junctions are correctly paired off in multi-intron genes because splicing is concurrent with transcription. Some specificity may also be allowed by the hnRNPs and SR-rich proteins that bind to the pre-mRNA. Capping, splicing and poly-A factors are associated with the tail of RNA polymerase II: the enzyme complex is a factory, both producing mRNA and processing it ready for export.
 +
 +
Splicing seems to be an expensive waste of time for the most part; however, under certain circumstances, it has advantages in producing more than one gene product from a single stretch of DNA. In B-lymphocytes, early in development, the cell produces membrane-tethered immunoglobulin (Ig) receptors by transcription and splicing of the entire gene:
 +
 +
Long transcript:
 +
 +
5′---Ig---donor---stop---acceptor---hydrophobic---stop---3′
 +
 +
After splicing:
 +
 +
5′---Ig----hydrophobic----stop----3′
 +
 +
Protein:
 +
 +
H3N+-Ig-hydrophobic-COO−
 +
 +
The longer transcript splices out the first stop codon, producing an antibody with a hydrophobic carboxy terminus, which allows it to be tethered into the cell membrane. However, after the cell has recognised an antigen, it begins to produce soluble antibodies instead. Recognition of antigen increases the amount of CStF in the cell, producing a shorter transcript of the gene, which lacks the intron acceptor sequence:
 +
 +
Short transcript:
 +
 +
5′---Ig---donor---stop---3′
 +
 +
No splicing, because of lack of acceptor junction, so the protein is:
 +
 +
H3N+-Ig-COO−
 +
 +
The shorter transcript produces a protein with no hydrophobic peptide, which is plasma-soluble.
 +
 +
We've mostly been speaking about mRNA modification, but the other types of RNA are even more heavily modified (edited) than mRNA…
 +
 +
tRNAs are c. 80 bases long, and in eukaryotes, there are at least 31 in the cytoplasm, 30 in chloroplasts and 22 in mitochondria. All tRNAs have an acceptor stem with a 3′ CCA sequence, which (covalently) binds amino acids. They all also have a triplet anticodon, which (transiently) binds mRNA. The general structure is often termed a 'cloverleaf', which is true of the secondary structure, but somewhat misleading for the tertiary.
 +
 +
[[Image:Trna_phe_cartoon.png|right|thumb|tRNA encoding Phenylalanine, tRNA has 'L'-shaped tertiary structure.]]
 +
 +
Modification of the tRNA anticodon produces gibberish proteins. Chemical modification of tRNAcys to tRNAala also produces non-functional proteins. The anticodon/amino-acid pairing is essential to correct translation of mRNA.
 +
 +
tRNA contains unusual nucleosides. Inosine (needed for wobble). Pseudouridine (only in Eukaryotes and Archaea). Dihydrouridine. Thymine (unusual for RNA). Wybutosine (found just after the anticodon).
 +
 +
Chemical modification of tRNAs is performed by proteins, unlike mRNA splicing. One major effect of modification is to allow wobble. Wobble is the non-Watson-Crick pairing at the third base of a codon, which means that fewer anticodons (tRNAs) are needed than the 64 possible codons: 64 codons. 20 amino acids. 30 is the compromise made.
 +
 +
There is generally more wobble in bacteria (see bracketed bases below). Mitochondria even have wobble at the second base of the codon.
 +
 +
{| class="wikitable"
 +
|-
 +
! Wobble codon base
 +
! Possible anticodon bases
 +
|-
 +
| U
 +
| A), G, I
 +
|-
 +
| C
 +
| G, I
 +
|-
 +
| A
 +
| U, (I)
 +
|-
 +
| G
 +
| C, (U)
 +
|}
 +
 +
In trypanosome mitochondria, guide RNA (gRNA) guides excision and addition (insertion/deletion or indel) of bases (mostly U) to mRNA.
 +
 +
gRNA above; mRNA below:
 +
 +
3′---UU AUCUCAACCGACCA---5′
 +
 +
5′---AAGUAGAG~~GGCUGGU---3′
 +
 +
Note that the is G displaced and the AA causes stretching, so the edited RNA looks like:
 +
 +
5′---AA|UAGAGUUGGCUGGU---3′
 +
 +
With the G excised and UU inserted in the stretch.
 +
 +
  
 
== Bibliography ==  
 
== Bibliography ==  

Revision as of 13:56, May 4, 2007

This article is currently undergoing a major revision. Please do not edit this page until this template is removed.


