1. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach RNA Classification Messenger RNA (mRNA) is produced by protein- encoding genes and is a short-lived intermediary between DNA and protein It is the only type of RNA that undergoes translation Transcription of mRNA is often followed by post- transcriptional processing 1

2. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Functional RNAs Functional RNAs do not encode proteins, but instead perform functional roles in the cell Transfer RNAs (tRNAs) are encoded in dozens of forms and are responsible for binding an amino acid and depositing it for inclusion into a growing protein chain Ribosomal RNA (rRNA) combines with numerous proteins to form ribosomes 2

3. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Additional Functional RNAs Small nuclear RNA (snRNA) of various types is found in the nucleus of eukaryotes and plays a role in mRNA processing Micro RNA (miRNA) is active in plant and animal cells and is involved in postranscriptional regulation of mRNA 3

4. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Additional Functional RNAs Small interfering RNA (siRNA) protects plant and animal cells from production of viruses and movement of transposons Certain RNAs in eukaryotic cells have catalytic activity; these are called ribozymes 4

5. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach 8.3 Eukaryotic Transcription Uses Multiple RNA Polymerases Eukaryotes have three different RNA polymerases that recognize different promoters and produce different types of RNAs 5

6. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Eukaryotic Transcription Eukaryotic genes carry introns and exons, and require processing to remove introns Eukaryote DNA is associated with proteins to form chromatin; the chromatin composition of a gene affects its transcription Chromatin thus plays an important role in gene regulation of eukaryotes 6

7. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Eukaryotic Polymerases RNA polymerase I (RNA pol I) transcribes three ribosomal RNA genes RNA polymerase II (RNA pol II) transcribes protein coding genes and most small nuclear RNA genes RNA polymerase III (RNA pol III) transcribes tRNA, one small nuclear RNA, and one ribosomal RNA 7

8. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Eukaryotic Polymerases, continued Each eukaryotic (and archaeal) RNA polymerase contains units that share homology with the 5 subunits of the bacterial polymerase Arachaea and eukaryotes have 6 to 11 additional subunits All RNA polymerases share a similar “hand” shape with “fingers” that grasp DNA and a “palm” where RNA synthesis takes place 8

9. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Cis-Acting Regulatory Sequences Bind Trans-Acting Regulatory Proteins to Control Eukaryotic Transcription Activator proteins bind regulatory sequences to stimulate transcription Repressor proteins bind other sequences to hinder transcription The regulatory proteins are found in large complexes in eukaryotes 9

10. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Transcriptional Regulatory Interactions Three sets of regulatory DNA sequences are commonly involved in eukaryotic gene regulation The core promoter region, containing the TATA box and other sequences, is immediately adjacent to the start of transcription; these bind RNA polymerase II and its associated transcription factors Upstream of the core promoter region are various proximal elements that bind regulatory proteins The last group of regulatory sequences are “enhancers” 10

11. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Cis-Acting Regulatory Sequences and Trans-Acting Proteins All three regulatory regions contain cis-acting regulatory sequences, which regulate transcription of genes on the same chromosome as the sequences RNA polymerase II (Pol II) and various general transcription factors (GTFs) bind the core promoter; these are trans-acting regulatory proteins, which can bind to their target sequences on any chromosome At enhancers, aggregations of multiple proteins form large complexes called enhanceosomes 11

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13. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Promoter Elements The most common eukaryotic promoter consensus sequence is the TATA box, or the Goldberg- Hogness box, located at about position  25 The consensus sequence is 5-TATAAA-3 A CAAT box is often found near the -80 position A GC-rich box (consensus 5-GGGCGG-3) is located at  90, or further upstream 13

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15. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Eukaryotic Promoter Elements Eukaryotic promoters display a high degree of variability in type, number, and location of consensus sequence elements The TATA box is most common, whereas the CAAT box and GC-rich box are more variable 15

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17. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Promoter Recognition RNA pol II recognizes and binds to promoter sequences with the aid of proteins called transcription factors (TFs) TFs bind to regulatory sequences and interact directly, or indirectly, with RNA polymerase; those interacting with pol II are called TFII factors The TATA box is the principle binding site during promoter recognition 17

18. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Promoter Recognition, continued At the TATA box, TFIID, a multisubunit protein binds the TATA box sequence The assembled TFIID bound to the TATA box forms the initial committed complex Next, TFIIB, TFIIF, and RNA pol II join the complex to form the minimal initiation complex 18

19. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Promoter Recognition, continued The minimal initiation complex is joined by TFIIE and TFIIH to form the complete initiation complex The complete initiation complex contains multiple proteins commonly referred to as “general transcription factors” The complete complex directs RNA pol II to the  1 position, where it begins to assemble mRNA 19

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21. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Enhancers and Silencers Promoters alone may not be sufficient to initiate eukaryotic transcription Two categories of DNA regulatory sequences lead to differential expression of genes These are enhancer sequences and silencer sequences 21

22. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Enhancer Sequences Enhancer sequences increase the level of transcription of specific genes They bind proteins that interact with the proteins that are bound to gene promoters, and together the promoters and enhancers drive gene expression Enhancers may be variable distances from the genes they affect and may be upstream or downstream of the gene 22

23. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Enhancer Sequences and DNA Bending Enhancer sequences bind activator proteins and associated coactivators that form a “protein bridge” that links the proteins at the enhancer sequence to the initiation complex at the promoter This bridge bends the DNA so that the proteins at both locations are brought close enough together for them to interact This bridge is known as an “enhanceosome” 23

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25. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Silencer Sequences Silencer sequences are DNA elements that act at a distance to repress transcription of their target genes Silencers bind transcription factors called repressor proteins that induce bends in DNA These bends reduce transcription of the target gene Silencers may be located variable distances from their target genes, either upstream or downstream 25

26. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Insulator Sequences Insulator sequences are cis-acting sequences located between enhancers and the promoters of genes that need to be protected from the action of the enhancers Insulators ensure that only the target gene is regulated by the enhancer Insulators may allow formation of DNA loops that contain the enhancers and their intended target promoters while excluding nontarget genes 26

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28. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Transcription Factors and Signal Transduction Tissue-specific and developmental gene expression are also affected by the presence of specific transcription factors and signal transduction pathways These pathways communicate the need for the specific regulatory molecules, such as the specific transcription factors needed for a particular gene Distinct transcription factor proteins are pivotal in regulating transcription of such genes 28

29. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Transcription Factor Synthesis Transcription factor availability is dependent on their transcription and translation in the cell Synthesis of transcription factors is tightly regulated and different cell types have distinct arrays of transcription factors The availability of activated transcription factors is also controlled through signal transduction pathways 29

30. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Signal Transduction Signal transduction pathways are sequential events that release regulatory molecules inside a cell in response to events outside the cell They utilize transmembrane proteins, which receive signals externally through an extracellular interaction domain They transmit signals within the cell via a binding domain inside the cell; this activates a transcription factor needed for expression of a target gene 30

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32. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Enhancer-Sequence Conservation Implies selection to retain function Enhancers for particular proteins, such as  - interferon, are conserved among mammals 32

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34. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Yeast Upstream Activator Sequences In the yeast Saccharomyces cerevisiae, transcription of genes in the galactose utilization pathway are carefully regulated by enhancer-like sequences When galactose is the only sugar available, wild-type yeast induce transcription of four enzyme-producing genes, GAL1, GAL2, GAL7, and GAL10 Together these import and then break down galactose 34

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36. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Upstream Activator Sequences Each of the GAL genes has its own promoter and similar enhancer-like sequences that are bound by a regulatory protein, Gal4, encoded by the GAL4 gene The enhancer-like element is called the upstream activator sequence (UAS, or UAS G ) Gal4 is continuously present in cells and interacts with the Gal80 protein that binds Gal4 and keeps it inactive in the absence of galactose 36

37. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Gal4 Function In the Absence of Galactose Each UAS G element contains two 17-bp repeat sequences that are binding sites for Gal4 Gal4 functions as a homodimer with each polypeptide forming two active domains One binds the 17-bp target sequence and the other interacts with Gal80 When Gal4 is bound to Gal80, the Gal4 DNA-binding domain is unable to bind UAS G 37

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39. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Gal4 Function in the Presence of Galactose When galactose is present, galactose and Gal3, encoded by the GAL3 gene, bind to Gal80 Gal80 releases Gal4, freeing the DNA-binding domain of Gal4 to recognize and bind to the UAS G sites The transcriptional activation domain then activates transcription of the GAL genes 39

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41. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Gal4 Activation of Transcription Gal4 binding of UASG leads to formation of a multiprotein complex called the Mediator The Mediator is an enhanceosome that induces formation of a DNA loop, making contact with the general transcription apparatus at the GAL gene promoters 41

42. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Repressor Proteins and Silencer Sequences Eukaryotic repressors inhibit transcription through different mechanisms than those seen in bacteria One mechanism is the binding of repressor proteins to silencer sequences, cis-acting regulatory sequences that directly prevent enhancer-mediated transcription The GAL genes utilize such a mechanism; the protein Mig1 is produced in the presence of glucose and binds silencer sequences upstream of the GAL genes 42

43. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Repression of the GAL Genes The silencer sequences of the GAL genes are located between the UAS G sequences and the promoters When bound, Mig1 attracts the protein Tup1 and the two together form a repressor complex that prevents UAS G from initiating transcription 43

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45. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Locus Control Regions The human  -globin gene encodes the  -globin polypeptide, two copies of which join with two copies of  -globin to form hemoglobin It is one of six closely related globin genes forming the  -globin complex, with a locus control region close to it A locus control region (LCR) is a highly specialized enhancer that regulates transcription of multiple genes packaged into complexes of closely related genes 45

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47. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach The  -Globin Locus Control Region The LCR regulating the  -globin complex contains four distinct sequences, named HS1 to HS4 These work together to produce the correct expression of each type of  -globin gene throughout development Each gene of the complex produces a distinct polypeptide with unique oxygen-carrying capacity 47

48. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach The  -Globin Locus Control Region, continued The HS1 to HS4 components bind regulatory proteins that direct the formation of small DNA loops These serve as a bridge to the promoters of the  -globin complex genes The composition of enhanceosomes bound to the LCR at different developmental stages is varied, such that the appropriate genes are expressed at each stage 48

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51. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Enhancer Mutations Mutation in the  - and  -globin genes produce hereditary anemias called thalassemia Some thalassemia patients were identified with no discernible mutations in the protein-coding part of the globin genes, nor in their promoters It was found that some thalassemia cases were due to deletions that altered the LCR regions, causing abnormal expression of the globin genes 51

52. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Transcriptional Regulation by Enhancers and Silencers Occasionally the same sequence can act as an enhancer or a silencer, depending on which regulatory proteins are present and bind to the sequence Sometimes the enhancer or silencer sequence is very distant from the gene it regulates A model for understanding transcriptional regulation must include a mode of action of enhancers and silencers and accommodate the variable distance and position they occupy relative to the target gene 52

53. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach A Model for Control of Eukaryotic Transcription The Sonic hedgehog (SHH) gene in humans and mammals directs limb formation under an enhancer 1 million base pairs away from the SHH gene SHH is expressed in tissue-specific fashion due to the action of two different enhancers One combination of regulatory proteins binds the brain enhancer in brain tissue, but a different combination binds the limb enhancer in developing limbs 53

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55. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach RNA Polymerase I Promoters RNA polymerase I transcribes genes for rRNA using a mechanism similar to that of RNA pol II RNA pol I is recruited to upsteam promoter elements following binding of transcription factors, and transcribes ribosomal genes found in the nucleolus The nucleolus is a nuclear organelle containing rRNA and multiple copies of genes encoding rRNA 55

56. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach RNA Polymerase I Promoters, continued Promoters recognized by RNA pol I have two functional sequences near the start of transcription The core element stretches from  45 to  20, and is needed for initiation of transcription; it is bound by sigma-like factor 1 (SL1) protein The upstream control element spans from  100 to  150, and increases the level of transcription; it is bound by upstream binding factor 1 (UBF1) 56