Genes are segments of DNA that contain the information required to make a protein. Gene expression refers to all the processes involved in converting genetic information from a DNA sequence, or protein. In prokaryotes, there are just two processes required: transcription and translation. In eukaryotes, there is an additional step RNA processing (splicing), which intervenes.

Transcription

The synthesis of a single-stranded RNA molecule using DNA as a template is referred to as transcription. The enzyme that catalyzes this reaction is known as RNA polymerase. Although the subunit structure and details of the process differ significantly in prokaryotes and eukaryotes, the chemical reaction is identical.

Prokaryotic Transcription

Prokaryotic RNA polymerase

Bacterial RNA polymerases are composed of multiple subunits. The core enzyme consists of 2 α, β and β′ subunits. This enzyme contains the catalytic activity for RNA synthesis. Association of the core enzyme with the σ subunit is required for initiation of transcription. This complex is referred to as the holoenzyme. The holoenzyme binds to DNA nonspecifically and scans for a promoter sequence. In E. coli, the most common type of promoter contains two conserved sequences centered at -10 and – 35. The σ subunit interacts specifically with these sequences in the double stranded DNA, positioning the active site at +1 (transcription start site). This complex is referred to as the closed complex.
The second step in transcription initiation involves the unwinding of the DNA between the -10 region and the start site of transcription. This is referred to as the open complex.

RNA synthesis
Termination of Prokaryotic Transcription

RNA Synthesis

The first two rNTPs complementary to the DNA bind to the active site of RNA polymerase and the first phosphodiester bond is formed. Synthesis is always in the 5’ to 3’ direction and involves the nucleophic attack by the 3’OH on the 5’ phosphate with the release of pyrophosphate. After approximately 8-10 ribonucleotides are added, σ factor is released and RNA polymerase core enzyme moves away from the promoter, synthesizing RNA as it goes.

Termination

Transcription continues until a termination signal is reached. The current model is that the RNA dissociates from the DNA – RNA hybrid to form a stem-loop structure. The remaining AU base pairs are not strong enough to keep RNA hybridized with the DNA. Dissociation of the RNA destabilizes RNA polymerases and transcription terminates.

Eukaryotic Transcription

Eukaryotic Polymerase

The basic features of RNA synthesis are shared between prokaryotes and eukaryotes; however, transcription in eukaryotes differs in that it is significantly more complex. First, rather than having a single RNA polymerase, eukaryotes have three different RNA polymerases, each of which transcribes a different set of genes. RNA polymerase I transcribes three types of rRNA (the 18S, 5.8S, and 28S species), RNA polymerase II transcribes mRNA, and RNA polymerase III transcribes tRNA and the smallest rRNA (the 5S species). The eukaryotic RNA polymerases consist of between eight and fourteen subunits, with two of them corresponding to the β and β′ subunits of prokaryotic RNA polymerases.

All three eukaryotic polymerases have some homology to the E. coli core polymerase (α2ββ′), but also contain additional subunits not found in prokaryotes. In total, approximately 12-17 subunits (varies with the species) are required for activity in vivo, making eukaryotic polymerases much more complex than prokaryotic polymerases.

Eukaryotic Polymerase II (PolII)

RNA Polymerase II

Near the carboxy end of Pol II’s largest subunit a unique region is found (CTD, carboxy terminal domain). This region contains a stretch of 7 amino acids that is repeated between 26 and 52 times (differences in the number of repeats occur in a species specific manner). During transcription initiation several amino acids in the repeat becomes phosphorylated.

Transcription of DNA with PolII

Promoters

A typical eukaryotic promoter is located from approximately -40 to +50 relative to the start site (+1). This is referred to as a “core promoter”, with additional regulatory sites being located nearby or at a large distance (enhancers and silencers). Within the core promoter several conserved sequences have been identified. The most prominent (though not found in all promoters) is the TATA motif (5'-TATAAA-3') or TATA box (Goldberg-Hogness box [1]). Located at approximately -30, this highly conserved sequence is easy to identify. However, some promoters are missing the TATA box and through the analysis of these promoters other less conserved sequences have been found. Once is located around +1 and is called the Initiator element (Inr).

Although Pol II is very large and complex, it is unable to neither recognize specific promoter sequences on its own nor catalyze all the steps required for transcription initiation. It requires the help of several factors known as general transcription factors (GTFs). The factors required at most promoters are TFIIA, TFIID, TFIIE, TFIIF and TFIIH.