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58. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach RNA Polymerase III Promoters RNA polymerase III is primarily responsible for transcription of tRNA genes, but also transcribes one rRNA and other RNA-encoding genes Small nuclear RNA genes have three upstream elements whereas the 5S rRNA and tRNA genes each have two internal promoter elements These are downstream of the start of transcription 58

59. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach RNA Polymerase III Promoters—The Upstream Elements The upstream elements of the snRNA genes are a TATA box, a promoter-specific element (PSE), and an octamer (OCT) A small number of transcription factors bind these elements and recruit RNA pol III RNA pol III initiates transcription in a manner similar to other polymerases 59

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61. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach RNA Polymerase III Promoters—The Internal Promoter Elements Genes for 5S rRNA and tRNAs have internal promoter elements called internal control regions (ICRs) These are two short DNA sequences, called either box A and box B for some genes, or box A and box C for other genes These are located between positions  55 and  80 61

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63. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach 63 ANIMATION: mRNA Production in Eukaryotes

64. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Post-Transcriptional Processing The initial eukaryotic gene mRNA is called the pre- mRNA whereas the fully processed mRNA is called the mature mRNA; modifications include 1. 5 capping 2. 3 polyadenylation 3. Intron splicing 64

65. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Capping 5 mRNA After the first 20 to 30 nucleotides of mRNA have been synthesized, a special enzyme, guanylyl transferase, adds a guanine to the 5 end of the pre-mRNA Additional enzyme action methylates the newly added guanine and may also methylate nearby nucleotides of the transcript The addition of the guanine to the mRNA is called 5 capping 65

66. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Functions of the 5 Cap 1.Protection of mRNA from rapid degradation 2.Facilitating transport of mRNA out of the nucleus 3.Facilitating subsequent intron splicing 4.Enhancing translation efficiency by orienting the ribosome on the mRNA 66

67. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Polyadenylation of 3 Pre-mRNA Termination of transcription by RNA pol II is not fully understood The 3 end of the pre-mRNA is created by enzyme action that removes a section of the 3 message and replaces it with a string of adenines This is thought to be associated with the subsequent termination of transcription 67

68. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Steps of Polyadenylation 1.Cleavage and polyadenylation specificity factor (CPSF) binds near the polyadenylation signal sequence—5-AAUAAA-3—which is downstream of the stop codon This is quickly followed by binding of cleavage- stimulating factor (CstF) to a uracil-rich region downstream of the polyadenylation signal sequence Two other cleavage factors, CFI and CFII, and polyadenylate polymerase (PAP) also bind 68

69. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Steps of Polyadenylation, continued 2.The pre-mRNA is cleaved 15 to 30 nucleotides downstream of the polyadenylation signal sequence 2.The 3 end of the cut pre-mRNA undergoes enzymatic addition of 20 to 200 adenines through the action of CPSF and PAP 69

70. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Steps of Polyadenylation, continued 5.After addition of the first 10 adenines, molecules of poly-A-binding protein (PABII) join the adenine tail and increase the rate of addition of adenines 70

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72. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Functions of Polyadenylation 1.Facilitating transport of mature mRNA across the nuclear membrane to the cytoplasm 2.Protecting the mRNA from degradation 3.Enhancing translation by enabling the ribosomal recognition of mRNA Some eukaryotic transcripts (e.g., the histone genes) do not undergo polyadenylation 72

73. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Pre-mRNA Intron Splicing Intron splicing requires great precision to remove intron nucleotides accurately Errors in intron removal would lead to incorrect protein sequences Roberts and Sharp shared the 1993 Nobel Prize for their codiscovery of “split genes,” i.e., the presence of intron and exon sequences 73

74. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Splicing Signal Sequences Specific short sequences define the junctions between introns and exons The 5 splice site is at the 5 intron end and contains a consensus sequence with an invariant GU dinucleotide at the 5-most end of the intron The 3 splice site at the opposite end of the intron has an 11 nucleotide consensus with a pyrimidine rich region and a nearly invariant AG at the 3-most end 74

75. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach The Branch Site A third consensus region, called the branch site, is 20 to 40 nucleotides upstream of the 3 end of the intron It is pyrimidine-rich and contains an invariant adenine called the branch point adenine near the 3 end of the consensus Mutation analysis shows that all three consensus sequences are required for accurate splicing 75

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77. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Splicing Introns are removed from the pre-mRNA by an snRNA-protein complex called the spliceosome Like molecular “workbench” The 5 splice site is cleaved first Then the 3 splice site is cleaved and the exon ends are ligated together 77

78. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach 78 ANIMATION: RNA Splicing

79. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Accurate Splicing Spliceosome components are recruited to 5 and 3 splice sites by SR proteins (pathfinders) SR proteins bind to sequences in exons called exonic splicing enhancers (ESEs), and ensure accurate splicing May play a roll in alternative intron splicing 79

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81. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Coupling of Pre-mRNA Processing Steps Introns appear to be removed one by one, but not necessarily in order The three steps of pre-mRNA processing are tightly coupled The carboxyl terminal domain (CTD) of RNA polymerase II functions as an assembly platform and regulator of pre-mRNA processing machinery 81

82. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Gene Expression Machines Current models suggest that RNA pol II and an array of pre-mRNA processing proteins function as “gene expression machines” The proteins that carry out capping, intron splicing, and polyadenylation associate with the CTD of pol II All three processes are carried out simultaneously 82

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84. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach 84 ANIMATION: RNA Processing Control

85. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Alternative Transcripts of Single Genes It is common for large eukaryotic genomes to express more proteins than there are genes in the genome For example, human cells produce over 100,000 distinct polypeptides but contain  22,000 genes Three transcription-associated mechanisms can explain this 85

86. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach The Human Genome Humans produce more than 100,000 distinct peptides Assumed 80,000 -100,000 genes We were wrong ~22,000 genes in humans What’s going on?

87. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Alternative pre-mRNA Processing 1.pre-mRNA can be spliced in alternative patterns in different cell types 2.Alternative promoters can initiate transcription at distinct start points 1.Different start point 3.Alternative localizations of polyadenylation can produce different mature mRNAs 1.Different stop point 87

88. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Alternative Intron Splicing Alternative intron splicing: processing of identical transcripts in different cells can lead to mature mRNAs with different combinations of exons and thus different polypeptides Approximately 70% of human genes are thought to undergo alternative splicing It is less common in other animals and rare in plants 88

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90. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Dscam The Drosophila Dscam gene has one of the most complex patterns of alternative splicing Of the 24 exons, numbers 4, 6, 9, and 17 have numerous alternative sequences More than 38,000 different polypeptides can be produced through alternative splicing Not all of the possible arrangements are observed, however 90

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92. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Alternative Processing Alternative splicing is mainly controlled by variation in SR proteins in different cell types Use of alternative promoters can occur when more than one sequence upstream of a gene can initiate transcription Alternative polyadenylation requires more than one polyadenylation signal in a gene Alternative promoters of polyadenylation are controlled by variable expression of regulatory proteins in specific cell types 92

93. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Rat α-tropomyosin 14 exons With 4 alternates Two promoters Five polyadenylation sites Yields 9 mature mRNA

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95. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Intron Self-Splicing RNAs can contain introns that catalyze their own removal There are three categories of self-splicing introns, group I, group II, and group III Group I introns were discovered in 1981 in the laboratory of Thomas Cech 95

96. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Post-Transcriptional RNA Editing In the mid-1980s, RNA editing was uncovered; that is responsible for post-transcriptional modifications to the nucleotide sequence (and the protein produced) of some mRNAs In one kind of RNA editing, uracils are added with the assistance of a guide RNA (gRNA), which contains a sequence complementary to the mRNA that it edits Editing may sometimes involve deletion of uracils, too 96

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98. Copyright © 2012 Pearson Education Inc. Genetics Analysis: An Integrated Approach Base Substitution A second type of RNA editing is base substitution, frequently replacement of cytosine with uracil This has been identified in mammals, most land plants, and some single-celled eukaryotes 98

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