Holoenzyme

Holoenzyme

TFIIE, TFIIF and TFIIH associate with RNA Polymerase II and another protein complex (the mediator) to form a holoenzyme. TFIIA and TFIIB are sometimes found together with the holoenzyme, and in other experiments they seem to bind independently. In any event, the holoenzyme is unable to recognize promoter squences and depends on TFIID binding at the TATA box. TFIIA and TFIIB bind to the DNA adjacent to the TATA box and also interact with TFIID.

TFIID

TFIID is composed of a single polypeptide known as the TATA binding protein or TBP, plus approximately 14 additional polypeptides known as TBP Associated Factors or TAFIIs. TBP specifically binds to the TATA box, causing a sharp bend in the DNA. It is this binding of TBP with its associated factors that begins the assembly of the remaining GTFs and the holoenzyme at the promoter.

TFIIE and TFIIH

After binding of the general transcirption machinery to the promoter, TFIIE and TFIIH assist Pol II in unwinding the DNA at the start site of transcription. One of the subunits of TFIIH is a helicase responsible for the unwinding and TFIIE binds to and stablilizes the single stranded DNA.

Another subunit of TFIIH contains a kinase activity responsible for phosphorylating the CTD region of Pol II. Although this activity can be influenced by many factors, in simple systems it is believed to correlate with the initiation of transcription and that movement of RNA polymerase away from the promoter.

Summery of Transcription

Although eukaryotic transcription is much more complex than prokaryotic transcription, initiation involves the same basic steps in both:

  1. Promoter Recognition
  2. Unwinding DNA around +1
  3. Synthesis of first few polynucleotides
  4. Release of polymerase from promoter

In prokaryotes, termination occures at discrete sites and translation can begin even before transcription has terminated. In eukaryotes, transcription continues past the site where poly-A addition occurs and then terminates randomly (frequently 500 bases or so down stream). RNA processing and splicing is completed in the nucleus before transported to the cytoplasm for translation.

Translation

Eukaryotic RNA Processing

Splicing

Transcripts in eukaryotes are c. 10 000 nt, but polypeptides are 400 aa = 1200 nt. 80% of transcript is not translated, i.e. pre-mRNA is generally 7800 bases longer than a typical 1200 bp mRNA. What happens to all this excess RNA? Eukaryotic genes (and some Archaeal gene) contain internal 'junk' (discovered in 1977). Exons (which are expressed, and exit the nucleus) are separated by introns (100 - 100 000 nt long), which are removed from the pre-mRNA and destroyed without ever leaving the nucleus. The size of introns is species-specific: yeasts have very few, some human genes can have 50 in a single gene! Typically there are four introns within five exons, but the introns (as mentioned above) are generally much larger than the exons. Introns must be spliced out from between the exons before the mRNA can leave the nucleus.

5′---E1---I1---E2---I2---E3---I3---E4---I4---E5---3′

As mRNA is transcribed, it is complexed by five small nuclear ribonucleoproteins (snRNPs, also known as 'U' particles).RNAse treatment of pre-mRNA degrades pre-mRNA into heterogeneous ribonucleoprotein fragments. These proteins have a common RNA binding motif (below). Some of these hnRNPs are U-proteins, directly involved in splicing out introns, others are important for labelling introns and exons.

H3N+---{KR}G{FY}{AG}{FY}VX{FY}---…---COO−

Sufferers of systemic lupus erythrematosus (SLE) produce autoimmune anti-snRNP antibodies, which can be used to reveal islands of snRNP in nucleus, which are know as Cajal bodies.

Splice sequences are quite variable across species, but in general:

Exon1 ends AG (the donor site). There must be internal A in branch site, which reacts chemically with the donor junction to effect the splice. Exon2 starts G (the acceptor site). 5′ donor branch acceptor 3′

5′---AGGURAGU---CURAYY---YNCAGG---3′

5′---AGG---3′

The intron sequence is spliced out of the transcript to generate the processed mRNA. This is the human consensus sequence.

The U-particles combine with one another in a variety of ways to for spliceosomes. In the most typical splicing reaction, U1 binds the 5′ donor. U2 then binds the 3′ acceptor and bridge sequence. U4 and U6 act together to form a lariat, and then U5 ligates the exons together. Note that U2, U5 and U6 remain attached to the lariat in U-catalysed splicing. This helps mark the lariat for degradation.

Eukaryotic RNA Processing

Tetrahymena (a ciliate) and some Archaea have self-splicing introns. The snRNP system described above probably evolved from (type-II) self-splicing systems. Note that there are several other varieties of splicing, using the U particles whose existence you might be wondering about.

Self Splicing

The donor and acceptor splice junctions are correctly paired off in multi-intron genes because splicing is concurrent with transcription. Some specificity may also be allowed by the hnRNPs and SR-rich proteins that bind to the pre-mRNA. Capping, splicing and poly-A factors are associated with the tail of RNA polymerase II: the enzyme complex is a factory, both producing mRNA and processing it ready for export.

Splicing seems to be an expensive waste of time for the most part; however, under certain circumstances, it has advantages in producing more than one gene product from a single stretch of DNA. In B-lymphocytes, early in development, the cell produces membrane-tethered immunoglobulin (Ig) receptors by transcription and splicing of the entire gene:

Long transcript:

5′---Ig---donor---stop---acceptor---hydrophobic---stop---3′

After splicing:

5′---Ig----hydrophobic----stop----3′

Protein:

H3N+-Ig-hydrophobic-COO−

The longer transcript splices out the first stop codon, producing an antibody with a hydrophobic carboxy terminus, which allows it to be tethered into the cell membrane. However, after the cell has recognised an antigen, it begins to produce soluble antibodies instead. Recognition of antigen increases the amount of CStF in the cell, producing a shorter transcript of the gene, which lacks the intron acceptor sequence:

Short transcript:

5′---Ig---donor---stop---3′

No splicing, because of lack of acceptor junction, so the protein is:

H3N+-Ig-COO−

The shorter transcript produces a protein with no hydrophobic peptide, which is plasma-soluble.

We've mostly been speaking about mRNA modification, but the other types of RNA are even more heavily modified (edited) than mRNA…

tRNAs are c. 80 bases long, and in eukaryotes, there are at least 31 in the cytoplasm, 30 in chloroplasts and 22 in mitochondria. All tRNAs have an acceptor stem with a 3′ CCA sequence, which (covalently) binds amino acids. They all also have a triplet anticodon, which (transiently) binds mRNA. The general structure is often termed a 'cloverleaf', which is true of the secondary structure, but somewhat misleading for the tertiary.

tRNA encoding Phenylalanine, tRNA has 'L'-shaped tertiary structure.

Modification of the tRNA anticodon produces gibberish proteins. Chemical modification of tRNAcys to tRNAala also produces non-functional proteins. The anticodon/amino-acid pairing is essential to correct translation of mRNA.

tRNA contains unusual nucleosides. Inosine (needed for wobble). Pseudouridine (only in Eukaryotes and Archaea). Dihydrouridine. Thymine (unusual for RNA). Wybutosine (found just after the anticodon).

Chemical modification of tRNAs is performed by proteins, unlike mRNA splicing. One major effect of modification is to allow wobble. Wobble is the non-Watson-Crick pairing at the third base of a codon, which means that fewer anticodons (tRNAs) are needed than the 64 possible codons: 64 codons. 20 amino acids. 30 is the compromise made.

There is generally more wobble in bacteria (see bracketed bases below). Mitochondria even have wobble at the second base of the codon.

Wobble codon base Possible anticodon bases
U A), G, I
C G, I
A U, (I)
G C, (U)

In trypanosome mitochondria, guide RNA (gRNA) guides excision and addition (insertion/deletion or indel) of bases (mostly U) to mRNA.

gRNA above; mRNA below:

3′---UU AUCUCAACCGACCA---5′

5′---AAGUAGAG~~GGCUGGU---3′

Note that the is G displaced and the AA causes stretching, so the edited RNA looks like:

5′---AA|UAGAGUUGGCUGGU---3′

With the G excised and UU inserted in the stretch.


Bibliography

  • Stryer, Lubert. Biochemistry, 4th ed. New York: W. H. Freeman and Company, 1995
  • Tijian, Robert. "Molecular Machines that Control Genes." Scientific American 272 (1995): 54–61.


References

  1. ↑ Lifton RP, Goldberg ML, Karp RW, Hogness DS. (1978). The organization of the histone genes in Drosophila melanogaster: functional and evolutionary implications. Cold Spring Harb Symp Quant Biol. 42, 1047-1051. PMID 98262
Retrieved from "https://www.conservapedia.com/index.php?title=Gene_expression&oldid=142907"